1

AUTHORS NAME – Deepak Kumar Upreti

GUIDE NAME – Prof. B. C. Joshi

Physics Department

S. S. J. Campus,

Kumaun University Almora

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KEYWORDS

Fluorescence, Emission Spectra, Absorption spectra, Energy transfer, Rare earths, Phosphate glass, Dyes, Polymer, dipole‐dipole interaction, dipole‐quadrupole interaction etc.

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CERTIFICATE

This is to certify that the thesis entitled “Sensitized

luminescence in ultraviolet and visible excited phosphors

doped with rare earth ions and phosphors doped with dyes”

submitted by Mr. Deepak Kumar Upreti for the award of Ph.D.

degree in Physics is the outcome of his own bonafide work.

He has carried out the research work under my supervision

and has put in the required attendance under Para (ii) of the

ordinances, in the Department of Physics, Kumaun University,

S.S.J. Campus, Almora while supplicating for the Ph.D. degree.

(Prof. B. C. Joshi)

Department of Physics

Kumaun University

S.S.J. Campus

Almora- 263601

Uttarakhand (India)

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ACKNOWLEDGEMENT

At the foremost, I wish to express my deep sense of gratitude to my

parents Sri Basav Datt Upreti and Smt.Puspa Upreti and Smt. Chandra Upreti

(Tai ji) who took great pains for my education, bringing me to the present stage,

their continuous support for my research activities and patience during the

course of the work.

I am indebted to my supervisor Prof. B.C. Joshi, Head, Department of

Physics, Kumaun University, Uttarakhand (India) for perpetual guidance and

everlasting inspiration throughout the progress of the research work presented

in the thesis. He always made my problem simpler. I hope his guidance and

blessing will support me to face the challenges of the life.

Thanks are due to Prof. K.L. Shah, Prof. M.C. Durgapal, Dr. O.P.S. Negi,

Dr. P.S. Bisht in the Department of Physics, S.S.J. Campus Almora, Dr. Sanjay

Pant, Dr. Bimal Pandey in the Department of Physics, D.S.B. Campus Nainital

for their fullest co-operation, suggestions and moral support during present

work.

I acknowledge special thank to Prof. Kavita Pandey, former Head,

Department of Physics, Kumaun University Nainital and Prof. M.C. Pant former

Head, Department of Physics, S.S.J Campus Almora for their encouragement

and moral support.

I also express my gratitude to Mr. K.P. Pandey (mama ji), Mrs. Pratibha

Pandey (mami ji), Pooja, Sonu, Payal and Gaurav providing the family like

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facilities at Almora during the research work. Without whom the work was not

possible for me.

I thanks to Mr. R.S. Rayal, Mr. J.C. Upadhayay, Mr. Heera Singh

Kharayat, Mr. Nailwal, Mr. Joga Ram and Mr. Rajendra Singh Rana the non-

teaching staff of the Physics department S.S.J. campus Almora for their help as

required during the research work.

It gives me pleasure to express my special thanks to Dr. Rajiv Lohani, Dr.

C. C. Dhondiyal, Dr. Dinesh Yadav, Gaurav Pant, Rakesh Pandey, Dr. B. B.

Bhatt, Bhawana Khulbey, Mrs. Pratibha Fuloria, Gaurav, Chandrashekhar,

Puspa, Himanshu, Jagat, Neeraj, Ranjan and Kanchan, for their help &

suggestion during the work. Friendly cooperation of all the members of research

group of Photophysics lab, Kumaun University, Nainital is also gratefully

acknowledged.

All the teaching and non-teaching staff of Govt. P.G. college Dwarahat

are gratefully thanked for their help and suggestion during the research work.

Not getting words for expressing my sentiments to my loving sister

Kusum whose support is to me from the bed of hospital in serious condition.

At Last but not least I would like thank to almighty God for giving me

strength and determination needed for the successful completion of the work

presented in this monograph.

DATE: (DeepakKumar Upreti)

S.S.J.Campus, Almora

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PREFACE

The luminescence of rare earth ions in glass has been subject of

interest since the advent of lasers, because unlike other luminescence

centers, in glasses, sharp bands occur under proper excitation. Since in the

RE earths ion the electronic transitions occur within the inner shielded 4-f

electron, their spectroscopic properties are not influenced by their

surroundings and therefore generally shift in emission bands do not occurs

in glass of varying nature.

The interaction among rare earth ions as well as with other ions (like

UO2++, Mn2+ etc.) doped in different crystals or glasses or solution causes

the transfer of excitation energy of one ion to another ion. Such interactions

thus help in increasing the emission of the acceptor ion. Glasses are known

to be non-crystalline solids. In the development of RE doped optical device,

the choice of the host glass matrix is very important factor to be considered.

The choice of suitable glass former and glass modifier system help to meet

the specific requirement.

Such an energy transfer between rare earth ions to rare earth ions

find wide application in sensitizing solid state and glass lasers, infrared

quantum counters as well as in infrared to visible converters.

Interest has been spurred both by the special spectral characteristics

of the dyes and by their wide range of applications. Polymers appropriately

doped with dye molecule, emitting in the visible spectrum provide stable

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sources of light for displays and illumination sources at a significantly lower

cost than semiconductor. Organic light emitting diodes may indeed evolve as

the most inexpensive alternatives to fluorescent light sources. Polymer fibers

doped with organic dyes have proved to be potential candidate for use in

fiber lasers and fiber amplifiers media of holographic recording and

permanent optical memory, solar energy converters etc.

In the present work, the study of emission and optical interaction

(energy transfer) between various pairs of RE ions as well as the emission

and absorption spectra of dyes doped polymer has been made. The work

was carried out in the physics laboratory of, S. S. J. Campus, Almora and in

the D.S.B. Campus Kumaun University, Nainital. The work has been

presented in six chapters.

Chapter 1 contains a review of spectroscopic properties of rare earth

elements, the mechanism of energy transfer and relaxation processes in

rare earth elements. Along with these a comparison between properties of

glasses and crystals and optical properties of rare earth ions in glasses and

crystals has been also discussed. The origin of molecular spectra and

broadness of molecular spectra has been discussed. Brief discussion on

dyes doped in polymer has also been taken.

Chapter 2 contains the details of the chemicals used, their

composition and methods of preparation of samples. The experimental

procedure and apparatus used to study the fluorescence spectra, absorption

spectra; fluorescence decay time measurement of various samples is also

included.

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Chapter 3 contains the three series Tb3+-Nd3+, Tb3+-Er3+and Tb3+-Pr3+

in zinc phosphate glass for an investigation of energy transfer. In all the

above mentioned system Tb3+ is taken as sensitizer (donor) and Nd3+, Er3+,

Pr3+ are taken as activator (acceptor). Fong-Deistler theory is used to find

the multipolar term responsible for the energy transfer mechanism. The

energy transfer mechanism in Tb-Nd systems is explained using cross

relaxation. The linear dependence of energy transfer probability vs square of

acceptor + donor concentration shows the electric dipole-dipole interaction in

the above mentioned systems. The average donor-acceptor distance (DD→A),

energy transfer probability (Pda), transfer efficiency (η) has been computed

for all the three above mentioned systems.

Chapter 4 describes the study of energy transfer in Sm-Sm, Sm-Eu

and UO2-Er systems in zinc phosphate glass. The nature of energy transfer

between them, energy levels involved in energy transfer, the mechanism of

energy transfer as well as the multipolar term responsible for the transfer

have been discussed. Various parameters necessary for quantitative study

have been also calculated. The quenching of Sm3+ ions is also discussed.

Chapter 5 contains Dy-Pr and Tm-Er system in zinc phosphate glass

for the study of energy transfer. Various parameters necessary for

quantitative study i.e. donor-acceptor distance, energy transfer efficiency

and energy transfer probability have been also calculated. The electric

dipole-dipole interaction is mainly found to be responsible for the energy

transfer.

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Chapter 6 contains the study of emission and absorption spectra of

different dyes. Spectroscopic parameter such as molar extinction coefficient,

oscillator strength etc. have also been calculated from emission and

absorption spectra of various dyes doped in polymers. A dye series of

fluorescein and erythrosin B is presented for the study of energy transfer.

Critical transfer distance, reduced concentration and overlap integral are

calculated for the series.

The work published during the period of study has been listed in the

Appendix.

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Chapter1: Introduction Page No.

1.1 General introduction 1 1.2 Rare earth elements 2 1.3 Energy levels of rare earth ions 3 1.4 Spectroscopic properties of rare earth ions 7

1.4. A) Forced electric dipole transitions

1.4. B) Magnetic dipole transitions

1.4. C) Electric quadrupole transitions

1.5 Excitation and relaxation processes in rare earth ions 9 1.5. A) Radiative decay 1.5. B) Non-radiative decay 1.5. B.1) Forester theory 1.5. B.2) Dexter theory 1.5. B.3) Inukuti and Hirayama theory 1.5. B.4) Peterson and Bridenbaugh theory 1.5. B.5) Fong-Diestler theory

1.6 Multiphonon relaxation 17 1.7 Energy migration 22 1.8 Structure of the glasses 23 1.9 The rare earth ions in glasses and crystal: 25

Properties and Applications 1.10 Molecular spectra 29 1.11 Dyes 30 1.12 Dyes doped in polymer 31 1.13 Polymer structure and classification 32 1.14 Polymer synthesis 34 1.15 Energy transfer probabilities and efficiencies 38 1.16 Objective of the work 41 Bibliography

Chapter2: Experimental techniques

2.1 Introduction: 59 2.2 Choice and preparation of materials 60

2.2. A) chemicals 2.2. B) glass composition & preparation 2.2. C) glass series 2.2. D) polymer compositions and preparation 2.2. E) polymer series doped with dyes

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2.3 Absorption spectroscopy 64 2.4 Fluorescence spectroscopy 64 2.5 Fluorescence decay time 66

Bibliography

Chapter3: Study of Sensitize luminescence and energy transfer process in Tb-Nd, Tb-Er and Tb-Pr systems in zinc phosphate glass

3.1 Introduction: 75 3.2 Experimental materials and methods 77 3.3 Theory 78 3.4 Results and discussion 86

3.4. A) The Tb-Nd system 3.4. B) The Tb-Er system 3.4. B) The Tb-Pr system

3.5 Concluding remark 96 Bibliography

Chapter4: Study of sensitize luminescence and energy transfer process in Sm-Sm, Sm-Eu and UO2-Er systems in zinc phosphate glass

4.1 Introduction: 122 4.2 Experimental materials and methods 125 4.3 Theory 126 4.4 Results and discussion 135

4.4. A) The Sm-Sm system 4.4. B) The Sm-Eu system 4.4. B) The UO2-Er system

4.5 Concluding remark 142 Bibliography

Chapter5: Study of sensitize luminescence and energy transfer process in Dy-Pr and Tm-Er systems in zinc phosphate glass

5.1 Introduction 167 5.2 Experimental materials and methods 169 5.3 Theory 170 5.4 Results and discussion 172

3.4. A) The Dy-Pr system 3.4. B) The Tm-Er system

5.5 Concluding remark 177 Bibliography

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Chapter6: Study of emission and absorption spectra of some dyes doped in polymer and study of process of energy transfer from fluorescein to erythrosin-B in poly vinyl alcohol

6.1 Introduction 196 6.2 Experimental materials and methods 197 6.3 Theory 198 6.4 Molecular structure of dyes 202 6.5 Results and discussion 204

1) Emission and absorption spectra of different

dyes doped in polymer 2) Study of energy transfer from fluorescein to

erythrosin B in Poly vinyl alcohol (PVA). 6.6 Concluding remarks 206

Bibliography

APPENDIX

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CHAPTER 1 INTRODUCTION

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CHAPTER 1

INTRODUCTION

1.1 GENERAL INTRODUCTION:

The spectroscopy referred to a branch of science in which light (i.e.

visible radiation) is resolved into its constituent component wavelength to

produce spectra. Spectroscopy has proved powerful tool for qualitative and

quantitative analysis. The meaning of spectroscopy has become broadened

to include studies not only with light but also with other types of

electromagnetic radiation such as x-ray, ultraviolet, infrared, microwave and

radio-frequency radiation.

The processes in which a molecule or atom, when undergoes a

transition from a state of high energy to a state of low energy state emits the

excess energy as photon is called Luminescence. Luminescent materials,

also called phosphors are a substance which converts certain type energy

into electromagnetic radiation over and above thermal radiation. The

electromagnetic radiation emitted by a luminescent material is usually in the

visible range, but can also be in other spectral regions.

In some cases, it useful to sensitize the luminescence centre with the

help of some other atoms / ions (donors or sensitizers), which when excited

transfers its excitation energy to the atoms/ions under study (acceptor or

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activator) and consequently the emission efficiency of the acceptor is

enhanced. This process is celled sensitization and the luminescence is

termed as sensitized luminescence.

1.2 RARE EARTH ELEMENTS:

Rare earth (RE) elements comprise two interesting group of

chemical elements characterized by the presence of f-electron in

configurations giving rise to their optical spectra. Two series are the

lanthanides, ranging from lanthanum (La, Z=57) to lutetium (Lu, Z=71) and

the actinides, ranging from actinium (Z=89) to lawrencium (Z=103)

(Fig.1.1).Both of the series are characterized by partially filled 4f or 5f shell in

their ground state respectively [1].

Lanthanides are usually found in tripositive ions and possesses xenon

like electronic configuration i.e. 1s2,2s22p6,3s23p63d10,4s24p64d10,5s25p6.

The electron configuration of the trivalent lanthanides, which tend to be most

stable in solid hosts is (Xe)4fN with N=0 for lanthanum to N=14 for lutetium,

where (Xe) is electron structure of Xenon. Thus the ground electronic

configuration of these ions is 4fN and first excited ion configuration is 4fN-15d.

It is principally the properties of the trivalent ions which are important rather

than those of neutral atoms. Most of the rare earth elements can also be

stabilized in the divalent state appropriate host by the use of special growth

and post growth treatments [2].

The actinides have properties similar to those of lanthanides but as 5f

orbits are more exposed to the external field and the levels are easily

16

disturbed. Most of the actinides are highly radioactive hence their uses are

limited .The term RE will be used to denote only the lanthanides series in our

study. Because of their special spectroscopic properties and various

industrial applications, the rare earths have become one of the most

extensively studied groups of elements in the periodic table (Fig1.1)

1.3 ENERGY LEVELS OF RARE EARTH IONS:

The positions of energy levels arise from a combination of the

Coulomb interaction among the electrons, the spin orbit coupling and the

crystalline electric field [1]. The resultant splitting of the 4fN configurations

are shown schematically in Fig.1.2. The electrostatic interaction (Coulomb

interaction) yields terms 2S+1L with the separations of order of 104 cm-1. The

spin –orbit interaction (spin orbit coupling) then splits these terms into J

states with typical splitting of 103 cm-1. Finally, the J degeneracy of free ion

states is partially or fully removed by the crystalline stark field, yielding a

stark manifold usually extending over several hundred cm-1.

Fig.1.3 (energy level diagram) shows the location of the J states of

the trivalent rare earths. The order and the separation of within the J

manifolds vary, however small from host to host.

Dieke [2] and Wybourne [1] successfully solved the complex energy

level structures of the rare earths using tensor operator techniques and

crystal-field theory. The free ion states, obtained by diagonalizing the

combined electrostatic and spin-orbit matrices, are linear combination of

Russell-Saunders sates of the form

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[ ] ( ) SLfSLcSLf N

SL

N γγγγ∑= (1.1)

Where S, L, J are spin, orbital and total angular momentum

respectively. γ is other quantum number introduce to specify the states.

The crystal field reduces the (2J+1)-fold degeneracy of the above

free-ion states and causes a small admixing of J states. Because of the

shielding effects of the outer 5S and 5P shell electrons, the crystal field

interaction within the inner 4f electron is weak. The crystalline field can thus

be treated as perturbation on the free ion states. The crystal field potential is

extended in a series of spherical harmonic terms of the form

( )∑=i

ik

qk

qCF CBV (1.2)

Where the factor kqB are parameters describing the strength of crystal-field

components, kqC are tensor operator components which transform as

corresponding spherical harmonics, the summation is over ‘i’ electrons of the

free ions. The number and types of the terms appearing in the expansion in

eq.(2) are derivable using group theory and the point symmetry at the rare

earth site. For the f shell, k is limited to value ≤6.

In the above approach the strength of the crystal field is given by a

small number of kqB parameters. Attempts to calculate these parameters

using lattice sums and including covalency and overlap effects have

achieved only limited success. Therefore the values have been determined

experimentally. To do this the energy matrix including the crystal field

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interaction is diagonalzed using an estimated set of starting parameters. The

resulting predicted energy levels are compared with observed levels and, by

an iterative fitting procedure, the parameters are adjusted to obtained best

overall fit to experiment. When the positions of many levels have been

measured, and the site symmetry is high so that only a few terms appear in

the expansion in eq. (2), root mean square deviations of observed and

calculated energies as small as 10 cm-1 have been obtained.

The crystal-field parameters have also been interpreted using a

superposition model proposed by Newman [3]. The field is assumed to arise

from a sum of independent contributions from the other ions in the crystal.

Once the crystal-field parameters for an ion-host system have been

determined, a complete set of energy levels and eigenstates can be

computed. These state can labeled by an additional crystal quantum number

µ and are of the form

[ ] ( ) zN

SLz

N SLJJfSLJJcJSLf γγγγ

µ ∑=

(1.3)

These states can be used to calculate matrix elements for radiative

and nonradiative transitions between any rare earth fN energy levels of

interest.

For a free ion, the lowest multiplets of fN configurations are pure L-S

multiplets while the upper level deviates from L-S coupling. As the number of

f-electrons increases, the spin-orbit coupling constant increases more rapidly

than electrostatic parameter and the breakdown in L-S coupling becomes

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greater. The density of the levels plays an important role in the breakdown of

L-S coupling. For the dense levels, the average spacing between levels of

same total angular momentum (J) decreases resulting in mixing in of spin-

orbit interaction of same J but with different L-S. This fact improves the

energy level scheme of fN configuration. Such as 4fN-15d or 4fN-15g further

improves the energy level scheme. Other interactions such as orbit-orbit and

spin-orbit and spin-other orbit in fN shell should be considered for precise

treatment of the calculation of energy levels of a rare earth atom [4].

The energy levels of free rare ions is (2J+1) fold degenerate, because

there exists a spherical symmetry, while in perturbing medium such as

crystal field it reduces the (2J+1) fold degeneracy. This causes the small

admixing of J state [5]. Because of the shielding effects of the outer 5s and

5p shell electrons, the crystal field interaction with inner 4f electron is weak.

The crystal can, thus be treated as perturbation on the free ion states. The

crystal field is essentially electrostatic and that the magnetic field in the

crystal is negligible is explicitly exhibited by the fact that in all the ions with

odd electrons the crystal levels retain an ultimate double degeneracy

(Kramer’s degeneracy) which is removed by external magnetic field. These

results were solely supported by experimental evidences obtained by Bethe

and Kramer. Once the crystal-field parameters for a given ion host system

have been determined, a complete set of energy levels and eigen states can

be computed. However, the locations of higher lying configurations are not

well established because most of the levels are at energies beyond the

readily accessible optical region and above the fundamental absorption edge

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of many materials. For 5d states there is a strong interaction between outer

5d states and the static and dynamic crystal fields. As a consequence, the

locations of 5d states vary by many thousands of cm-1 in different materials

and the optical transitions have line widths.

1.4 SPECTROSCOPIC POPERTIES OF RARE EARTH IONS:

The optical absorption spectrum of the RE ions from the UV to the

mid-infrared originates from transition between electronic states in the

partially filled 4f orbital [2] shielded by filled 5s and 5p shells. These partially

filled shells of f electron give rise narrow a localized electronic transition that

occurs not only in visible parts of the electromagnetic spectrum but also in IR

and ultraviolet region. The electronic transitions between fN configuration,

which are responsible for the crystal spectra are all, in principle strictly

forbidden as electric dipole transitions. This is because the parity of

wavefunction of electrons does not change. This is true for the free ion. The

parity prohibition can be lifted only by the influence of the crystal lattice,

which leads to a mixing of states of opposite parity.

The absorption at low temperature takes place from the lowest stark

component of the ground state to various stark components of excited state.

The fluorescence spectrum arises due to the transitions from an excited

state of an ion to a lower state. Sometime the emission takes place from

more than one excited state, the terminal state may be the ground state or

any other higher state. At very low the temperatures the fluorescence

originates from lowest stark component of excited state. At higher

temperature the higher stark components becomes populated according to

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Maxwell-Boltzman law and these states may also participates in absorption

and fluorescence.

The presence of lines in the spectra of RE ions in crystal may be due

to one or more of following mechanism [6, 7]

1. Forced electric dipole transitions

2. Magnetic dipole transitions

3. Electric quadrupole transitions.

The transitions due to higher multipoles are negligible.

1.4. A FORCED ELECTRIC DIPOLE TRANSITIONS

According to Laporte’s selection rule the electric dipole transition

between f states are forbidden as these states have the same parity, while

electric dipole transition require a change of parity of the electron

wavefunction. But if rare earth ions are placed in a perturbing field such as

crystals, glasses or solutions, which generally lacks a centre of symmetry,

which causes the wave functions to be of mixed parity because of interaction

of 4fN level with the remote states of opposite parity such as 4fN-15d or 4fN-

15g.

1.4. B MAGNETIC DIPOLE TRANSITIONS:

Magnetic dipole transitions can only take place between

components of the same L-S coupling multiplets (∆L=0&∆S=0) for which

∆J=0 or ±1 with J=0 ↔ J=0. ∆J= ±1 indicates transition between

neighboring multiplet components and ∆J=0 has a meaning only in an

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external field. These transitions are not forbidden by the presence of a

centre of symmetry in the environment around the ion and are therefore

allowed between the states of same parity. When the lanthanide ion is

placed in a centre of symmetry, only MD transitions are possible. The

magnetic dipole transitions may thus be expected for many lines and also

been confirmed by experimental data.

1.4. C ELECTRIC QUADRUPOLE TRANSITION:

Selection rule for parity allowed electric quadrupole transition

between fN states are ∆S=0 and both ∆L & ∆J ≤2.The probabilities of these

transitions are usually several orders of magnitudes smaller than those for

dipole processes some transitions having distinctive polarization and angular

dependence have considerable intensities.

1.5 EXCITATION AND RELAXATION PROCESSES IN RARE EARTH

ELEMENTS:

Paramagnetic ions in solids can be treated as isolated ions only

when they are well separated. As concentration is increased or if non-

random distribution occurs, the ion spacing may become sufficiently small

for ions to interact. Such ion-ion interaction is important in the operation of

lasers and other fluorescence devices. When an ion (donor) is excited into a

metastabe state (level), the excited ions may relax to ground state as follows

(a) Luminescence from sensitizer (S).

(b) Radiationless decay in sensitizer (S).

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(c) Energy transfer from sensitizer (S) to another centre of type

sensitizer (S)

and

(d) Energy transfer from sensitizer (S) to activator (A).

Fig.1.4 shows a host lattice in which a sensitizer (S) and an

activator (A) are presented. In this scheme, four different processes can be

distinguished after excitation of S.

The relaxation of RE ions in excited electronic states includes:

(1) Radiative decay [8]

(2) Nonradiative decay [9] - which is again classified as:

(a) Non radiative decay wherein the excitation energy is converted

into vibrational quanta of surrounding. (i.e. ion – lattice interaction or

multiphonon emission).

(b) Non radiative transfer of energy between like and unlike ions

with possible degradation of excitation energy. (i.e. ion- ion interaction or

cooperative phenomenon).

1.5. A RADIATIVE DECAY:

When rare earths ion changes their electronic state by absorbing or

emitting photon, the optical transition is called radiative decay. Electric

dipole-dipole (EDD) interaction is dominantly responsible for the radiative

decay of rare earth ions. Magnetic dipole (MD) and electric quadrupole (EQ)

are allowed but their contributions are generally small and negligible.

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Radiative decay may take place as follow

(A) Radiative energy transfer between likes ions with possible

degradation of

excited energy by emission of photons (Phosphorescence).

(B) Radiative energy transfer between unlike ions by ED,MD and QD

interactions.

The radiative energy transfer process also called trival process

occurs merely by the absorption of photons emitted by donors. The

radiative transfer is easily treated by measuring the absorption and

emission characteristics of ions involves. Radiative transfer rate depends

on the number of ions between the excited volume and the absorber and

their emission or absorption strength [8].

1.5. B NON-RADIATIVE DECAY:

The non-radiative transfer from donor to acceptors depletes the

population of excited state of the donor and decreases the intensity and

lifetime from the excited state to lower state [10]. In order that the transfer is

significant and measureable, the rate of energy transfer must be of the same

order of magnitude as the radiative transition in donor ion. The non-radiative

transfer can be subdivided into following three categories [11]

(i) Multipolar resonance,

(ii) Multipolar transfer, and

(iii) Non-resonant energy transfer.

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In the simple case of two ions each with one excitable electronic

state separated from its electronic ground state by nearly equal energy and

coupled by suitable interaction between two electron system, the excitation

will jump from one ion to another before one is able to emit a quantum of

fluorescence. The mutual interaction is the Coulomb interaction between two

ions and the process is known the non-radiative transfer of energy from

donor to acceptor ions. In early attempts Perrin [12] formulated a classical

theory of excitation energy transfer between molecules in solutions. Transfer

distances of more than 100Å were predicted by this theory. Other attempts

are as follows

1.5. B.1) FORSTER THEORY:

Forster [13] first treated the problem by quantum mechanically by

considering the dipole-dipole interaction assumed that the interaction

between two well separated ion is strongest if for both the ion’s electric

dipole transitions are permitted. The energy transfer probability from

sensitizer (S) to activator (A) is given by

η S A= ∫∞

046046

2 )()(128

10log9000 dvv

vvfNRn

K As

s

ετπ

(1.4)

While critical transfer distance R0 for which excitation transfer and emission

of the sensitizer are equally probable is given by

∫∞

=0

446

0260

)()(128

10ln9000dv

vvvf

NnK

R Asj επ

η (1.5)

26

Where v is the wave number f ( v ) is the spectral distribution of

fluorescence(measured in quanta and normalized to unity on a wave no.

scale). )(vAε is the molecular decade extinction coefficient, N is the avogadro

number, n is the refractive index of the host matrix, R is the distance

between molecules and 0sτ is the radiative lifetime of the excited sensitizer,

K is a constant that depends on a mutual orientation of two doped molecules

and 0jη is the donor fluorescence quantum yield in the absence of acceptor.

Therefore the transfer probability can be written as

6

00

1⎟⎠⎞

⎜⎝⎛=→ R

R

SAS τ

η (1.6)

This shows R -6 dependence of the transfer rate.

1.5. B.2) DEXTER THEORY:

Dexter [14] extended Forster’s theory and includes the case of

forbidden transition moments in donors and acceptors. However his theory is

applicable to rigid media only. He considered the following cases:

(i) Electric dipole-dipole (d-d) interaction - This is simply a

repetition of of Forster’s results and predicts a R-6

dependence of the transfer rate

(ii) Electric dipole-quadrupole (d-q) interaction – In this case the

transfer rate varies as R-8 and may give rise to sensitization

upto 102 lattice site.

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(iii) Exchange interaction – Energy transfer by this mechanism

occurs as a result of overlap of electronic clouds of donors

and acceptor and such transfer does not allow to occur more

than approximately 40 lattice site. The transfer probability by

exchange mechanism is given by Dexter as,

( ) ( )dEEff ASSA ∫ ΕΖ=Ρ 2

2

2h (1.7)

Where ⎟⎠⎞

⎜⎝⎛ −=Ζ

.2exp22

LRK

In this equation K is a constant with dimension of energy. L is the

effective Bohr radius. )(Efs and )(Ef A are donor and acceptor absorption

spectra respectively. He correctly concluded that direct exchange, with it’s

exponential radial dependence is probably too short range for effective

energy transfer in dilute materials.

Although Dexter theory describes reasonably the concentration

dependence of transfer rate, the radial dependence and dominant

interactions in some cases are often ambiguous. At concentration large

enough to show substantial ion pair decay, resonant transfer can be

exceedingly fast among donor ions, particularly since the degree of

resonance for ion pair decay can be expected to generally small. Therefore,

when donor ions are surrounded by a greater number of acceptor ions, the

average will dominant the decay and the short range (stronger) interaction

will be enhanced. At the low concentrations, where the average separation is

28

large, the longer range interactions such as electric dipole-dipole will be

dominant. Thus in a given material, different interaction will dominant for

different concentration ranges.

1.5. B.3) INOKUTI AND HIRAYAMA THEORY:

In the Dexter’s theory it was assumed that luminescence was

dominated by the transfer to the nearest acceptor ion .An extension of the

entire environment including dynamics of the transfer was formulated by

Inokuti & Hirayama [15].

The emission intensity of the donor decays as a result of

electrostatic multipolar interactions with acceptors, when donor & acceptor

ions are randomly distributed and the donor ions are excited by a flash light,

according to the following equation:

( ) ( )⎪⎭

⎪⎬

⎫

⎪⎩

⎪⎨

⎧

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛ −Γ−−=

StCC

Stt

3

000

31exp0ττ

φφ (1.8)

Where C is the concentration of acceptor, C0 is the critical transfer

concentration.τ0 is the donor decay time in the absence of acceptor and

s=6, 8, 10 corresponding to EDD, EDQ& EQQ interaction between donor &

acceptor respectively. The Inokuti-Hirayama theory also treats the direct

exchange interactions. Where the decay function is given by:

where( ) ( ) ( )

LR

tgCCtt

0

00

3

0

2

expexp0

=

⎭⎬⎫

⎩⎨⎧

⎟⎟⎠

⎞⎜⎜⎝

⎛−−= −

γ

τγγ

τφφ

(1.9)

29

1.5. B.4) PETERSON AND BRIDENBAUGH EXPLAINATION:

The cooperative relaxation of rare earth ions can also occur by

following ways: [16, 17, 18, and 19]

1. Cross relaxation of energy between the same ions.

2. Cross relaxation of energy between different ions.

Cross relaxation of energy between the same ions is observed in

the concentration quenching of some rare earth ions which occurs due to the

production of pairs of the same ions. This mechanism of excited state

relaxation was first given Varsanyi and Dieke [20] and later by Peterson and

Bridenbaugh [21]. If a level ’A’ above a stable fluorescent level ‘B’ is

observed and the difference (A-B) is used for multiple phonon excitation. If

(A-B) becomes very large, the energy is not transferred directly from ‘A’ to

‘B’ and ‘A’ becomes a stable fluorescent level. On other hand, if (A-B)

approach the excitation energy of lower level of another ion of same species,

the transfer is again occurs, that is, fluorescence from ‘B’ is again observed.

The difference in energy (A-B) is then used to excite the second ion. In this

way the ion pair resonance causes to quench the fluorescence intensity

(Fig.1.5).

30

1.5. B.5) FONG-DIESTLER THEORY:

The problem of energy transfer by the use of many body interaction

mechanisms is given by Fong & Diestler [22]. According to which, at low

donor acceptor concentration the transfer rate per ion varies linearly with the

concentration and the transfer occurs by a pair wise (two body) interaction

.At higher concentration a higher order interaction mechanism can take

place. In general the per ion transfer rate PDA varies linearly with Cn-1 where

C is the concentration of the donor & acceptor ions and n determines the

order of the process.

1.6 MULTIPHONON RELAXATION:

Non radiative energy transfer between J states can occur by the

simultaneous emission of several photons sufficient to conserve the energy

of transition. These multiphonon processes arise from interaction of the rare

earth ion with fluctuating crystalline electric field. The crystal field at the ion

site is not static but undergoes oscillatory behavior due to the vibrations of

lattice or molecular group. For one phonon relaxation process, the energy

gap between the electronic levels of RE ion should be equal to the energy of

one phonon of the host matrix. If the energy gap is large compared to the

maximum energy of one phonon, then the relaxation can occur by emission

of several phonons of the host, simultaneously to conserve the energy [23].

Huang and Rhys [24] developed a single configuration coordinate

model based on the Franck-Condon principle. Which is extended to arbitrary

31

oscillator parabola and parabola offsets by Struck & Fonger [25, 26]. They

derived an eqn. for the multiphonon relaxation rate, which is given here in an

approximation valid for f-f transitions in RE ions [26].

Wmp=( )[ ] ( )[ ]12exp

!1

00 +−

+B

pB nSp

nSN (1.10)

Here N is an empirical factor of the order of 1013 S-1. S0 is Huang-

Rhys Pekar number, which measures the parabola offset. nB the Bose-

Einstein occupation number of the effective phonon mode = [exp (ħω / KT)

– 1]-1 and p is the smallest integer no. of phonons needed to bridge the

energy gap.

Kiel [27] considered the mode of rare earth ions in crystals relative

to nearest neighbours and then expanded these oscillators in terms of the

lattice modes. When lattice and the ion are treated as a coupled system,

optical transitions are considered to occur between vibrational-electronic or

vibronic states. The wavefunction ψ of these states can be written as

product of the ion electronic state eψ and the network normal vibrational

modes each characterized by its occupation number, ni

ii

e n∏= φφ

And the crystal field Hamiltonian HCF can be expanded in a Taylor

series about the equilibrium ion position, as

∑∑ ++=ij

jijii

iiCF QQVQVVH ,0 (1.11)

32

Qi represents ith normal mode coordinate. VCF is equilibrium or

static field. Vi, j are the partial derivative of crystal field.

The one phonon transition rate (W1) involves matrix elements of

those modes that couple to the ion through the second term of equation

(1.11) and also conserved energy between the initial electronic state aψ and

the final state bψ

2

12 ∑ ∏ ∏

≠

=i j ij

jiijaib nnQnVW φφπh

(1.12)

For large energy separation between the initial and final state is

greater than highest energy of one phonon of the matrix, the perturbation

calculations must be done in higher order to allow emission of two or more

phonon.

Kiel, higher order time-dependent perturbation theory, which was

extended by Riseberg and moos [28]. The three prediction of the

multiphonon relaxation theory are

(i) The exponential dependence of multiphonon rates on energy

gap.

(ii) The importance of the highest energy phonons in the material.

and

(iii) The temperature dependence of multiphonon decay.

33

Glasses differ from crystals, as glasses have lack of symmetry and

have the molecular character of high energy vibrations. Layne et.al. [29]

modify the Kiel approach of multiphonon theory for the glasses. In glasses

few independent oscillators are attach to ion, while in crystal oscillator

(modes) are considered only relative to nearest neighbour.

Considering that glasses lack both the point group symmetry and

space group symmetry of periodic crystal lattice and taking the single

phonon term in the expansion of potential in a p-order perturbation

calculation, the rate for a transition across an energy gap E∆ accompanied

by emission of p photons all of energy pEh ∆=ν and simplifying the

calculation is given by

( )( ) )1(2

2

22

)1(2 12

22 −− +⎟

⎠⎞

⎜⎝⎛= p

p

ppppp W

bVaMn

MWW

h

h

h

π (1.13)

Where ‘a’ and’ b’ denotes the intermediate state and M is he reduce

mass.

The rate of multiphonon relaxation is temperature dependent. This

rises from its dependence on the Bose-Einstein occupation number of

phonon mode

11/exp()( −−= kThTn ν

For a p-order process, the temperature dependence of (n+1)p identifies the

order of the process and the energy of the photon νh .

34

The dependence of the multiphonon rate on the energy gap to be

bridged results from comparing the rates of a p phonon decay process with a

(p-1) phonon decay process. The ratio of Wp and Wp-1 is, thus,

( )2

2

2

1 2)1(4

W

bVaMW

mnWW

p

p

h

h+=

−

For weak perturbation

11

⟨⟨=−

εp

p

WW

This result leads to the following exponential dependence of the rate

of energy gap:

EW

PWWW np

p ∆⎟⎠⎞

⎜⎝⎛==h

)(exp00

εε (1.14)

Considering equations (D) and (E), the rate for p-order multiphonon

decay is

( )EThcW pp ∆−+= αexp1)(

Where, c is a constant depending upon the host. This result

expresses both the exponential dependence of multiphonon relaxation rate

on the energy gap which the decay takes place and the explicit temperature

dependence. In spite of the difference between crystals and glasses this

theory gives good results for both mediums.

35

1.7 ENERGY MIGRATION:

Multiphonon relaxation and cooperative relaxation described in

above articles are one step processes involving resonant energy transfer

between donors and acceptors. Relaxation by energy migration is a

multistep process involving resonant energy transferred from one ion to

another of the same species in a random walk manner and finally to an

acceptor which acts as a quenching centre or energy sinks (Fig. 1.7).

Migration becomes increasingly dominant with the increasing RE content. In

concentrated materials or where the RE is a constituent of the host, the

probability for energy transfer between donor ions may be large. Rapid

energy diffusion can lead to a spatial equilibrium of excitation within the

donor system. The rate limiting step for the donor relaxation becomes either

the donor acceptor transfer rate or the acceptor relaxation rate. A simple rate

equation model for the donor system relaxation predicts a simple

exponential decay in the limit of fast diffusion.

When the rate of energy diffusion within the donor system is slow

but still comparable to the intrinsic decay rate, the donor decay is composed

of two competing processes (i) Excited donors near acceptors relax

predominantly by direct ion pair energy transfer (ii) Distant excited donors

first diffuse into the vicinity of an acceptor before relaxation occurs.

Yakoto & Tanimoto [30] obtained a general solution for the donor

fluorescence decay function including both diffusion within the donor system

& donor exception energy transfer. Experimental evidences from migration

36

of energy have been reported by a number of workers [31-34]. Weber [35]

used Yakota- Tanimoto equation to analyze his data to explain energy

transfer between Eu3+ and Cr3+ in phosphate glass. Another developed

technique for the observation of energy migration in rare earth systems

involves excitation with a narrow band laser in a system with significant

inhomogeneous broadening [36, 37]. In such a system excitation is

selective, involving only a narrow energy bands within the inhomogeneously

broadened system. A narrowed fluorescence signal is observed,

corresponding to emission by that class of ions that has excited. Migration

from these ions to other ions can then be studied by observation of the line

narrowed signal.

Many higher lying excited levels of rare earth ions give

luminescence in various crystalline matrices such as LaCl3 and CaF2 [38]

while these levels do not luminance in some glasses such as borate and

phosphate. Such a quenching of fluorescence of these levels occurs due to

the transfer of electronic excitation energy to the vibration of the surrounding

medium.

1.8 THE STRUCTURE OF THE GLASSES:

Though different in composition & structure, the common property of

all glasses is that they don’t nucleate during the process of cooling from

molten to rigid solid [39]. Like crystals, some glasses consist of a random

three dimensional continuous network of ions [40] and other contain like ions

whose locations are preferentially close to one another and not completely

37

random. Also, some glasses consist of regions of preferred structural group

[41], The chemical bonding forces within the group being stronger than those

between the groups even metals may be produced in the amorphous or a

microcrystalline state but this lack of crystalline structure does not put them

in the category of glasses .A rigid microcrystalline state is called a glass if on

heating it turns into a liquid in a reversible fashion [42].

According to the present theories of the glass structures, oxide

glass of our type chosen for our work are formed from networks of glass with

ions as B,P or Te, which are strongly linked by bridging oxygen ions. Distinct

structural units, such as BO4, PO4, SiO2 tetrahedral exist in glasses with

random orientation [43]. On adding network modifier, such as alkaline ions

(CaO, ZnO, BaO), break up the three dimensional network, thereby

introducing non-bridging oxygen ions. The MO4 tetrahedral (M=B or P) are

undistorted, since their covalent bonding strongly favours preservation of the

tetrahedral geometry. On other hand, the relative positions of the tetrahedral

can be changed in relation to RE ions.

Reisfeld et.al. [44] have proposed that the RE ion in glasses is

coordinated by four MO4 tetrahedra, each tetrahedral contributing two

oxygens to the coordinate with the rare earth (RE). Reisfeld et.al. [45] have

also shown that the behavior of RE ions in glasses is similar to that in

inorganic crystals of low symmetry except for the inhomogeneous

broadening of the spectra because of the site to site variation in crystal field

of the glass. According to their theory, a rare earth ion occupies the centre of

a distorted cube which is made of four tetrahedral of borate, phosphate or

38

silicate. Two oxygen belonging to such a tetrahedral, produce an edge of the

cube as shown in Fig.1.8. This model has been adopted for the present

study of the glass matrices chosen by us. The coordination of RE is such an

arrangement is eight oxygens. The site different can occur from the

existence of non-uniform, non-identical crystal field caused by slightly

different ranges of RE-oxygen distances. The average RE-oxygen distances

calculated by Reisfeld et.al. are 2.1Å and 2.2Å [46] for borate and phosphate

glass respectively. In these glasses, the symmetry is lower than cubic

symmetry.

The position of rare earth ion in borate or phosphate glass can be

assumed as shown in Fig. The RE ion is situated in a cube with eight oxygen

ions at the corners of the cube. Each edge of the cube is common to the

cube and tetrahedron. The cube is not a regular one, but is distorted by the

relative twisting of the tetrahedral which can be situated at angles other than

900 relative to each other [47]. Therefore a low symmetry Cs for the site is

formed. From the inhomogeneous broadening, we can conclude that while

this low symmetry is conserved for all the sites, the position of given site

relative to the surrounding oxygen may depend on the exact of the doped

rare earth ion the cube.

1.9 THE RARE EARTH IONS IN GLASSES AND CRYSTALS:

PROPERTIES AND APPLICATION

The properties of glasses differ from those of crystals and liquids.

The crystals have orders atomic structure, while liquids and glasses

39

possessing a degree of atomic disorder. In glasses (solids), the atom

maintains permanent positions with respect to the location of their

neighbours. Like liquids, glasses are good host matrices for the rare earth

ions.

The luminescence of rare earth ions in glass has been a subject of

renewed interest since the advance of Laser, because, unlike the other

luminescence centre, in glasses, sharp bands occur under the proper

excitation. Since in the RE ions the electronic transition occur among the

inner, shielded 4f-electron their spectroscopic properties are not greatly

influence by their surrounding, and therefore large shifts in emission bands

do not occur in glasses of varying nature [39].

With the upsurge of interest in the optical properties of ions in

glasses and crystals, trivalent rare earths ions have received wide attention

by virtue of their enormous technological applications in the field of

phosphors. There are some phosphors, which have good absorption

properties, but they emits in the undesired wavelength region. The

properties of such phosphors may be improved by the introduction of an

activator which can received energy from the emitting centre already present

and emit it in desired wavelength region. The narrow band absorption and

emission in lanthanides arise from parity forbidden weak transitions,

whereas the post transition group ions, such as Tl+, Pb2+,Sb3+,Sn2+, Bi3+,

generally correspond to emission of broad bands due to parity allowed

transitions. Other hand such as UO2++, Mn++ etc. have high fluorescence out,

even if involving weak transitions. All such ions may sensitize and increase

40

the population of the excited levels of lanthanides by one or two orders of

magnitude compared with the direct absorption of light into the rare earth

ions. The sensitized luminescence is not only useful in providing several

useful photoluminescence phosphors for florescent lamps, but also are of

immense important in improving laser efficiency, infrared quantum counters,

infrared to visible up-converters as well as luminescent solar concentrators

and optical fibers.

The emissions of RE ions in glasses are usually broad in

comparison to crystalline matrices due to different environment at different

rare earth sites which causes a variation in crystal field and, hence giving an

inhomogeneous broadening of the doped RE ion energy levels. Such an

inhomogeneous broadening of spectral bands in glasses is important in

study of energy transfer since these causes in the overlap between donor

and acceptor levels [48]. Also the phonon energy of glasses is higher [49],

which also help in increasing the energy transfer. It is, therefore, necessary

to have an insight in the structure of glasses and position of the doped ion in

the glass.

Due to lack of periodicity in the atomic arrangements, potential

luminescence centers may find themselves in a wide variety of energetically

different environment within a given glass. In general this leads to a

broadening of the luminescence bands compared to the branch of similar

centers in crystals [50]. The doped luminescent ions in glass donor exist as

a part of the glass structure by rearrangements of the constituent atoms.

41

The thermodynamic equilibrium doesn’t exist in glasses. While the

free energy of a glass is below to that of a super cooled liquid. It is higher

than that of its crystalline counterpart. However glass formation is not

dependent upon the difference in the free energies of the glass and crystals

[51]. The height of the energy barrier between the liquid and the crystalline

states at the liquid state temperature determine whether the liquid will revert

to a glass or to a crystal upon cooling .The energy relationships [51] are

shown schematically in Fig. 1.9. This plot of free energies as a function of

atomic structure locate glasses in a flat cavity, which means that a glass of a

given composition can exist in different energy states depending on its

structure and consequently the properties of a glass cannot be defined by

specifying the usual parameters of composition, pressure and temperature.

Weber [52] used laser induced fluorescence line narrowing

technique to investigate the local fields and interactions of paramagnetic

ions in oxide glasses. Since the paramagnetic ion enters into glasses as a

network modifier cations and because the differences in the bonding to

nearest neighbouring anions in multicomponent glass composition, the local

fields at individual ion sites vary. These result in the variation of radiative and

non-radiative transition probabilities as is evident from the inhomogeneous

broadening of absorption and emission lines and non-exponential excited

state decays observed for glasses [53]. In the fluorescence line narrowing

technique, short laser pulse of narrow spectral width is used to excite

selectively an energetically specified subset of ion environment within an

inhomogeneously broadened absorption line. If the emission is measured

42

before substantial cross relaxation between ions in different sites occurs.

The resulting spectrum characterizes only that subset of ions, which were

originally excited. Thus by exciting at a sequence of different frequencies

within the absorption line, one probes the variation of the spectroscopic

properties of the entire ensemble of environments.

1.10 MOLECULAR SPECTRA

Absorption spectra of atoms consist of sharp lines, whereas

absorption spectra of molecules show broad bands in UV/VIS region. These

may exhibit some vibrational structure, particularly in case of rigid molecule.

Polyatomic molecule possessing a large number of normal vibrational

modes of varying frequencies having closely spaced energy levels. As a

result of line broadening due to inhomogeneity of the interactions between

solute molecule and solvent, to hindered rotation, and to the short lifetimes

of the higher excited states, the vibrational structure may be either

unresolved or only partly resolved (Fig.1.10).

The vibration structure may be explained as follows. For each state

of molecule there is a wavefunction that depends on time, as well as on the

internal space and spin coordinates of all electrons and all nuclei, assuming

that the overall translational and rotational motions of the molecule have

been separate from internal motion. A set of stationary states exits whose

observable properties, such as energy, charge density etc. do not change in

time. These states may be described by time-independent part of their wave

functions alone. Their wave functions are the solutions of time independent

43

Schrödinger equation and depend only on the internal coordinates q=q1,

q2,……….. of all electrons and internal coordinates Q=Q1,Q2,………. , of all

nuclei.

Within the Born-Oppenheimer approximation, the total

wavefunction Tψ of stationary state is written as

( ) ( )Qq vj

jQ

vjT χψψ =, 1.15

Where ‘j’ characterizes the electronic state and ‘v’ the vibrational sublevel of

that state (Fig.1.11). And the energy of a stationary state of wavefunction of

equation (1.15) is written as

)()( vibel EEE += 1.16

As a result, each electronic state of molecule with energy E(el) = EQj carries a

manifold of vibrational sublevels, and the energy of an electronic excitation

may be separated into an electronic component and vibrational component

according to

)()( vibel EEE ∆+∆=∆ 1.17

1.11 DYES

Dyes are usually a coloured organic compound or mixture that has

the ability to impart their colour to materials to be dyed in an aqueous

medium. However all the colored substances are not necessarily dyes nor

44

that all dyes are necessarily colored therefore optical brightness or whiteners

(or white dyes) are also included in the term dye.

According to the molecular orbital theory (MO) a molecule, in ground

state the electrons are present in sigma (σ ), pi (π ), or non-bonding

molecular orbitals. When it absorbs radiations there is electronic excitation

i.e., electrons are transferred from bonding to anti-bonding orbitals (σ *, π *).

In M.O. theory the transitions of electron is considered singly. Out of these

transitions σσ → * and σ→n * transitions are difficult and take place only by

the absorption of U.V. radiations. Hence saturated compounds are

colourless. But the ππ → * and π→n * transitions are easier as they

required less energy.

1.12 DYES DOPED IN POLYMER

Polymers are high molecular weight compounds which are composed

of a large number of simple repeating units of one or different type

substances of low molecular weight. These small units are called monomers.

Unmatched in the diversity of their properties, polymers such as nylon,

cotton, wool, rubber, teflon and all plastics are used in nearly every industry.

Natural and synthetic polymers can be produced with a wide range of

stiffness, strength, heat resistance and even price. With continued research

into the science and applications of polymers, they are playing an ever-

increasing role in society.

Incorporation of dyes in solid materials instead of liquid is as old as

dye lasers itself. From application point of view polymers are the first choice

45

as solid host material due to the well-established production techniques and

the very low price. The dye doped polymers are used to develop light

emitting diodes, Lasers, luminescence solar collector, optical sensors,

biological sensor used to investigate the dynamics of protein folding, DNA

structure etc..

Most of the natural and synthetic polymers don’t show luminescence

properties [54]. However luminophoric properties can be introduced in

polymer by doping dyes in it. The photo induced excited state relaxation

processes in various polymers are useful for the study of different specific

interactions between dye and host polymer, which change the photophysics,

and photochemistry of excited molecules due to various microenvironment

effects [54-56].

1.13 POLYMER STRUCTURE AND CLASSIFICATION

The physical structure of chain is also an important factor that

determines the macroscopic properties. The geometrical structure of

polymer is described using the term configuration and conformation [57].

Configuration: Configuration refers to the order that is determined by

chemical bonds and cannot be altered unless chemical bonds are broken

and reformed. The two types of polymer configuration are cis and trans. Cis

configuration arises when substituent groups are on the same side of a

carbon-carbon double bond. Trans refers to the substituents on opposite

sides of the double bond.

46

Conformation: Conformation refers to order that arises from the rotation of

molecules about the single bonds .If two atoms are joined by a single bond

then rotation about that bond is possible since unlike a double bond, it does

not require breaking the bond. The ability of an atom to rotate this way

relative to the atoms, which it joins, is known as an adjustment of the torsion

angle. If the two atoms have other atoms or groups attached to them then

configurations that vary in torisional angle are known as conformations.

Since different conformations represent varying distances between

the atoms or groups rotating about the bond, and these distances determine

the amount and type of interaction between adjacent atoms or groups,

different conformation may represent different potential energies of the

molecule. Three possible conformations are: Anti (Trans), Eclipsed (Cis)

and Gauche (+ or -).

The geometric arrangement of the bonds is not the only way the

structure of a polymer can vary. A branched polymer is formed when there

are side chains attached to main chain. In star branching polymerization

starts with a single monomer and has branches radially outward from this

point. Polymers with high degree of branching are called dendrimers. Often

in these molecules braches themselves have branches. This tends to give

the molecule an overall spherical shape in three dimensions.

Often it is possible to obtain polymers with new and desirable

properties by incorporating more than one kind of monomers into their chain.

Such polymers are known as copolymer. Three main types of copolymer

are random, block and graft. A random copolymer contains a random

47

arrangement of the multiple monomers. A block copolymer contains blocks

of monomers of the same type. Finally a graft copolymer contains a main

chain polymer consisting of one type of monomer with branches made up of

other monomers.

Cross linking: In addition to the bonds which hold monomers together in a

polymer chain, many polymers form bonds between neighboring chains,

these bonds can be formed directly between the neighboring chains, or two

chains may bond to third common molecule. Though not as strong as the

bonds within the chain, these crosslink have an important effect on the

polymer. Polymers with a high enough degree of cross-linking have memory.

When the polymer is stretched the crosslink prevent the individual chains

from sliding past with each other. The chains may straighten out once the

stress is removed they return to their original position and the object returns

to its original shape.

1.14 POLYMER SYNTHESIS

Polymer synthesis is a complex procedure and can take place in a

variety of ways [58]. Addition polymerization describes the method where

monomers are added one by one to an active site on the growing chain.

Addition polymerization The most common type of addition polymerization

is the free radical polymerization. A free radical is simply a molecule with an

unpaired electron. The tendency for the free radicals to gain an additional

electron in order to form a pair makes it highly reactive so that it breaks the

bond on another molecule by stealing an electron, leaning that molecule with

48

an unpaired (which is another free radical). Free radicals are often created

by the division of a molecule (known as an initiator) into two fragments along

a single bond. The following diagram shows the formation of a radical from

its initiator, in this case benzoyl peroxide.

The stability of a radical refers to the molecule΄s tendency to react

with other compounds. An unstable radical will readily combine with many

different molecules. However a stable radical will not easily interact with

other chemical substances. The stability of free radicals can vary widely

depending on the properties of the molecules. The active center is the

location of the unpaired electron on the radical because this is where the

reaction takes place. In free radical polymerization, the radical attacks one

monomer, and the electron migrates to another part of the molecule. This

newly formed radical attacks another monomer and the process is repeated.

Thus the active center moves down the chain as the polymerization. There

are three significant reactions that take place in addition polymerization:

Initiation (birth), Propagation (growth) and termination (death).

Initiation reaction: The first step in producing polymers by free radicals is

the initiation. This step begins when an initiator decomposes into free

radicals in the presence of monomers. The instability of carbon-carbon

double bonds in the monomer makes them susceptible to reaction with the

49

unpaired electrons in the radical. In this reaction, the active center of the

radical grabs one of the electrons from the double bond of the monomer,

leaving an unpaired electron to appear as a new active center at the end of

the chain. Addition can take place at either end of the monomer. In a typical

synthesis, between 60% and 100% of the free radicals undergo an initiation

reaction with a monomer. The remaining radicals may join with each other or

with an impurity instead of with a monomer. "Self destruction" of free radicals

is a major hindrance to the initiation reaction. By controlling the monomer to

radical ratio, this problem can be reduced.

Propagation reaction: After a synthesis reaction has been initiated, the

propagation reaction takes over. In the propagation stage, the process of

electron transfer and subsequent motion of the active center down to the

chain proceeds. In free radical polymerization, the entire propagation

reaction usually takes place within this time. The entire process stops when

the termination reaction occurs.e.g. In this example (Chain) refers to a chain

of connected monomers, and X refers to a substituent group (a molecular

fragment) specific to the monomer. For example if X were a methyl group,

the monomer would be propylene and the polymer, polypropylene.

In free radical polymerization, the entire propagation reaction usually

takes place within a fraction of a second. Thousands of monomers are

50

added to the chain within this time. The entire process stops when the

termination reaction occurs.

Termination reaction: Theoretically, the propagation reaction could

continue until the supply of monomers is exhausted. However this outcome

is very unlikely. Most often the growth of a polymer chain is halted by the

termination reaction. Termination typically occurs in two ways: combination

and disproportionation.

Combination occurs when free electrons from two growing chains that

join and form a single chain stop the polymer growth. The following diagram

depicts combination, with the symbol (R) representing the rest of the chain.

Disproportionation halts the propagation reaction when a free radical

strips a hydrogen atom from an active chain. A carbon-carbon double bond

takes the place of the missing hydrogen.

51

Living polymerization: it does not undergo termination reaction and

continues until the monomer supply has been exhausted. When this

happens, the free radicals become less active due to interaction with the

solvent molecules. If more monomers are added to the solution, the

polymerization will resume.

The bulk polymer is characterized by a wide distribution of molecular

weights and chain lengths. The degree of polymerization (DP) refers to the

number of repeat units in the chain, and gives a measure of molecular

weight. Many important properties of the final result are determined primarily

from the distribution of lengths and the degree of polymerization.

1.15 CALCULATION OF NON-RADIATIVE ENERGY TRANSFER

PROBABILITIES (PDA) & EFFICIENCIES (η):

Reisfeld [59] has proposed a method to calculate the energy

transfer parameters for any two rare earth ions having well defined electronic

levels. These derivations are based on the rate equations applicable to a

system consisting of a pair of unlike RE ions in a glass medium. The system

is shown in Fig. 1.11.

Here the no. specifies the levels of the donor ion and the letters the

level of acceptor ion. P's are transition probability between various levels

designated in the Fig. 1.9. The superscripts ‘r’ and ‘nr’ refers to the radiative

and non-radiative transition respectively.

'sφ are quantum efficiencies defined as:

52

rnr

r

1010

101 Ρ+Ρ

Ρ=φ

c221

212 Ρ+Ρ

Ρ=φ (1.18)

If P1c is the probability of energy transfer between lower levels &

P20 is the probability of energy transfer between the higher levels, than the

donor emission quantum yield in the presence of c (concentration of

acceptor) dη from level 1> to 0> as :

( )21

221

0

1(1ΡΡ

+Ρ+= Dcd

d

d φτηη

) (1.19)

Here 0dη Is the donor emission quantum efficiency when no acceptor

is present.

From the equation it is possible to determine whether one or more

energy transfer probability is proportional to some power of concentration

and a single channel transfer process would give a linear dependence on

this power. where a single channel is operative, as in most practical cases

and P2D« P21.

( )cdd

d1

0

1 Ρ+= τηη

(1.20)

c1Ρ = ⎥⎦

⎤⎢⎣

⎡−11 0

d

d

d ηη

τ (1.21)

When P2D« P21 the efficiency of energy transfer is given by:

53

−=Ρ+

Ρ= 1

1 1

1

dc

dc

ττητ 0

d

d

ηη

(1.22)

These equation 1.26 and 1.27 enable us to calculate Pda and ητ

using known experimental values of donor emission intensity (yield and

fluorescence lifetimes, A simple operational definition of ητ in terms of the

lifetimes can be given as

do

d

ττητ −= 1 (1.23)

where τd0 is the intrinsic (radiative and radiationless) decay time of

the donor excitation & τd is the donor lifetimes in the presence of the

acceptor .If the total donor decay probability is P then:

P=0

11

dd ττ= +Pda (1.24)

where τdo-1

and τd-1 are the donor decay rates in absence and in

presence of the acceptor ion respectively.Pda is the energy transfer

probability given by:

Pda= ⎥⎦

⎤⎢⎣

⎡−=−

00

1111

d

d

ddd ττ

τττ (1.25)

We have frequently exploited the use of equation 1.24 and 1.25 in

our computation.

54

1.16 OBJECTIVE OF THE WORK

An ion or molecule has its own characteristic absorption and

emission spectrum. The optical properties of an ion or molecule are

dependent on ion / molecule itself and the matxics in which the luminescent

ion / molecule is doped. Luminescent ions /molecules can be selected at will

which give emission in the desired wavelength range. Matrix in which ions

/molecules are doped can be selected for which one will get better emission

intensity. A large number of phosphors have been identified by now a days

which have found applications in the field of lighting and television. Still there

is vast scope in this field.

Rare earths have specials characteristic absorption and emission

spectrum. These spectrum having high intensity, small band width and

longer life times but due low oscillator strength they can’t absorb significant

amount of incident energy. This drawback can be overcome by sensitizing

such ions (called activator or acceptor) by other ions (called donor or

sensitizer).

The applications of sensitized phosphors are in sensitizing solid

state glass laser, IR up-converter, solar energy converters, optical amplifiers

etc. So it is useful to find out the circumstances under which the emission

can occur with high efficiency.

With the increasing use of dye doped polymer found various

application as light emitting diodes, lasers, optical sensor and biological

sensor, the study of these dyes are new area for the research.

55

Keeping above points in mind the present work is carried out to

investigate the interaction involved and energy transfer process in various

ion-ion system. The present work we chose zinc phosphate glass as host in

which Na2O is network former and ZnO is the modifier. For the study of

emission and absorption spectrum of different dyes, we take PVA as host.

Chapters 3, 4, 5, 6 present energy transfer process between rare earth to

rare earth and between dye to dye particularly involve the study of

fluorescence spectra, energy level involved, multipolar term and mechanism

responsible for the energy transfer. Different theories are used to explain the

experimental results.

56

BIBLIOGRAPHY

1 B.G. Wybourne: Spectroscopic properties of rare earths, Wiley Inter-science,

New York (1965).

2 G.H. Dieke: Spectra and energy levels of rare earths ions in crystal, John

Wiley and Sons, Inc. New York (1968).

3 D.J. Newman: Adv. Phys. 20 (1971) 197.

4 H.A. Bethe and E.E. Salpeter: Quantum Mechanics of one and two electron

atoms, Academic Press, New York (1957).

5 H.A. Bethe: Ann. Physik, 3 (1929) 133.

6 J. H. Van Vleck: J. Phys. Chem., 41 (1937) 67.

7 B. R. Judd: Phys. Rev., 127 (1962) 750

8 H. F. Ivey: Proc. Int. Conf. on luminescence., 20 (1966) 2027.

9 Th. Forster: Z Naturefosch., 49 (1949) 321.

10 D.L. Dexter: J. Chem. Phys., 21 (1953) 836.

11 D.L. Dexter: Phys. Rev., 126 (1962) 1962.

12 J.Parrin: Second Council de Chime Solvav, Gantier and villar, Paris

(1925).

13 Th.Forster: Ann. Physik, 2 (1948) 55.

14 D.L. Dexter: J. Chem. Phys., 2 (1953) 836.

15 M. Inokuti & F. Hirayama: J. Chem. Phys., 43 (1965) 1978.

16 L. G. Van Uitert and L. F. Johnson: J.Chem.Phys.,44 (1966) 3514

17 G. E. Peterson and P. M. Bridenbaugh: J. Opt. Soc. Am. 53 (1963)

1129.

57

18 F. Varsanyi & G. H. Dieke: Phys. Rev. Lett., 7 (1961) 442.

19 R. Reisfeld and Y. Eckstein: J. Non Cryst. Solids, 11 (1973) 261.

20 F. Varsanyi and G.H. Deike: Phys. Rev. Lett., 7 (1961) 442.

21 G. E. Peterson and P. M. Bridenbaugh: J. Opt. Soc. Am., 53 (1963)

1129.

22 F. K. Fong and D. J. Diestler: J. Chem. Phys., 56 (1972) 2875.

23 R. Reisfeld: Struct and bonding, 13 (1973) 53.

24 Kun Huang and Avnl Rhys: Proceedings of the Royal Society of

London, 204 (1951) 406.

25 C. W. Struck & W.H. Fonger: Journal of Luminescence, 10 (1975) 1.

26 W. H. Fonger and C. W. Struck: Journal of luminescence, 17 (1978)

241.

27 A. Kiel: Quantum electronics, Columbia University press, New York

(1964).

28 L. A. Riseberg and H.W. Moos: Physical review, 74 (1968) 438

29 C.B. Layne, W.H. Lowdermilk and M.J. Weber: Phys. Rev., B15

(1977)179.

30 M. Yakota and O.Tanimoto: J. Phys. Soc. Japan, 22 (1967) 779.

31 R. K. Watts and H. J. Ritcher: Phys. Rev., B6 (1972) 1584.

32 Van Der Ziel, J.P., L.Kopf and L.G. Van Uitert: Phys. Rev., B6

(1972)615.

33 E.Okamota: J. Luminsc., 12/13 (1976) 763.

34 N. Krasutsky and H. W. Moos: Phys. Rev., B38 (1973) 1010.

35 M. J. Weber: Phys. Rev., B4 (1971) 2932.

36 L. A. Riseberg, Phys. Rev., A7 (1973)671.

58

37 N. Motegi and S. Shionoya: J. Luminescence, 8(1973)1.

38 M. J. Weber: Radiative and Non radiative transitions of RE Ions: in

Phys. of quantum electronics (McGraw Hill Book Co., NY), (1966).

39 W. A. Weyl and E. C. Marrboe: The constitution of glasses, A

Dynamic interpretation, Vol 1 (Wiley Interscience, NY) ,1962.

40 B.E. Warren: J. Am. ceram. Soc., 17 (1937)249.

41 A. S. Prebus and J. W. Michner: Ind. Eng. Chem. 46(1952)147.

42 G. E. Rindone: Luminescence in glassy state, Luminescence of

Inorganic Solids (Academic Press, Newyork), 1965.

43 G.O. Karapetyan: Ann. SSSR Izu. Ser Fiz., 27 (1963)779.

44 R. Reisfeld & Y.Eckstein: J.Solid State Chem., 5(1972)174.

45 R. A. Velapodi, R. Reisfeld and L. Boehm: Ninth Rare earth Conf.,

2(1971)488.

46 R. Reisfeld, R.A. Velapodi and L. Boehm and M. Ish. Shalom:

J.Chem. Phys., 75(1971)3980.

47 M.J. Weber, J. Hegarty and D.H. Blackburn:in Boron in Glass and

Glass Ceramics ,Plenum Press, New York (1977).

48 R. Reisfeld: Struct and bonding, 30 (1976)65.

49 R. Reisfeld: Struct and bonding, 22 (1975)123.

50 W. A. Weyl: Coloured glasses, The Society of Glass Technology,

Sheffield, England , (1951).

51 W. A. Weyl and E. C. Marrboe: The constitution of glasses, A

Dynamic interpretation, Vol 1 (Wiley Interscience, NY) ,1962.

52 M. J. Weber: Proc. 7th Int. Conf. on Amorphous and liquid

Semiconductor, Edinburgh, (1977).

59

53 C. B. Layne: Unpublished Ph.D. thesis submitted to university of

California,USA (1975).

54 N. N. Barashkov, O. A. Gunder N. N. Barashkov, O.

A. Gunder: Fluorescent polymers, Ed. By Eliss Herwood, New York, (1994).

55 Photophysics of polymers, ASC, Symposium series

38, edited by Hoyle C.E. and J. M. Torkenson, (1987).

56 T.G. Dewey: Biophysics and biochemical aspects of

fluorescence spectroscopy, Ed., Plenum Press NY (1991).

57 Fred W. Billmeyer: Textbook of polymer science,

John. Wiley & Sons, New York (1968).

58 B. A. Swartz, T. Cole and A. H. Zewail:

Opt.Lett.,1(1977)73.

59 R. Reisfeld: structure and bonding ,13(1973)53

60

Figure caption

Fig.1.1 Rare earth element in periodic table.

Fig.1.2 Schematic diagram of the splitting of rare earth energy levels

due to the electrostatic, spin-orbit, crystal field interactions.

Fig.1.3 G.H.Dieke. Spectra and energy levels of rare earth ions in crystals (Interscience Publisher, New York, 1968).

Fig.1.4 A Sensitizer – activator system with activator (A) and sensitizer (S).Excitation of S (Exc (S)) is followed by one or more of the following processes:

1) Emission from S. 2) Radiation less decay in S. 3) Energy transfer to another centre of type S. and 4) Energy transfer to a centre of type A. Here emission

from A can observed.

Fig.1.5 Excited state relaxation and energy transfer in RE ions.

Fig.1.6 Schematic representation of migration of energy among similar ions.

Fig.1.7 Schematic representation of relaxation of excited state population by cascade and by multiphonon.

Fig.1.8 Proposed rare earth site model.

Fig.1.9 SSchematic picture of the free energies of crystals, supercooled liquid and glasses.

Fig. 1.10: Energy level and molecular spectra.

Fig. 1.11: Schematic representation of potential energy curve and vibrational evel of molecule.

61

Fig. 1.1: Rare earth elements in periodic table.

62

Fig 1.2: Schematic diagram of the splitting of rare earth energy levels due to the electrostatic, spin – orbit, crystal field interactions.

COULOMB

SPIN ORBIT

CRYSTAL FIELD

~1O4 Cm‐1

2S+1LJ

2S+1LJ(µ)

2S+1L

~1O2 Cm‐1

~1O3 Cm‐1

63

Fig 1.3: G.H.Dieke. Spectra and energy levels of rare earth ions in

crystals (Interscience, Publisher, new York, 1968).

64

Fig. 1.4: A sensitizer – activator system with activator (A) and sensitizer (S).Excitation of S (Exc (S)) is followed by one or more of the following processes:

1) Emission from S. 2) Radiation less decay in S. 3) Energy transfer to another centre of type S. and 4) Energy transfer to a centre of type A. Here emission

from A can observed.

SA

HOST SS

1

2

3

4

Exc(S) Em(A)

65

Fig. 1.5: Excited state relaxation and energy transfer in RE ions.

0

1

2

P21

P20

Pr10

Pnr10

Donor Acceptor

C

B

D

A

PCA

PDC2

PCB

PnrBA

PrBA

66

Fig. 1.6: Schematic representation of migration of energy among similar ions.

SIMILAR IONS

ION 1 ION 2

67

Fig. 1.7: Schematic representation of relaxation of excited state

population by cascade and by multiphonons.

MULTIPHONON RELAXATION

SINGLE PHONON

CASCADE

ENER

GY

ABSORP

TION

Emission

68

Fig 1.8: Proposed rare earth site model.

BORON

OXYGEN

C2

69

Fig. 1.9: Schematic picture of the free energies of crystals, supercooled liquid and glasses.

SUPERCOOLED

FREE ENER

GY

GLASS

CRYSTAL CRYSTAL

70

Fig. 1.10: Comparison between atomic and molecular spectra schemes.

71

Fig. 1.11: Schematic representation of potential energy curve and vibrational level of molecule.

72

CHAPTER 2

EXPERIMENTAL TECHNIQUES

73

CHAPTER 2

EXPERIMENTAL TECHNIQUES

2.1 INTRODUCTION:

The luminescence of the rare earth ions in glasses has been subject

of interest since the advent of lasers because sharp bands occur under

proper excitation [1]. Energy transfer between rare earth ions find wide

application in sensitizing solid state glass lasers & optical amplifiers [2-9]

infrared quantum counters as well as in infrared to visible convertors [10].

Recently, the transfer of excitation energy among dyes is drawing

attention because of its applicability in solar collectors [11, 12]. Dye doped in

transparent polymers can serve as the purpose of luminescent solar

collectors.

The experimental methods to observe the energy transfer process

need the fluorescence studies of donor ions in presence and in absence of

acceptor ions, the absorption spectra of both the ions and the decay time

measurements of donor ions in presence and absence of acceptor ions. A

sufficient knowledge is, therefore, needed to work out the dynamics involved

in energy transfer processes. In this chapter the details of the chemicals

used, their composition, experimental procedure and the apparatus used are

given.

74

2.2 CHOICE AND PREPARATION OF MATERIALS:

The interaction among rare earth ions causes the transfer of

excitation energy of one ion / molecule (donor) to another ion / molecule

(acceptor) [13]. Such interactions help in increasing the emission of the

acceptor ions, which either do not get excited with the direct pumping or get

so weakly, indirectly via the donor ions. In glass matrices, due to

inhomogeneous broadening of the energy levels of doped ion, the probability

of energy transfer increases [14].

The energy transfer parameters are greatly affected by the purity of

the chemicals used in manufacturing the glass for the study of the energy

transfer between the doped ions, viz., donor and acceptor. An efficient

general research on phosphors requires very pure inorganic and organic

substances as the constituents of the glass matrix. Besides, one or more

furnace capable of attaining a temperature of at least 10000C, a source of

high energy phonons and the means of controlling and determining the

energies and number of these particles are the other basic requirements for

an efficient research in this field.

2.2 A) CHEMICALS:

The details of the chemical used in the present work are as follows:

1. Sodium dihydrogen orthophosphate (NaH2PO4.2H2O) was of E Merck

(India) Ltd. Make and was reagent grade pure (R.P.)

75

2. Zinc oxide (ZnO) was of Ferak Berlin (Germany) make and was 99.9%

pure.

3. Rare earth Oxides such as Terbium oxide (Tb2O3), Europium Oxide

(Eu2O3), Erbium oxide (Er2O3), Praseodymium oxide (Pr2O3), Samarium

oxide (Sm2O3), Thulium oxide(Tm2O3),each with a purity of 99.9%,

Dysprosium oxide (Dy2O3)99% pure were obtained from Prof. Fortne

GTE Sylvania, U.S.A.

4. Uranyl acetate [UO2 (C2H3O2.2H2O](phosphor grade), Sodium borate

(Na2B4O7) Calcium oxide (CaO), Magnesium carbonate (MgCO3), Borax

(H 3 BO3) were obtained from Rare earth India ltd.

5. Polyvinyl alcohol (PVA) soluble in cold water and dichloromethane was

obtained from central drug house (P) ltd., Bombay.

6. Fluorescein, Saffranine T, Malachite green, erythrosin B, Eosin was

obtained from BDH chemicals ltd., Poole England.

7. Double distilled water used in the work was obtained from department of

chemistry, S.S.J. campus Almora, U.A. India.

2.2 B) GLASS COMPOSITION & PREPARATION:

Different glass samples with or without rare earth ions were prepared

as follows:

All the components of the glass with requisite composition were taken

in a beaker and were thoroughly stirred with the help of an electrically driven

stirrer till the mixture becomes homogeneous. Then the mixture is placed in

a platinum crucible. The crucible was then put inside an electric furnace

76

(which is capable of achieving temperature up to 12000C at a temperature

ranging from 8500C to 9500C) for half an hour. The molten mass was taken

poured into a brass ring (mould) resting on an aluminum plate. The glass so

formed was then allowed to cool at room temperature. The annealing

conditions for each series of glass samples were kept identical. In this way it

is possible to get glass samples of almost equal size & surface area.

The compositions of glasses prepared for the study were as follows:

1. To study the energy transfer in different rare earth doped series sodium

dihydrogen orthophosphate and zinc oxide were used in the ratio 3:1 by

weight to make zinc phosphate glass.

2. To study the absorption spectra of acceptor RE ion one reference glass

sample of zinc phosphate glass is made by mixing sodium dihydrogen

orthophosphate and zinc oxide in the ratio 3:1 by weight.

The size of glass sample for absorption spectra is taken as

1cmx1cmx3 cm. The presence of zinc oxide in the glasses makes them less

hygroscopic.

2.2 C) GLASS SERIES:

By doping the different donor-acceptor rare earth ion different glass

series were made to study the energy transfer between them. The details of

series of samples so prepared have been presented in the respective

chapters. In general we take 1 wt % doping of donor and 0.2, 0.4, 0.6, 0.8,

1.0 wt% doping of acceptor ion. However for some glass series the acceptor

77

doping is taken 0.5, 1.0, 1.5, 2.0, 2.5 wt% by weight.. UO2++ as a donor is

taken 0.1 wt % for UO2 - Er system.

2.2 D) POLYMER COMPOSITIONS AND PREPARATION:

Thin films of PVA soluble in cold water with or without doping of dyes

were prepared as follows:

PVA and appropriate concentration of dyes were dissolved in distilled

water of known volume by stirring for half an hour at room temperature. This

dye-PVA mixture in water was slowly heated in an incubator to a

temperature of approximately 400 C with intermittent stirring. The

homogeneous mass was then poured in the appropriate container

(polypropylene dish) to obtain transparent sheets of required shape and size

and allowed to dry for approximately 2-3 days at 300K.

Then by doping the different dyes in host PVA were made to study

the luminescence properties of the dye doped polymer samples. The details

of samples than prepared have been presented in the chapter 6.

2.2 E) POLYMER SERIES DOPED WITH DYES:

Following polymer series doped with dyes were prepared as

1. Eosine of molar concentration 0.63 x 10-5M, 1.25 x 10-5M, 2.5 x 10-

5M and

5 x 10-5M doped in PVA.

2. Saffranine T of molar concentration 1.25 x 10-5M and 5 x 10-5M

doped in PVA.

78

3. Erythrosin B of molar concentration 0.63 x 10-4M, 0.80 x 10-4M and

2.5 x 10-4M in PVA.

4. Malachite green of molar concentration 2.5 x 10-5M, 1.25 x 10-5M

5. Fluorescein of molar concentration of 10-4M and Erythrosin B of

molar 0.31 x 10-5M, 0.63 x 10-5M, 0.80 x 10-5M, 1.0 x 10-5M and 2.5 x

10-5M doped in PVA concentration.

2.3 ABSORPTION SPECTROSCOPY:

The absorption spectra of the samples (RE doped glass as well as

dye doped polymer sample) were taken by PC controlled double beam UV-

VIS spectrophotometer (ECIL’s UV 5704 SS) having specifications as

follows:

Optics used in the spectrophotometer is double beam with quartz

coated optics .The light source used in the spectrophotometer are deuterium

and tungsten halogen lamp. It has an inbuilt silicon photo detector to cover

the entire wavelength range as given below. The power supply needed to

operate the unit is 230 V AC ±10 %, 50 Hz. The specification of wavelength

and optics involved is as follows:

Wavelength range: 190-1100 nm with wavelength accuracy ± 0.3 nm.

And band pass < 2 nm and .1 nm wavelength resolution.

A Pentium PC with SVGA colour monitor with standard PC

configuration and printer is attached with the unit. All the absorption spectra

were taken at room temperature.

2.4 FLUORESCENCE SPECTROSCOPY:

79

The emission intensity of each sample of the sample series are taken

by the current meter using following arrangement:

A high-pressure mercury lamp excited the samples in a fixed

geometry on frontal illumination with wood’s glass filter. Frontal illumination

of samples was made with the help of a concave mirror having a circular

aperture at its center. The arrangement for emission spectra is shown in

Fig.2.1. As shown in the figure a convex or a cylindrical lens on the entrance

slit of a monochromator focused the emission given by samples. The

dispersing system was either a grating monochromator with a dispersion 3.3

nm/mm and Czerny turner mounting (CEL, model HM 104) or a constant

deviation prism for low light levels. A photomultiplier tube (RCA 1P21)

(Circuit diagram is shown in Fig. 2.2) was attached with the exit slit of

monochromator to scan the dispersed light obtained from monochromator.

Regulated high power supply was used to operate the photomultiplier tube at

750 V. The output of the photomultiplier tube was fed to a current meter

(least count 1×10-9 amp.). All the graphs are drawn in MS excel 2007.

The emission spectra of rare earths-doped glass and dye-doped

polymer were taken by Jasco made FP-777 spectroflurometer at

photophysics laboratory, DSB Campus, Kumaun University Nainital.

The light source used in FP-777 is 150 W xenon lamp. The light from

the light source was focused on to the entrance slit plane of the excitation

monochromator by the electrical mirror m1 and a concave mirror m0. The

incident light from the slit is dispersed by the grating j1 and than arbitrary

monochromatic beam is selected by the exit slit. A part of the

80

monochromatic beam is lead to the ministering PMT by quartz plate beam

splitter and the diffuser plate DG. The monochromatic beam that has

transmitted the BS is focused on to the center of sample cell by plane mirror

m3 and toroidal mirror m4. The fluorescence radiation from the sample is

focused on to the entrance slit plane of the emission monochromator by

toroidal mirror m5 and plane mirror m6 and m7. The emission

monochromator is identical with exit monochromator in a structure and the

beam from the exit slit is lead to the PMT tube pm2 by concave mirror.

Wavelength range: 220-750 nm with wavelength accuracy ± 1.5 nm. Oth

order light can be expanded to 200 to 850 nm by the optical detector.

Spectral bandwidth changed over in 5 steps of 1.5,3,5,10 & 20 nm,

simultaneous scan is also possible.

2.5 FLUORESCENCE DECAY TIME:

The decay times of the sample, wherever necessary were taken from

the published work of Reisfeld [14] and published and unpublished work of

Joshi et.al. [15]. Joshi et. al. have used the single flash technique for the

fluorescence decay time measurements, which uses the following

experimental arrangements:

81

82

BIBLIOGRAPHY

1. G.O. Karapetyan: Ann SSSR Izu Ser, 27, (1963) 779.

2. R. Reisfeld and C.K.jorgenson: Lasers and excited states of rare earth

(springer-Verlag Berlin, Heidenberg, New York), 1977.

3. X.X.Zhang, P.Hong, M.Bass, B.H.T.Chai, Phys. Rev., B 51, (1995) 9298.

4. A.Bjarkev, in: Optical Fiber Amplifiers: Design and system Applications,

Artech house, Boston, London., (1993).

5. H. Higuchi, M. Takahahsi, Y.Kawamoto, K. Kadono. T.Ohtsuki,

N.Peyghambarian, N.Kitamura, J. Appl. Phys., 8319 (1998).

6. R. Reisfeld, Inorg. Chem. Acta, 140(1987) 345.

7. M.Tsuda, K.Soga, H.Inoue, S. Inoue, A. Makishima. J. Appl. Phys. 85,

(1999) 29

8. J.E.Roman, P.Camy. M.Hempstead, W.S.Brocklesby. S.Nouth, A.Beguin,

C.Lerminiaux, J.S.Wilkinson, Electron.Lett., 31, (1995) 1345.

9. E.Snoeks, G.N.Van den Hoven, A.Polman: IEEE J. Quantum Electron., 32,

(1996) 1680.

10. D. Rose: Laser light amplifiers and oscillator (Academic press, New York,

1969).

11. W. H. Weber and J. Lambe: Appl opt (USA), 15 (1976) 2299.

12. A. Geatzberger and V. Wittwer: Adv Solids St Phys (Neatherland), 19

(1979) 427.

13. Th.Forster and D.L.Dexter: Structure and bonding, 22(1975) 123.

14. R.Reisfeld: structure and bonding, 30(1976) 65.

83

15. J.C.Joshi, B.C.Joshi, Janardan Joshi and R.Lohani: Personal

communication.

84

FIGURE CAPTION

Fig. 2.1: Plan for scanning fluorescence spectra.

Fig. 2.2: Base connection of multiplier tube 1P21.

Fig. 2.3: Design for Sample holder.

Fig. 2.4: Block diagram of spectrophotometer JASCO FP-777

85

Fig.2.1 Plan for scanning fluorescence spectra

TO POWERSUPPLY

CONCAVE MIRROR

D.C.AMPLIFIER

RECORDER

PMT

EARTH

365 NM SOURCE

SAMPLE

‐1500 V

86

R

R

R

R

R

R

R

R

RR

E

D8

D9

D7

R

D1

K

R=220KΩ

D5

D4

D2

D6

D3

200V

87

Fig 2.2 Base connection of photomultiplier tube RCA 1P21

88

Fig 2.3 (a) Design for Sample holders for glass samples

1. for fluorescent or emission spectra

2. for absorption spectra

WOODEN HANDLE

IRON

SPACE FOR SAMPLE

BRASS

89

Fig 2.3 (b) Design for Sample holders for polymer samples for

fluorescent or emission spectra

OPEN SPACE FOR POLYMER SAMPLE

CARD BOARD

90

Fig.2.4 Block diagram of spectrophotometer JASCO FP-777.

91

92

CHAPTER 3

STUDY OF SENSITIZE LUMINESCENCE AND ENERGY TRANSFER PROCESS IN Tb-Nd,

Tb-Er AND Tb-Pr SYSTEMS IN ZINC

PHOSPHATE GLASS

93

CHAPTER 3

STUDY OF SENSITIZE LUMINESCENCE AND ENERGY TRANSFER PROCESS IN Tb-Nd, Tb-Er AND

Tb-Pr SYSTEMS IN ZINC PHOSPHATE GLASS

3.1 INTRODUCTION

The rare earth ions are characterized by their sharp absorption lines,

which occur due to transitions within 4fN configuration, and consequently the

transitions involved are immune to environmental changes[1-3], however,

the presence of two or more RE ions in a given host matrix affects the

luminescence properties of each other. Thus the interactions among the RE

ions have importance in studying the energy transfer phenomena as well as

in enhancing the emission of a given RE ion arising due to energy transfer

from another RE ion.

The terbium ion is well known for its relatively large absorption

among rare earth ions and has bright green emission. Therefore it is a

suitable choice for sensitizing other RE ions. Chrysochoos & Evers observed

energy transfer between Tb3+ to Eu3+ in dimethyl sulfoxide (DMSO) [4]. Joshi

et. al. [5] reported diffusion limited energy transfer at low Ho3+ concentration

and electric dipole-dipole interaction at higher concentration while the same

authors in other paper nonradiative energy transfer is reported from Tb3+ to

Eu3+ in zinc phosphate glass and found energy transfer mechanism is mainly

94

electric dipole- dipole in nature [6]. Some recent work on studying the

properties of terbium are cited in references [7-9].

Neodymimum finds its wide applications in glass lasers. Diffusion

limited energy transfer from Eu3+ to Nd3+ in borate glass has been studied by

Joshi et. al.[10]. Joshi observed transfer of energy from Dy3+ to Nd3+ in

calibo glass and Tm3+ to Nd3+ in zinc phosphate glass [11]. A. Sureshkumar

et.al. [12] studied the spectral properties of different conc. of Nd3+ ion in

barium lead borophosphate glass. Many other researchers also studied

neodymium properties with or without other ions in different glass matrices

[13-15].

In recent years, the erbium is the most studied RE ions in different

host materials [16-22]. Lohani [23] have reported the transfer of energy from

Mn2+ to Er3+ while Reisfeld et. al. [24] have observed non-radiative energy

transfer between Tm3+ and Er3+ ions in phosphate and borate glasses and

their result shows a mutual migration of energy between Tm3+ and Er3+ ions

. R.Lohani. [23] have reported non-radiative energy transfer from Dy3+ to Er3+

in zinc phosphate glass.

The importance of Pr3+ in infrared emission is well known. Joshi et.

al. [25] sensitized the praseodymium ion (Pr3+) by Mn2+ in phosphate glass

and reported that the energy transfer from Mn2+ to Pr3+ takes place non-

radiatively and the mechanism of energy transfer observed by them was

electric dipole-dipole interaction. Joshi has also reported non-radiative

energy transfer from Dy3+ to Pr3+ in calibo glass by electric dipole-quadrupole

interaction [26]. R.Lohani [23] has reported non-radiative energy transfer

95

from Sm3+ to Pr3+ in zinc phosphate glass. Some of the workers studying

properties of Pr3+ are cited in references [25-30].

In view of the above properties of Tb3+, Nd3+, Er3+ and Pr3+, we have

chosen the Tb3+ - Nd3+, Tb3+ - Er3+ and Tb3+ - Pr3+ systems in zinc phosphate

glass to investigate the following points:

(a) Nature of the energy transfer from Tb3+ - Nd3+, Tb3+ - Er3+ and

Tb3+ -Pr3+

(b) The mechanism of energy transfer between the ions in each of

the systems mentioned above.

(c) The levels between which the energy transfer takes place.

(d) Calculation of parameters related to energy transfer as a

function of concentration (e.g. average donor –acceptor

distance (DD-A), transfer probabilities (PDA), transfer efficiencies

(η) etc.)

(e) Comparison of the energy transfer rate in Tb-Nd, Tb-Er and

Tb-Pr systems.

3.2 EXPERIMENTAL MATERIALS AND METHODS:

Sodium dihydrogen phosphate 2-hydrate (NaH2PO4 .2H2O) and

zinc oxide (ZnO), both of reagent grade, were used in a proportion of 3:1 by

weight, were used as the constituents of the glass matrix .The method for

preparing the glass pallets has already been discussed in chapter 2.

96

The following series of glasses were prepared by doping the above

mentioned rare earth ions for studying the energy transfer.

Series 3.I: This series consists of glasses doped with 1 wt % (fixed)

of Tb3+ ions codoped with 0.2 wt %, 0.4 wt%, 0.6 wt%,

0.8 wt% and 1.0 wt% and 1.2 wt% of Nd3+ ions.

Series 3.II: This series consists of glasses doped with 1 wt % (fixed)

of Tb3+ codoped with 0.2 wt %, 0.4 wt% ,0.6 wt%,0.8 wt

% ,1.0 wt% of Er3+.

Series 3.III: This series consists of glasses doped with 1 wt %

(fixed) of Tb3+ codoped with 0.5 wt %, 1.0 wt% ,1.5

wt%,2.0 wt % ,2.5 wt% of Pr3+.

In addition to these glasses with 1 wt% of Nd3+,1 wt% of Er3+, 1 wt%

of Pr3+ were prepared to study the absorption spectra of these ions in

isolation.

Emission & absorption spectra were taken according to the method

described in chapter 2.

3.3 THEORY:

The transfer of excitation energy from one ion to another ion can

occur either via radiatively or non-radiatively. Most frequently observed

energy transfer energy transfer is non-radiative. In radiative transfer the life

time of the excited donor ion is not affected in presence of acceptor ion,

whereas in non-radiative transfer process life time of donor ion decreases on

97

increasing the acceptor concentration. In case of radiative decay the

magnitude and the shape of spectrum changes as acceptor concentration,

whereas in the case of non-radiative transfer only the magnitude not the

shape changes.

Theory of non-radiative resonance energy transfer from one molecule

(sensitizer or donor) to another molecule (activator or acceptor) was first

given by Forster [31]. The energy transfer rate (PDA), according to this

theory, is proportional to the overlap of the donor emission and acceptor

absorption spectra and R-6, where R is the donor acceptor distance. Forster

[31] theory was extended by Dexter [32] for the ions in inorganic crystals and

for higher multipolar interaction.

Interactions involving two or more ions, which provide the means for

energy transfer and co-operative relaxation, are:

1) Electric multipolar coupling arising from the interaction of

multipoles e.g. D-D, D-Q etc. of different ions.

2) Magnetic dipole-dipole interaction.

3) Exchange interaction.

The interaction between two like or two unlike ions lead to energy

transfer and relaxation in the following manner.

a. An ion A in an excited state 2 decays to its ground state 1 with

corresponding excitation of a neighboring ion B from its ground state 1 to

excited state 2. If A and B are identical ions, this process involves resonant

transfer of energy from ion to ion while this does not lead to net relaxation it

98

does give rise to spatial energy migration resonant energy transfer becomes

fast in concentrated RE ions.

b. An excited ion A decays from state 2 to 2′ while ion B initially in

its ground state is excited to 1′. A and B may or may not be identical .In the

absence of any additional interactions energy conservation imposes the

constraint of resonance i.e.

E2 – E2′= E1- E1′.

In liquids and solids, however due to the presence of ion –lattice

coupling, any energy mismatch may be taken up by the emission or

absorption of one or more phonons.

In Fig. 3.1, a schematic energy level diagram for the sensitizer (D)

and an activator (A) centre is shown. The donor is raised from the ground

state 1 to the excited state 2. In order to energy transfer to activator (A),

donor (D) has to return from state 2 to state 1/,while activator (A ) at the

same time should move from state 1 to a higher energy level. According to

Forster–Dexter theory [31, 32] this is possible only if one of the levels of A

lies at the same height as level of D, if this happens it is called resonance.

The energy transfer can take place from D→A in two essentially different

ways in following ways.

1. The transfer can be brought about by the coulomb interaction

between all charged particle of D and A. In case the distance

between D and A is so much that there charge clouds do not

overlap this is the only possible way of resonant nonradiative

energy transfer.

99

2. If the charge clouds of D and A overlap, transfer process is

possible by exchange interaction between the centers of donor

and acceptor.

The essential difference between above mentioned processes is that

while an electron remains with their respective ion or ionic groups i.e. no

charge is transferred from D to A in the first process whereas in the second

process electrons are exchanged between D and A.

The probability of energy transfer by dipole-dipole interactions in a

simple case, where Born-Oppenheimer approximation holds, is given by

Dexter [32] as:

( ) ( ) ( )dE

EEfEf

KnRQCh

ddP AD

cd

aDA ∫⎥

⎥⎦

⎤

⎢⎢⎣

⎡⎥⎦

⎤⎢⎣

⎡= 4

4

46

44

43

εε

τπ (3.1)

here h is Planck const., C is speed of light, R is the donor-acceptor

separation, cε is the electric field within the crystal, ε is the electric field in

vacuum, n is the refractive index of medium. D & A refer to donor and

acceptor ions, E is energy, ( )Ef D is the observed shape of the emission

band normalized to unity, ( )Ef A is the normalized function of the acceptor

band, dτ is decay time.

The ratio of dipole quadrupole and dipole-dipole transition

probabilities is given by the following relationship:

( )( )

2

⎟⎠⎞

⎜⎝⎛≅

Ra

ddPdqP

(3.2)

100

where a is the atomic radius of RE ion. In addition to multipolar

transfer mechanism, the Dexter theory also treated energy transfer due to

exchange processes at very short inter-ionic distances.

The probability of this exchange transfer is given as:

( ) ( ) ( )dEEfEfZexchangeP AD∫=h

2

(3.3)

where Z2 is proportional to exp (LR

− ). Here L is the effective Bohr radius of

donor and acceptor ions for the excited and unexcited state.

The multipolar interactions are electrostatic in nature, while the

exchange interactions arise from the anti-symmetry requirements of the

electronic wave functions for a system consisting of a donor and an

acceptor.

In equation (3.3) the optical properties of D and A are not considered.

Exchange transfer, then, unlike transfer by Coulomb interaction, is not

dependent on the oscillator strength or transition probabilities of the relevant

transitions, and may even not take place to a level from which a return to

ground state is forbidden.

Inokuti & Hirayama [33] further extended the work of Forster [31] &

Dexter [32]. They take exchange coupling and incorporate the time

dependence of the fluorescence decay in the presence of ion pair

interactions.

101

Van Uitert et.al. [34] observed of the variation of donor intensity or life

time with varying acceptor concentration and used the following relationship

for the quantum efficiency:

13

00

1−

⎟⎠⎞⎜

⎝⎛ +===

θβ

ττη C

II

(3.4)

where 0I and 0τ are the radiative intensity and decay time,

respectively, of donor in the absence of acceptor, I and τ are the above

parameter in the presence of acceptor at acceptor concentration ‘C’ , β is

constant and =θ 6 or 8. Reisfeld and Boehm [35] also observed similar

behavior of rare earth ions in glasses and interpreted the =θ 6 or 8

dependence of C1/3 in term of the dipole-dipole and dipole-quadrupole

interaction respectively.

The concentration dependence of non-radiative transfer process

given by Dexter was re-examined by Fong and Diestler [36] in term of

ensemble statistical mechanics for many body interactions. At low

concentration, regardless of the nature of interaction, the dependence of per

ion transfer rate on ‘C’ is linear so long so the interaction is pair-wise. Here

‘C’ is total ionic concentration of donor and acceptor. For three body

interaction mechanism arising from dipole-dipole perturbation Hamiltonian of

second order, the per ion non-radiative transfer rate is proportional toC2 and

in general for n-body it varies as Cn-1. The C2 dependence is for =θ 6

dependence observations by Van Uitert et al. Whereas the C3 dependence

102

accounts for =θ 8 observations which were presumed to arise due to the

pair-wise dipole-quadrupole interaction.

Fong-Diestler theory applicable for the case where there is no

spectral overlap between donor-emission and acceptor-absorption and

irrespective of the nature of energy level involved, the non resonant energy

transfer processes can be easily interpreted in terms of many body

interactions.

Another way by which the rare earth ion can be relaxed from its

excited state is cross relaxation of energy, as shown in Fig. 3.2

This mechanism of excited state relaxation was first given by

Varsanyi & Dieke [37] and later by Peterson & Bridenbaugh [38]. If a level A

above a stable fluorescent level B is observed and the difference of energy

level (A-B) is used for multiple phonon excitations. If (A-B) becomes very

large, the energy is not transferred directly from A to B and A becomes a

stable fluorescent level (metastable) on the other hand if A-B approaches

the excitation energy of a lower level of another ion of the same species the

transfer again occurs i.e. fluorescence from B is again observed .This

difference in energy (A-B) is then used to excite the second ion. In this way

the ion pair resonance causes to quench the fluorescence intensity (Fig.

3.2.A).The quenching of 5D3 level emission of Tb3+ at high concentration

occurs by cross relaxation .

Again if the energy difference between the fluorescent level at lower

level of donor ion becomes equal to the energy gap between two levels of

103

the acceptor ion. The excited state of donor ion is relaxed by transferring its

energy to the acceptor by cross relaxation (Fig. 3.2 B).

Cross relaxation between different ions are given by Reisfeld and

Eckstein [24] in

Er3+[ 2P3/2 → 4G11/2] → Tm3+ [3H6→ 3H4]

Eu3+[ 5D0 → 7F6] → Nd3+ [7F7/2→ 3I9/2].

104

3.4 RESULTS AND DISCUSSION

3.4. A) The Tb - Nd system in zinc phosphate glass

Fluorescent spectra:

Fig.3.2 (A) shows the emission spectra of Tb3+ (1 wt %) in zinc

phosphate glass. It clearly shows four peaks arising from the 5D4 -7F

manifold transitions. The broadening of the lines is characteristic of the glass

matrix. Emission from the 5D3 level was not observed in our case due to high

concentration of Tb3+ ions and ions previously excited to the 5D3 level cross

relax to the 5D4 state.

Energy level diagram:

The energy level diagram of these ions is shown in Fig. 3.3. The

incident radiation (365 nm group of mercury lines) excites the Tb3+ ions. The

Tb3+ ions rapidly depopulate to luminescent 5D4 level and Nd3+ to the ground

level 4I9/2.

Nature of energy transfer:

Fig.3.4 shows the variation of Tb3+ emission intensity with the varied

concentration of Nd3+ ions and in Fig. 3.2(B) shows the emission spectra of

Tb3+ (1wt%) + Nd3+ (1wt%). Both the observations indicate that there is

overall decrease of emission intensity of Tb3+. The overall decrease of the

emission intensity of Tb3+ ion suggested that there is non-radiative energy

transfer from Tb3+ to Nd3+ ion.

105

Energy transfer by exchange process is negligible in our case

because it needs acceptor-donor separation of about 0.3-0.4nm with overlap

of wavefunction, while in our case donor-acceptor separation varies from

1.80 to 2.16 nm.

The absorption spectrum of Nd3+ in zinc phosphate glass is shown in

Fig. 3.7. The absorption peak which lies at 583nm falls partially at the

emission (5D4-7F6 transition) 592nm of Tb3+. Hence the possibility of small

radiative transfer from Tb3+ to Nd3+ can’t be ruled out.

Mechanism of energy transfer:

In order to find out the mechanism of energy transfer, we proceed

as follows:

Careful observation of energy level diagram of Tb3+ and Nd3+ in Fig.

3.3 shows that there is no energy level of Nd3+ close to 5D4 level of Tb3+. The

4G9/2 level of Nd3+ is 267 cm-1 above the 5D4 level of Tb3+ while 2G9/2 level of

Nd3+ is 1333 cm-1 below the 5D4 level of Tb3+. Hence phonon assisted

energy transfer is less possible [39]. Therefore we suggest that the only way

of energy transfer from Tb3+ to Nd3+ is the ion pair resonance or cross

relaxation. This can be explained as follows:

Tb3+ and Nd3+ are randomly distributed in glass matrix. When this

glass matrix is excited to 365nm group of mercury lines, Tb3+ ions rapidly

depopulate to luminescent 5D4 level and Nd3+ to ground level 4I9/2. On adding

Nd3+ to Tb3+ all the four peaks decreases with same proportion. As energy

gap between 5D4 & 7F0 of Tb3+ is equal to energy gap of 2G7/2 & 4I9/2 of Nd3+,

106

this may cause the energy transfer from 5D4 of Tb3+ to 4I9/2 of Nd3+.

Symbolically this may be shown as

Tb3+ (5D4-7F0 ) → Nd3+ ( 2G7/2 -4I9/2 ) (Fig.3.5).

Such processes (cross relaxation) become appreciable if energy is living in

long-lived metastable state. In present case decay time of metastable state

5D4 is 2.5 ms, hence have enough time to transfer its energy to 4I9/2 level of

Nd3+. Van Uitert et. al. also interpreted their result of self quenching of Sm3+

by cross relaxation of energy [40] . Reisfeld et. al. [24] also used this

process to explain their result of energy transfer in Tm-Er system.

Energy level involved in energy transfer:

As explained above ,the energy level between which energy

transfer can take place are 5D4 level of Tb3+ to 2G7/2 level of Nd3+ .This may

be written symbolically as Tb3+ (5D4-7F0 ) → Nd3+ ( 2G7/2 -4I9/2 ).

Multipolar term responsible for energy transfer:

To find out which of the multipolar term is responsible for energy

transfer, a graph is drawn between energy transfer probabilities (Pda) and

square of the concentration (donor & acceptor) which gives a straight line

(Fig. 3.6). The linear dependence of Pda on the square of the concentration

of donor & acceptor is attributed to because of dipole- dipole interaction

between donor & acceptor [36]. The dipole -dipole mechanism of energy

transfer is further supported by the average donor to acceptor distance

which varies in this system between 1.87nm & 2.55 nm (Table 3.1) which is

107

in the range of electric dipole- dipole interaction between donor & acceptor in

accordance with Forster’s [31] & Dexter’s [32] theories of multipolar

interactions.

Other parameters involved in the energy transfer:

In this series, the average donor acceptor distance along with the

energy transfer probabilities & transfer efficiencies are presented in table 3.1

which are calculated by using the following formulae:

DD→A = 1 / (Cd + Ca) 1/3

where Cd & Ca are donor and acceptor ion concentration per cm3 in the host

matrix.

Pda =1/τ0 (Id0/Id - 1),

η= 1 – Id/Id0.

Critical transfer distance (R0), at which the energy transfer

probability is equal to the radiative transition probability, in our system is

2.16nm. This value can be compared with those obtained by Eyal et al [41],

R0=2.1nm in Mn-Tm system in metal fluoride glass & by Joshi et. al. [42]

R0=2.50nm in Sm-UO2 system in zinc phosphate glass for electric dipole-

dipole interactions.

108

3.4. B) The Tb - Er system in zinc phosphate glass

Fluorescent spectra:

Fig.3.9 (A) shows the emission spectra of Tb3+ (1 wt%) in zinc

phosphate glass. It clearly shows four peaks arising from the 5D4 -7F

manifold transitions. The broadening of the lines is characteristic of the glass

matrix. Emission from the 5D3 level was not observed in our case due to high

concentration of Tb3+ ions and ions previously excited to the 5D3 level cross

relax to the 5D4 state.

The erbium ions also get excited by 365nm radiation and give

visible emission in some glasses, like tellurite [43] and in borate glasses by

382nm excitation source due to transition 3H11/2→3I15/2 (524nm) in tellurite

glass and due to 4S3/2→4I11/2 (548nm) in borate glasses [44]. In our case a

very weak and uncorrected spectra could be observed because of the fact

that the selection rule ∆ J = 2 is not always applicable for the fluorescent

spectrum because of the possibility of non-radiative transitions between

levels, which are dominant when energy gap is less than 3000 cm-1 [44].

This energy corresponds to about three phonons in our glass lattice. This

number of phonons can be available easily in the glass at the temperature

around it. As a result the levels 3H11/2 and 4S3/2 undergoes depopulation and

hence a weak or no emission is observed from these levels. Emission from

other levels was not observed in our study.

109

Energy level diagram:

The energy level diagram of these ions is shown in Fig. 3.8. The

incident radiation (365 nm group of mercury lines) excites the Tb3+ ions. The

Tb3+ ions rapidly depopulate to luminescent 5D4 level and Er3+ to the ground

level 4I15/2.

Nature of energy transfer:

As shown in Figs 3.9(B) and 3.10, the overall emission of Tb3+

decreases when doped along with Er3+ ions. Such an overall decrease in

donor emission indicates that there is non- radiative energy transfer from

Tb3+ to Er3+. This decrease of Tb3+ emission is more pronounced with

increasing Er3+ concentration. This decrease in Tb3+ emission can be

explained as - on increasing Er3+ concentration there are more Er3+ ions

available for receiving the excited energy of Tb3+ ions, which reduce the

radiative energy of Tb3+ ions.

Since the absorption peak (490nm) of Er3+ ions as can be seen from

Fig.3.12 falls on emission peak (490nm) of Tb3+ , the possibility of small

radiative energy transfer from Tb3+ to Er3+ can’t be neglected.

Mechanism of energy transfer:

A close look at the energy level diagram of Tb3+ and Er3+ presented

in Fig. 3.8 shows that 4F7/2 level of Er3+ is almost at same height to the 5D4

level of Tb3+. The energy can easily be transferred from Tb3+ to Er3+ as

110

follows. Small mismatch of the energy level can be compensated by the low

energy phonon.

Tb3+ and Er3+ are randomly distributed in glass matrix. When the glass

matrix is excited by 365 nm group of mercury lines, Tb3+ ions rapidly

depopulate to luminescent level 5D4 and Er3+ to ground state 4I15/2. As the

decay time of the metastable state 5D4 of Tb3+ is 2.5 ms, it has enough time

to transfer its energy to 4F7/2 of Er3+.

Energy level involved in energy transfer:

As explained above, the energy level between which energy

transfer can take place can be 5D4 level of Tb3+ to 4F7/2 of Er3+.

Multipolar term responsible for energy transfer:

The linearity of the graph drawn between energy transfer probabilities

(Pda) and square of the donor concentration + acceptor concentration (C2)

presented in Fig. 3.11 shows that electric dipole-dipole interaction is mainly

responsible for the energy transfer, which supports the Fong-Diestler theory.

The average donor acceptor distance varies from 1.45 nm to 1.71 nm (Table

3.2) also in support of electric dipole –dipole interaction suggested by

Dexter.

Energy transfer by exchange process is ruled out in present case

because it needs acceptor-donor separation of about 0.3 to 0.4 nm with

considerable overlap of wavefunction, while in present case donor acceptor

distance varies from 1.45 nm to 1.71nm.

111

Other parameters involved in the energy transfer:

The average donor acceptor distance along with the energy transfer

probabilities & transfer efficiencies are presented in table 3.2.

The critical transfer distance at which the probability of energy

transfer is equal to radiative decay in present case is equal to 1.59 nm, the

critical transfer distance can be compared with Nakazava and Shionoya [45]

in calcium metaphosphate glass for various rare earth ion pairs lying

between 0.3 nm and 1.2 nm., by Joshi et al.[6] (1.91 nm) for Dy3+-Ho3+ in

zinc phosphate glass.

3.4. C) The Tb - Pr system in zinc phosphate glass

Fluorescent spectra:

Fig.3.13 (A) shows the emission spectra of Tb3+ (1 wt%) in zinc

phosphate glass. It shows four peaks arising from the 5D4 -7F manifold

transitions. The broadening of the lines is characteristic of the glass matrix.

Emission from the 5D3 level was not observed in our case due to high

concentration of Tb3+ ions and ions previously excited to the 5D3 level cross

relax to the 5D4 state.

Energy level diagram:

The energy level diagram of these ions is shown in Fig. 3.13. The

incident radiation (365 nm group of mercury lines) excites the Tb3+ ions. The

Tb3+ ions rapidly depopulate to luminescent 5D4 level. Pr3+ is not excited by

the 365nm radiation so the ions will be in the ground level 3H4.

112

Nature of energy transfer:

On studying the figure presented in 3.13(B) and 3.15, it is observed

that the emission of Tb3+ decreases when doped along with Pr3+ ions. The

decrease in emission is noticed when the concentration of Pr3+ ion is

increased. Such an overall decrease in donor (Tb3+) emission indicates that

there is non- radiative energy transfer from Tb3+ to Pr3+. This decrease in

Tb3+ emission can be explained as, on increasing Pr3+ concentration there

are more Pr3+ ions available for receiving the excited energy of Tb3+ ions,

which reduces the radiative energy of Tb3+ ions.

Since the absorption peak (490nm) of Pr3+ ions as can be seen from

Fig.3.17 falls on emission peak (490nm) of Tb3+. Hence there is possibility of

small radiative energy transfer from Tb3+ to Pr3+.

Mechanism of energy transfer and multipolar term responsible for

energy transfer:

A close look at the energy level diagram of Tb3+ and Pr3+ presented

in Fig. 3.14 shows that 3P0 level of Pr3+ is at same height to the 5D4 level of

Tb3+.The small mismatch of the energy level can be explained by the low

energy phonon present in the lattice. Energy transfer phenomena from Tb3+

to Pr3+ is explained as

Tb3+ and Pr3+ are randomly distributed in glass matrix. When the glass

matrix is excited by 365 nm group of mercury lines, Tb3+ ions rapidly

depopulate to luminescent level 5D4 and Pr3+ to ground state 3H4. As the

113

decay time of the metastable state 5D4 of Tb3+ is 2.5 ms, it has enough time

to transfer its energy to 3P0 of Pr3+.

Energy transfer by exchange process is negligible in present case

because it needs acceptor-donor separation of about 0.3 to 0.4 nm with

overlap of wavefunction, while in present case donor acceptor distance

varies from 1.16 nm to 1.56nm.

The linearity of Fig. 3.16 (Pda versus C2) shows that electric dipole-

dipole interaction is mainly responsible for the energy transfer, which

supports the Fong-Diestler theory. The average donor acceptor distance

varies from 1.16 nm to 1.56 nm (Table 3.3) also in support of electric dipole

–dipole interaction suggested by Dexter

Energy level involved in energy transfer:

As explained above, the energy level between which energy

transfer can take place can be 5D4 level of Tb3+ to 3P0 of Pr3+.

Other parameters involved in the energy transfer:

The average donor acceptor distance along with the energy transfer

probabilities & transfer efficiencies are presented in table 3.3.

The critical transfer distance at which the probability of energy

transfer is equal to radiative decay in present case is equal to 1.40 nm, the

critical transfer distance can be compared with the values obtained by

Nakazava and Shionoya in calcium-metaphospate glass [46] for various rare

114

earth ion pairs (lying between 0.3nm and 1.2nm) and Reisfeld &Boehm for

Sm-Eu pair in phosphate glass (1.44nm) [29].

3.5 CONCLUDING REMARKS:

In this chapter the study of energy transfer in Tb-Nd, Tb-Er and Tb-

Pr systems in zinc phosphate glass has been done. In all the above

mentioned series Tb3+ is taken as sensitizer (donor) and Nd3+, Er3+ , Pr3+

are taken as activator (acceptor). Tb3+ emission is observed decreasing with

the increasing concentration of Nd3+, Er3+, and Pr3+. This is explained by

non-radiative energy transfer from Tb3+ to Nd3+, Er3+, and Pr3+. However the

absorption peak (583nm) of Nd3+ and absorption peak (490nm) of Pr3+ falls

partially at the emission peak at 592nm(5D4-7F6 transition) of Tb3+ ions, small

possibility of radiative energy transfer can’t be ruled out. We calculated the

various parameters as average donor-acceptor distance (DD-A), energy

transfer probability (Pda) and transfer efficiency (η ), which are necessary for

the study of non-radiative energy transfer.

The energy transfer mechanism in Tb-Nd system is explained using

the cross relaxation. Fong-Deistler theory is used to find the multipolar term

responsible for the energy transfer. The linearity of the graph plotted

between energy transfer probability and square of donor + acceptor

concentration in all the above series suggested that the electric dipole-dipole

interaction is mainly found for the energy transfer.

115

Table 3.1: Energy transfer and energy transfer probabilities and donor-

acceptor distances between donor (Tb3+) and acceptor (Nd3+)

Cdonor Cacceptor DD-A Ido Id η Pda x 102 S-1

(wt%) (wt%) (nm) (±1) (±1) (±0.05) (±0.05)

1.0 0.2 2.25 70 54 0.23 1.18

0.4 2.33 47 0.33 1.95

0.6 2.18 36 0.48 3.67

0.8 2.05 28 0.60 6.00

1.0 1.95 23 0.67 8.17

1.2 1.87 17 0.75 12.00

Where Cdonor is the donor concentration, Cacceptor is the acceptor

concentration, DD-A is the average donor-acceptor distance, Ido is the donor

intensity in the absence of acceptor, Id is the donor intensity in presence of

acceptor, η is the energy transfer efficiency (=do

d

II

−1 ) and Pda is the energy

transfer probability (= ⎟⎟⎠

⎞⎜⎜⎝

⎛−11 0

d

d

II

τ )

116

Table 3.2: Energy transfer and energy transfer probabilities and donor-

acceptor distances between donor (Tb3+) and acceptor (Er3+)

Cdonor Cacceptor DD-A Ido Id η Pda x 102 S-1

(wt%) (wt%) (nm) (±1) (±1) (±0.05) (±0.05)

1.0 0.2 1.71 90 63 0.30 1.71

0.4 1.63 48 0.47 3.50

0.6 1.56 44 0.51 4.18

0.8 1.50 42 0.53 4.57

1.0 1.45 36 0.60 6.00

Where Cdonor is the donor concentration, Cacceptor is the acceptor

concentration, DD-A is the average donor-acceptor distance, Ido is the donor

intensity in the absence of acceptor, Id is the donor intensity in presence of

acceptor, η is the energy transfer efficiency (=do

d

II

−1 ) and Pda is the energy

transfer probability (= ⎟⎟⎠

⎞⎜⎜⎝

⎛−11 0

d

d

II

τ )

117

Table 3.3: Energy transfer and energy transfer probabilities and donor-

acceptor distances between donor (Tb3+) and acceptor (Pr3+)

Cdonor Cacceptor DD-A Ido Id η Pda x 102 S-1

(wt%) (wt%) (nm) (±1) (±1) (±0.05) (±0.05)

1.0 0.5 1.56 80 53 0.33 2.04

1.0 1.41 42 0.47 3.62

1.5 1.30 30 0.62 6.67

2.0 1.23 25 0.68 8.80

2.5 1.16 21 0.73 11.20

Where Cdonor is the donor concentration, Cacceptor is the acceptor

concentration, DD-A is the average donor-acceptor distance, Ido is the donor

intensity in the absence of acceptor, Id is the donor intensity in presence of

acceptor, η is the energy transfer efficiency (=do

d

II

−1 ) and Pda is the energy

transfer probability (= ⎟⎟⎠

⎞⎜⎜⎝

⎛−11 0

d

d

II

τ )

118

BIBLIOGRAPHY

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crystals (John Wiely and Sons., New York) 1968.

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122

Figure Caption

Fig. 3.1: A) Schematic representation of multipolar resonance.

B) Schematic representation of multipolar transfer.

C) Schematic representation of non resonant energy transfer.

Fig.3.2 Emission spectra of (A) Tb3+ (1.0 wt % fixed) (B) Tb3+ (1 wt%) + Nd3+ (0.4 wt%

Fig.3.3 Energy level diagrams of Tb3+ and Nd3+ .

Fig.3.4 Emission spectra of Tb3+ (1.0 wt %) in presence of varing

concentration of Nd3+.

Fig.3.5 Schematic representation of cross relaxation betweenTb3+ and

Nd3+ .

Fig.3.6 Variation of energy transfer probability (Pda) with square of

donor + acceptor concentration.

Fig.3.7 Absorption spectra of Nd3+ (1.0wt%) in zinc phosphate glass.

Fig. 3.8 Energy level diagram of Tb3+ and Er3+.

Fig 3.9 Emission spectra of (A) Tb3+ (1.0 wt %) and (B) Tb3+ (1.0wt %) + Er3+ (0.6wt %) in zinc phosphate glass.

Fig 3.10 Emission spectra of Tb3+ with varying concentration of Er3+

in zinc phosphate glass.

123

Fig 3.11 Variation of energy transfer probability, Pda, with square of

donor + acceptor concentration.

Fig. 3.12 Absorption spectra Er-3+ (1 wt %) in zinc phosphate glass.

Fig.3.13 Emission spectra of (A) Tb3+ (1.0 wt % fixed) (B) Tb3+ (1 wt%)

+ Pr3+ (1.0 wt%).

Fig. 3.14 Energy level diagram of Tb3+ and Pr3+.

Fig. 3.15 Variation of Tb3+ ion emission with the varied concentration of

Pr3+ in zinc phosphate glas

Fig.3.16 Variation of energy transfer probability (Pda) with square of

donor + acceptor concentration.

Fig. 3.17 Absorption spectra of Pr3+(1 wt%) in zinc phosphate glass.

124

Fig. 3.1: A) Schematic representation of multipolar resonance.

B) Schematic representation of multipolar transfer.

C) Schematic representation of non resonant energy transfer.

ION 1

ION 1

1 2

2 1

[A]

MULTIPOLAR RESONANCE

[B]

MULTIPOLAR

TRANSFER

[C]

NONRESONANT

TRANSFER

125

Fig.3.2 Emission spectra of (A) Tb3+ (1.0 wt % fixed) (B) Tb3+ (1 wt%) + Nd3+ (0.4 wt%)

126

Fig.3.3 Energy level diagrams of Tb3+ and Nd3+ .

127

Fig.3.4 Emission spectra of Tb3+ (1.0 wt %) in presence of varing

concentration of Nd3+.

128

Fig.3.5 Schematic representation of cross relaxation betweenTb3+ and Nd3+ .

129

Fig.3.6 Variation of energy transfer probability (Pda) with square

of donor + acceptor concentration.

130

Fig.3.7 Absorption spectra of Nd3+ ( 1wt%) in zinc phosphate glass.

131

Fig. 3.8 Energy level diagram of Tb3+ and Er3+.

132

Fig 3.9 Emission spectra of (A) Tb3+ (1.0 wt %) and (B) Tb3+ (1.0wt %) + Er3+ (0.6wt %) in zinc phosphate glass.

133

Fig 3.10 Emission spectra of Tb3+ with varying concentration of

Er3+ in zinc phosphate glass.

134

Fig 3.11 Variation of energy transfer probability, Pda, with square of

donor + acceptor concentration.

0

2

4

6

1 2 3 4 5C2, C is donor+acceptor concentration

Pdax 10

2S

-1

135

Fig. 3.12 Absorption spectra Er-3+ (1 wt %) in zinc phosphate glass.

136

Fig.3.13 Emission spectra of (A) Tb3+ (1.0 wt % fixed) (B) Tb3+ (1

wt%) + Pr3+ (1.0 wt%).

137

Fig. 3.14 Energy level diagram of Tb3+ and Pr3+.

138

Fig. 3.15 Variation of Tb3+ ion emission with the varied

concentration of Pr3+ in zinc phosphate glass

139

Fig.3.16 Variation of energy transfer probability (Pda) with square

of donor + acceptor concentration.

140

Fig. 3.17 Absorption spectra of Pr3+(1 wt%) in zinc phosphate glass

141

CHAPTER 4

STUDY OF SENSITIZE LUMINESCENCE AND ENERGY TRANSFER PROCESS IN SM-SM, SM-EU AND UO2-ER SYSTEMS IN ZINC

PHOSPHATE GLASS

142

CHAPTER 4

STUDY OF SENSITIZE LUMINESCENCE AND ENERGY TRANSFER PROCESS IN Sm-Sm, Sm-Eu

AND UO2-Er SYSTEMS IN ZINC PHOSPHATE GLASS

4.1 INTRODUCTION:

The sensitization of luminescence is a well known phenomena. Due

to its wide applications there has been a great interest for energy transfer

phenomena in rare earth activated phosphors. Because of the special

spectroscopic properties of the rare earth ions, RE doped glasses and fibers

are widely used in bulk and in fiber in lasers.

The self quenching of samarium ions in tungstate glass has been

reported by Van Uitert and Johnson [1] which occurs by electric dipole-dipole

mechanism, Joshi [2] studied the self quenching of samarium in calibo glass

occurred by electric dipole-quadrupole interaction and Reisfeld et.al. [3]

studied in borate and phosphate glass by electric dipole-quadrupole

interaction. Reisfeld and Boehm [4] observed energy transfer from Sm3+ to

Eu3+ in phosphate glass.

Europium (Eu) is one of the rarest, hard as lead and is quite ductile. It

is the most reactive of the RE metals. Studies of europium ion in various

host e.g. crystal & glasses have revealed that this ion with configuration 4f is

generally found in trivalent state [5]. The red emission of Eu3+ doped

phosphors has been extensively used in colour television and lasers [6]. Eu

activated yttrium vanadate is in commercial use as the red phosphor in

143

colour TV tubes. To enhance the Eu3+ emission many researchers have

sensitized it by RE and other ions in various matrices. Reisfeld et.al. [7] have

observed that the emission of Eu3+ is enhanced by two orders of magnitude

due to non-radiative energy transfer from UO2++ and from Bi3+ to Eu3+ in

glasses. Joshi [8] has also reported energy transfer from Tb3+ to Eu3+ in zinc

phosphate glass. Optical properties of Eu3+ ions in glasses have been

extensively studied for preparing materials for good optical devices [9-12].

In glasses the uranium ion exists in many forms such as trivalent

(U3+), tetravalent (U4+) and hexavalent (U6+) [13-14]. The emission of

uranium ions occur from an excited level situated at 20200cm-1 [15], from

which the probability of energy transfer to various ions such as rare earths

ions is high. Also due to strong and broad absorption bands in UV region (it

has large oscillator strength compare to trivalent rare earth ions) the large

portion of excitation energy is absorbed by it can easily be transferred to

other rare earth ions.

The uranyl ion (UO2++) has its maximum emission in green region.

This ion has found its application in various field such as indirect pumping

source application for RE ions in lasers, luminescence, photochemical

reactions, studying the nature of excited state solar energy converters [16-

20].

Cabezas and Deshazer [21] studied the energy transfer process from

uranyl to europium in borosilicate glass. They have shown that the addition

of uranyl to europium increases europium emission fivefold; however the

study is mostly confined to the radiative transfer. Joshi et.al.[22] observed

144

energy transfer from UO2++ to Eu3+ in H2O,D2O,potassium format and acetic

media. They found that a small portion of energy lost by uranyl ions is

transferred to Eu3+ ion via non-radiative process. V.V.SytKo et.al. [23]

studied the characteristic features of electron excitation energy transfer from

UO2++ to Ln3+ at an abnormally high concentration of active ions( from the

viewpoint of concentration quenching ) in phosphate matrices. Transfer of

energy from UO2++ to Ho3+ and Eu3+ has also been reported by many

workers [24-27].

The use of Er3+ ions due to their emission in infrared region of

spectrum is well known in laser and in solar energy concentrators. Van Uitert

[28] studied the energy transfer from Tb3+ ions to Er3+ ions in tungstate glass

by dipole-dipole interaction. Er3+ ions are used as activator in Dy3+-Er3+

system in zinc phosphate glass by Joshi et.al. [29].They found electric

dipole-dipole interaction is mainly responsible for the energy transfer

Keeping in view the above properties of Sm3+, Eu3+, UO2++ and Er3+,

we have chosen the Sm3+ - Sm3+, Sm3+ - Eu3+ and UO2++ - Er3+ systems in

zinc phosphate glass to investigate the following points:

(a) Concentration quenching of Sm3+ ions.

(b) Nature of the energy transfer from Sm3+ - Eu3+ and UO2++-

Er3+.

(c) The mechanism of energy transfer between the ions in each of

the systems mentioned above.

(d) The levels between which the energy transfer takes place.

145

(e) Calculation of parameters related to energy transfer as a

function of concentration (e.g. average donor –acceptor

distance (DD-A), transfer probabilities (PDA), transfer efficiencies

(η) etc.).

4.2 EXPERIMENTAL MATERIALS AND METHODS:

Sodium dihydrogen phosphate 2-hydrate (NaH2PO4 .2H2O) and

zinc oxide (ZnO), both of reagent grade, were used in a proportion of 3:1 by

weight, were used as the constituents of the glass matrix .The method for

preparing the glass pallets has already been discussed in chapter 2.

The following series of glasses were prepared by doping the above

mentioned rare earth ions for studying the energy transfer.

Series 3.I: This series consists of glasses doped with Sm3+ ions of

0.5 wt %, 0.8 wt%, 1.0 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%,

2.0 wt%, 2.5 wt% and 3.0 wt%.

Series 3.II: This series consists of glasses doped with 1 wt % (fixed)

of Sm3+ codoped with 0.2 wt %, 0.4 wt% ,0.6 wt%,0.8 wt

% ,1.0 wt% of Eu3+.

Series 3.III: This series consists of glasses doped with 0.1 wt %

(fixed) of UO2++ codoped with 0.2 wt %, 0.4 wt% ,0.6

wt%,0.8 wt % ,1.0 wt% and 1.2 wt% of Er3+.

146

Glass sample doped with 1 wt% of Er3+ is prepared to study the

absorption spectra.

Emission and absorption spectra were taken according to the method described in chapter 2.

4.3(A) THEORY

In the case of donor-donor interactions at high donor concentration,

the relaxation of electronic excitation energy occurs by migration among the

ions of same species in random walk manner and finally to quenching

centre. This multistep process was originally proposed by Botden [30] to

account for the concentration quenching of luminescence and later

developed by Dexter and Schulman [31]. They observed that with the

increase of concentration, radiative transitions become less likely than non-

radiative.

There are a number of methods for demonstrating the occurrence of

energy transfer as

1. By measuring the excitation spectrum of the emission from the

activator (A). This done by measuring the quantum yield of the

emission from A (identified by its wavelength region) as a function of

the wavelength of the incident radiation. A band in excitation

corresponds to a absorption band. If the excitation spectrum of

activator (A) emission shows excitation band of the sensitizer (D) in

addition of those of A, it is indicates energy transfer from D to A.

147

2. By measuring the decay time of the luminescence from ‘D’ as

fraction of concentration of ‘A’. If ‘D’ is situated in the host lattice in an

isolated position, the average life time ( )dτ of an excited state of ‘D’

(i.e. the decay time of the luminescence) is equal to the reciprocal

of rdP , where r

dP is the probability of radiative emission of donor ion.

The addition of ‘A’ ions, the donor ions can lose its energy and life

time ( )dτ will become shorter and too decay of the luminescence

from ‘D’. By measuring the life time ( )dτ as a function of the

concentration of ‘A’, we can obtain information about the energy

transfer probability from donor to acceptor.

The ratio of number of photons emitted by ‘A’ to that absorbed by

’D’ is called the quantum efficiency (q) of the emission from activator (A) in

case of the sensitizer (D). For high quantum efficiency, rdDA PP ⟩⟩ ( DAP is the

energy transfer probability).

The detailed study of the energy transfer can be done by considering

a system consisting of two type of ions viz., energy donor (D) and energy

acceptor (A). Let D & A are distributed randomly throughout the host matrix

and present in low concentration such that the distance between the

acceptor ions is much greater than the distance between donor ions. The

medium is assumed inactive. A small number of donor ions are initially

excited by a flash of light. The number of excited donor ions is assumed to

very small compared to the total number to donor ions and distributed

uniformly in matrix.

148

In the absence of any interaction between the excited donors, their

decay can be expressed as:

( ) ( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛−=

0

exp0τ

φφ tt (4.1)

where 0τ is the intrinsic donor decay time and ( )tφ and ( )0φ is the

donor excitation density initially and after time t respectively.

This expression denotes the probability of finding a donor at a

particular position in an excited state at time t.

In the absence of resonant transitions within the donor system, the

excited donor ion will decay to a random distribution of acceptor ion

therefore the environment of donor ions varies and the ion pair relaxation

rate depends on the particular environment of any excited donor ion. Thus

the decay of the system of excited donor ions appears non exponential.

In Inokuti –Hirayama [32] approach a donor ion (D) is considered to

be surrounded by a set of quenching ions (A) at distance rj the energy

transfer rate from a donor ion to the ith acceptor ion is Wdai (ri). The time

dependence of the donor ions excited state population is then:

( ) ( ) ∏=

×−=n

iidai rWtttr

10

expexp,τ

φ (4.2)

where N is the total number of acceptor ions.

Statistical average φ (t) of φ (r, t) of an infinitely large no. of donors is

given by:

149

( ) ( )⎪⎭

⎪⎬

⎫

⎪⎩

⎪⎨

⎧

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛ −Γ−−=

stsC

Ctt

3

000

31exp0ττ

φφ (4.3)

For the interaction by the direct exchange between donor and the acceptor

ions, the functions Ф(t) has the following form:

( ) ( )⎭⎬⎫

⎩⎨⎧

⎟⎟⎠

⎞⎜⎜⎝

⎛ ×−−= −

00

3

0

exp0τ

γτ

φφγ teg

CCtt (4.4)

where γ=LR02 is the effective Bohr radius.

We can discuss the result in the following manner:

Irrespective of the scattering by the sample, the observed emission

intensity (Iob) is expressed as the multiplication of the rate of formation of the

excited activator ion (∆I) and the probability of the emission of excited

activator ion(η),[33,34] i.e.

Iob= ∆I × η (4.5)

The rate of formation of the excited activator ion (∆I) can be obtained

from Lambert’s absorption law as follows:

∆I =Ii –It (4.6)

where Ii is the incident radiation intensity and It is the transmitted

radiation intensity. But

It = Ii exp (-K× C×D) (4.7)

150

where K = extinction coefficient, C is the concentration and D is the

path length in the glass.

Therefore equation (4.6) can be written as:

∆I=Ii- Ii exp (-K× C×D)

= Ii (1- exp (-K× C×D)) (4.8)

In the case of weak absorption (low oscillator strength of RE ions)

K ×C<< 1, therefore using the expansion

ex = 1+x+x2+x3+------------and so on

≈ 1+x (when x<<1)

∆I= Ii 1-(1 -K× C×D)

Or ∆I= Ii × K× C×D (4.9)

Now the probability of an excited activator will undergo emission i.e. η

is equal to 3

1

PP where 1P is the probability of emission & 3P is the probability

of deexcitation of activator ion which is equal to the sum of probabilities for

emission

( 1P ) and of non radiative loss 2P .

i.e. 213 PPP += (4.10)

151

thus as Iob= ∆I × η

= Ii × K× C×D×3

1

PP

= Ii × K× C×D×21

1

PPP+

= Ii × K× C×D×

1

21

1

PP

+ (4.11)

Now if non radiative losses are because of multipolar transfer, then

[ ] 32

θα ∗= C

CP (4.12)

Where α is a constant, C is the concentration of acceptor and

( )1

3

34 −

∗∗

⎭⎬⎫

⎩⎨⎧ Π

=θ

RC (4.13)

where R* is the critical transfer distance between donor and acceptor

ions. Critical transfer distance is defined as that distance where the

probability of donor emission is equal to the probability of non radiative

relaxation θ=6, 8, 10 for D-D, D-Q,Q-Q interactions respectively between

donor and acceptor ions.

So Iob= ∆I × η

152

( ) 131

−

∗⎭⎬⎫

⎩⎨⎧ +×××=

θβ C

CDCKIiIob (4.14)

where =β1P

α is called constants of interaction.

Now defining I0= Ii × K× C×D as the emission intensity of the

donor ion in the absence of acceptor.

( ) 13

0 1−

∗⎭⎬⎫

⎩⎨⎧ +=

θβ C

CIIob (4.15)

Taking log on both sides and simplifying the above equation

can be written as :

( )∗+=−

CC

III

ob

ob log3loglog 0 θβ (4.16)

It shows that if a curve is plotted between log( 10 −obII ) and log ( )∗C

C it

comes out to be a straight line of slope is ( )3θ .this can be used to study the

multipolar term responsible for interaction between donor and acceptor ion.

According to Van-Uitert theory [33] the value of θ is 6,8,10 for electric

dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interaction

respectively.

153

4.3(A) GENERALIZED ENERGY LEVEL DIAGRAMS FOR THE

PHOSPHORS CENTERS

The luminescence phenomena that occur in solids can be

discussed with the energy level diagram [35]. The generalized and simplified

energy level diagram is presented in Fig.4.1. The figure shows the potential

energy curve of phosphors centre as the function of configurational

coordinate r, which represents three dimensional changes in average

internuclear spacing and possible changes in the geometrical arrangement

of the atoms in the centre [36-39]. Equilibrium ground state and equilibrium

excited state of the phosphor centre are represented by two curves. There

may be following possibilities for the excited phosphor centre

(A) At the certain points, say f, the ground state and the excited state

curves come very close. The high probability is that the excited

energy stored to such points is delivered to the centre and will

quickly dissipate as heat (non-radiatively) to surrounding lattice.

(B) At room temperature, the unexcited centre may be in its ground

state vibrational level, say Ea. This centre is raised to some

excited state level Eb* by absorbing energy (Eb

* - Ea). Let the

excited centre gives up some energy as heat large (Eb* - Ec

*) in

about 10-12 seconds and rests in the excited Ec*. In that case of

favorable selection rule, the centre may make a spontaneous

radiative emission by emitting the energy (Ec* - Ed) as a luminous

photon therefore the emission consists of a broad band.

154

In some exceptional case, when the configurational coordinate curves

are identical in shape and have the same equilibrium distances as e.g. in the

case of rare earth ions one can found line emission. Because of the heat

dissipation, the emission always lies at a lower energy than the absorption.

This displacement of emission with absorption is known as stokes shift.

The above mentioned theory is used by Mott [37] and Seitz [40] for

explaining the thermal quenching of luminescence. Dexter, Klick and Russel

[41] also used similar approach to explain the quenching of solids at higher

temperature.

In all the above mentioned theories, at sufficiently high temperature

emission of lattice vibration occurs in completion with the phonon emission.

Luminescence efficiency η is given by

21

1

PPP+

=η

where 1P is the probability of luminescence emission and 2P that of

the non radiative transitions. It is assumed that 1P is independent of

temperature while 2P rises with temperature.

In Fig.4.2 at the certain points, say S, the ground state and the

excited state curves intersect each other. If the luminescent centre is in

equilibrium configuration of the excited state, it may also, as a result of

thermal activation, attain a vibrational level situated at the point S. Having

achieved it, the centre will return non-radiatively to the equilibrium

configuration of the ground state, dissipating as the heat in the process.

155

Pearson et.al.[42] have explained the fall of decay time of Tb3+ in glasses

and Pringsheim [43] has explained the thermal quenching of uranyl emission

in glass.

4.4 RESULTS AND DISCUSSION

4.4.A.1) The Sm - Sm system in zinc phosphate glass.

The emission spectrum of Samarium (Sm3+) ions in zinc phosphate

glass is shown in Fig.4.3. The three emission peaks shown in the spectrum

arise due to the transitions 4G5/2 →6H5/2 (560nm), 4G5/2 →6H7/2 (595nm) and

4G5/2 6H9/2 (645nm). In glasses, due to high phonon energy the first two

levels undergo a rapid depopulation and the Sm3+ ion luminesces only from

4G5/2 level. The decay time of emitting level in phosphate glass is found 1.8

[44].

4.4.A.2) The concentration quenching

For obtaining high emission efficiency, it is obvious to make the

activator concentration high. As the concentration of the ions is increased,

the emission intensity increases. But in many case it found that the

fluorescence intensity of the ions increases with the concentration to the

maximum value, after getting maximum value emission intensity decreases

with increasing concentration. This phenomenon is known as concentration

quenching.

Some of the ions such as Mn2+ show concentration quenching.

Bingham and Parke [45] obtained concentration quenching of Mn2+ emission

156

at 1.0 mol% in sodium silicate and 0.5 mol% in borate glass. Joshi et.al.

found concentration quenching of Mn2+ at 0.2 wt% in zinc phosphate glass

[46].

4.4.A.3) The concentration quenching in Sm3+ ions.

Variation of emission intensity of Sm3+ ion with the concentration of

Sm3+ is presented in Fig.4.4. It is observed that the quenching starts above

the 1.8 wt% concentration of Sm3+.

The concentration quenching observed in present case can be

explained using Dexter and Schulman [31] which states that if the

concentration of activator becomes so high that the probability of energy

transfer exceeds the probability of emission. In such condition the excitation

energy repeatedly goes from one activator ion to another. Since the lattice is

not perfect, it contains all kinds of sites where the excitation energy may be

lost at the surfaces and to the dislocations due to impurities. In traversing the

host matrix the excitation energy will sooner or later encounter such as site,

where dissipated as heat, it makes no contribution to the luminescence and

thus causing a decrease in the luminescence intensity.

As suggested by above theory, we calculated energy transfer and

energy transfer probabilities, donor- donor distances and energy transfer

efficiency. These values are presented in Table 4.2. The linearity of the

graph (Fig.4.5) drawn between energy transfer probability and square of

donor + donor concentration suggested that electric dipole-dipole interaction

is mainly responsible for the quenching.

157

4.4.B) The Sm - Eu system in zinc phosphate glass

Fluorescent spectra:

The samarium (Sm3+) and europium (Eu3+) ions get excited by 365

nm radiation of excitation. The electronic configuration of the trivalent Sm3+ is

4F6. The Sm3+ ion luminance from 4G7/2, 4F3/2 and 4F7/2 levels to the ground

6H multiplet [47]. The emission spectrum of Sm3+ ion in zinc phosphate glass

is presented in Fig. 4.6(A). The three emission peaks shown in the spectrum

arise due to the transitions 4G5/2 →6H5/2 (560nm), 4G5/2 →6H7/2 (595nm) and

4G5/2 →6H9/2 (645nm). In glasses, due to high phonon energy the first two

levels undergo a rapid depopulation and the Sm3+ ion luminance only from

4G5/2 level. The Eu3+ ions show emission in red region through 5D0 →7F1

(590 nm) and 5D0 →7F2 (617 nm). The emission spectrum of Eu3+ ion in zinc

phosphate glass is presented in Fig. 4.6(B). The energy level diagram of

these ions is shown in Fig. 4.7.

Nature of energy transfer:

Fig.4.8 shows the variation of Sm3+ emission intensity with the

varied concentration of Eu3+ ions. The Fig 4.8 and Fig. 4.6(C) shows that

there is overall decrease of emission intensity of Sm3+ ions and the

increased the emission intensity of Eu3+ ions. The overall decrease of the

emission intensity of Sm3+ ion suggested that there is non-radiative energy

transfer from Sm3+ to Eu3+ ion.

Energy transfer by exchange process is negligible in our case

because it needs acceptor-donor separation of about 0.3-0.4nm with overlap

158

of wavefunction, while in our case donor-acceptor separation varies from

1.41 to 1.67 nm.

Mechanism of energy transfer:

In order to find out the mechanism of energy transfer, we proceed

as follows:

The observation of energy level diagram of Sm3+ and Eu3+ in Fig.

4.7 shows that there is no energy level of Eu3+ is exact match to 4G5/2 level of

Sm3+. The 5D0 level of Eu3+ is close to 4G5/2 level of Sm3+. So the emission

energy of Sm3+can be transferred to Eu3+. Small mismatch of energy can be

compensated by the emission of phonon.

Sm3+ and Eu3+ are randomly distributed in glass matrix. When this

glass matrix is excited to 365 nm group of mercury lines, Sm3+ ions rapidly

depopulate to luminescent 4G5/2 level and Eu3+ to 5D0. On increasing Eu3+ to

Sm3+ all the three peaks decreases with same proportion.

Such processes (cross relaxation) become appreciable if energy is

living in long-lived metastable state. In present case decay time of

metastable state 5D4 is 1.8 ms, hence have enough time to transfer its

energy to 7F6 level of Nd3+.

Joshi et.al [48,49] uses the theory of cross relaxation in Eu-Er and

Eu-Tm system in zinc phosphate glass.

159

Multipolar term responsible for energy transfer:

To find out which of the multipolar term is responsible for energy

transfer, a graph is drawn between energy transfer probabilities (Pda) and

square of the concentration (donor & acceptor) which gives a straight line

(Fig. 4.9). The linear dependence of Pda on the square of the concentration

of donor and acceptor is attributed to because of dipole- dipole interaction

between donor and acceptor [50]. The dipole -dipole mechanism of energy

transfer is further supported by the average donor to acceptor distance

which varies in this system between 1.41nm and 1.67 nm (Table 4.3) which

is in the range of electric dipole- dipole interaction between donor & acceptor

in accordance with Forster’s [51] & Dexter’s [52] theories of multipolar

interactions.

Other parameters involved in the energy transfer:

In this series, the average donor acceptor distance along with the

energy transfer probabilities & transfer efficiencies are presented in table 4.3

which are calculated by using the following formulae:

DD→A = 1 / (Cd + Ca) 1/3

where Cd & Ca are donor and acceptor ion Concentration per cm3 in the host

matrix.

Pda =1/τ0 (Id0/Id - 1),

160

η= 1 – Id/Id0.

4.4. C) The UO2 - Er system in zinc phosphate glass

Fig.4.10 (A) shows the emission spectra of U02++ (0.1 wt%) in zinc

phosphate glass. When excited with 365 nm of mercury lines, the uranium

ions in zinc phosphate glass gives bright green luminescence. The emission

spectrum extends in the visible region from blue to red with maximum

intensity in green region corresponding to 525nm. The five bands shown in

the emission spectrum correspond to the transition from the excited level

situated at around 20200 cm-1 above the ground state to the five vibrational

levels situated above ground state with equal energy intervals ( )120855 −± cm

[53]. The emission intensity of uranyl ion in zinc phosphate glass is higher

than in borate and silicate glasses [44]. The reason for this lies in the fact

that phonon energy of phosphate glass (~ 1100cm-1) is lower than in borate

glass. This lower phonon energy results in a higher radiative transition

probability in glass. The broad lines of uranyl emission are characteristic of

the vibrational frequencies of the ion which are further broadened by the

glassy matrix. The energy level diagram of these ions is shown in Fig. 4.11.

Nature of energy transfer:

Fig. 4.10(A) and Fig.4.10(B) shows the emission spectra of uranyl ion

in the absence and presence of the Er3+ ion in zinc phosphate glass

respectively. A comparison of both the curve indicate that the uranyl ion

decreases, when added Er3+ ions. The decrease in intensity of uranyl ions is

overall i.e. its decrease is same for the entire wavelength. This fact results

161

that there is a non radiative energy transfer from UO2++ ions to the Er3+ ions.

The result is also supported by the Figure at Fig. 4.12, which shows the

emission of UO2++ with the varying concentration of Er3+ ions.

The erbium ions have unobservable emission in visible region in zinc

phosphate glass by the excitation with 365 nm of radiation. So the possibility

of back transfer of energy from Er3+ to UO2++ is extremely low. Thus the

energy transfer from UO2++ to Er3+ is mainly non radiative in nature.

The possibility of small radiative transfer is not completely ruled out

as Er3+ shows absorption near the emitting level of UO2++ (Fig. 4.15).

Mechanism of energy transfer and multipolar term responsible for

energy transfer:

The energy level diagrams of UO2++ and Er3+ is presented in Fig.

4.11. The energy is being transferred from the lowest excited state of UO2++

at 20200 cm-1 to the 4F7/2, 2H11/2 and 4S3/2 recipient levels of Er3+. Small

mismatch of the energy level can be explained by the low energy phonon

present in the lattice.

Energy transfer by exchange process is negligible in present case

because it needs acceptor-donor separation of about 0.3 to 0.4 nm with

overlap of wavefunction, while in present case donor acceptor distance

varies from 1.71 nm to 2.88nm.

The linearity of Fig. 4.13 (Pda versus C2) shows that electric dipole-

dipole interaction is mainly responsible for the energy transfer, which

162

supports the Fong-Dieslter theory [50]. The average donor acceptor distance

varies from 1.71 nm to 2.88 nm (Table 4.4) also in support of electric dipole

–dipole interaction suggested by Dexter [52].

Dipole-dipole interaction is further corroborated by using Van-Uitert

theory [1]. From Fig. 4.14 we have obtained the value of C* as 0.38 wt% of

UO2++. The curve plotted between log (I0/Iob-1) and log C/C* has been

presented in Fig. 4.10, which comes a straight line indicating a multipolar

interaction between UO2++ and Er. The slope of the line i.e. θ/3 gives a value

around 6 to θ, suggesting dipole-dipole interaction between donor and

acceptor ions.

The average donor acceptor distance along with the energy transfer

probabilities & transfer efficiencies are presented in table 4.4.

The critical transfer distance at which the probability of energy

transfer is equal to radiative decay in present case is equal to 2.42 nm.

4.5 CONCLUDING REMARKS:

The energy transfer between Sm-Sm, Sm-Eu and UO2-Er in zinc

phosphate glass have been made in this chapter. The quenching of Sm3+

ions is observed at 1.8 wt% in zinc phosphate glass. The mechanism is

found to be dipole-dipole in nature for all the series .The average donor-

acceptor distance varies from 2.89 to 1.77 nm while energy transfer

efficiency varies from 0.38 to 0.83 while transfer probability varies from 3.00

× 103 to 24.33 ×103 s-1 respectively.

163

Table 4.1: Energy transfer and energy transfer probabilities and donor- acceptor distances between donor (Sm3+) and acceptor (Eu3+)

CSm Emission Intensity

(wt%) (arb.units)

0.2 27

0.5 45

0.8 59

1.0 66

1.2 70

1.5 73

1.8 76

2.0 74

2.5 69

3.0 61

Where Csm is the concentration of Sm3+ ions in wt%.

164

Table 4.2: Energy transfer and energy transfer probabilities and donor-

donor distances between donor (Sm3+) and acceptor (Eu3+)

Cdonor Cacceptor DD-D Ido Id η Pdd x 102 S-1

(wt%) (wt%) (nm) (±1) (±1) (±0.05) (±0.05)

1.8 2.0 1.14 76 74 0.03 0.15

2.5 1.10 69 0.09 0.56

3.0 1.06 61 0.19 1.37

Where Cdonor is the donor concentration, Cacceptor is the acceptor

concentration, DD-A is the average donor-acceptor distance, Ido is the donor

intensity in the absence of acceptor, Id is the donor intensity in presence of

acceptor, η is the energy transfer efficiency (=do

d

II

−1 ) and Pda is the energy

transfer probability (= ⎟⎟⎠

⎞⎜⎜⎝

⎛−11 0

d

d

II

τ )

165

Table 4.3: Energy transfer and energy transfer probabilities and donor-

acceptor distances between donor (Sm3+) and acceptor (Eu3+)

Cdonor Cacceptor DD-A Ido Id η Pda x 102 S-1

(wt%) (wt%) (nm) (±1) (±1) (±0.05 ) (±0.05)

1.0 0.2 1.67 1379 1290 0.06 0.38

0.4 1.59 1226 0.11 0.69

0.6 1.52 1163 0.15 1.03

0.8 1.46 1139 0.17 1.17

1.0 1.41 1032 0.25 1.86

Where Cdonor is the donor concentration, Cacceptor is the acceptor

concentration, DD-A is the average donor-acceptor distance, Ido is the donor

intensity in the absence of acceptor, Id is the donor intensity in presence of

acceptor, η is the energy transfer efficiency (=do

d

II

−1 ) and Pda is the energy

transfer probability (= ⎟⎟⎠

⎞⎜⎜⎝

⎛−11 0

d

d

II

τ )

166

Table 4.4: Energy transfer and energy transfer probabilities and donor-

acceptor distances between donor (UO2++) and acceptor (Er3+)

Cdonor Cacceptor DD-A Ido Id η Pda x 103 S-1

(wt%) (wt%) (nm) (±1) (±1) (±0.05) (±0.05)

0.1 0.2 2.88 88 55 0.38 3.00

0.4 2.39 33 0.63 8.33

0.6 2.12 32 0.64 8.75

0.8 1.94 29 0.67 10.17

1.0 1.81 20 0.77 17.00

1.2 1.71 15 0.83 24.33

Where Cdonor is the donor concentration, Cacceptor is the acceptor

concentration, DD-A is the average donor-acceptor distance, Ido is the donor

intensity in the absence of acceptor, Id is the donor intensity in presence of

acceptor, η is the energy transfer efficiency (=do

d

II

−1 ) and Pda is the energy

transfer probability (= ⎟⎟⎠

⎞⎜⎜⎝

⎛−11 0

d

d

II

τ )

167

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171

Figure Caption

Fig.4.1: Generalized energy level diagram for a luminescence ion.

Fig.4.2: Schematic representation of the ground state and excited state of an ion in a solid.

Fig.4.3: Emission spectra of Sm3+ (1.0 wt %).

Fig.4.4: Variation of Sm3+ emission with varying concentration of Sm3+ in zinc phosphate.

Fig.4.5: Variation of Pda with the square of donor + acceptor concentration in wt% (for Sm-Sm series).

Fig.4.6: Emission spectra of (A) Sm3+ (1.0 wt % fixed)

(B) Eu3+ (0.8 wt% fixed)

(C) Sm3+ (1 wt%) + Eu3+ (0.8 wt%).

Fig.4.7: Energy level diagram of Sm3+ and Eu3+.

Fig.4.8: Variation of Sm3+ emission with the Eu3+ concentration.

Fig.4.9: Variation of Pda with the square of donor + acceptor concentration in wt%.

Fig.4.10: Emission spectra of (A) UO2++ (0.1 wt % fixed)

(B) UO2++ (0.1 wt %) +Er3+ (1.0 wt%).

Fig.4.11: Energy level diagram of UO2++ and Er3+.

Fig.4.12: Variation of UO2++ emission with the Er3+ concentration.

Fig.4.13: Variation of Pda with the square of donor + acceptor concentration in wt%.

Fig.4.14: The graph between log (I0/Iob-1) and log C/C*.

Fig. 4.15: Absorption spectra Er-3+ (1 wt %) in zinc phosphate glass.

172

Fig. 4.1: Generalized energy level diagram for a luminescence ion.

b

a

Equilibrium grond

c

d

Ro Re

Equilibrium excited

Energry, E

Configurational coordinate

Eb

E*

f

Eb**

e

173

Fig. 4.2: Schematic representation of the ground state and excited state of an ion in a solid.

r

Δr

Energy

r

Ue

S

Ug

(Configurational coordinate)

174

Fig.4.3 Emission spectra of Sm3+ (1.0 wt %).

175

Fig.4.4 Variation of Sm3+ emission with varying concentration of Sm3+ in zinc phosphate.

176

Fig.4.5 Variation of Pda with the square of donor + acceptor concentration in wt% (for Sm-Sm series).

177

Fig.4.6 Emission spectra of (A) Sm3+ (1.0 wt % fixed)

(B) Eu3+ (0.8 wt% fixed)

(C) Sm3+ (1 wt%) + Eu3+ (0.8 wt%).

178

Fig.4.7: Energy level diagram of Sm3+ and Eu3+.

179

Fig.4.8: Variation of Sm3+ emission with the Eu3+ concentration.

180

Fig.4.9 Variation of Pda with the square of donor + acceptor concentration in wt%.

181

Fig.4.10 Emission spectra of (A) UO2++ (0.1 wt % fixed)

(B) UO2++ (0.1 wt %) +Er3+ (1.0 wt%).

182

Fig.4.11 Energy level diagram of UO2++ and Er3+.

183

Fig.4.12 Variation of UO2++ emission with the Er3+ concentration.

184

Fig.4.13 Variation of Pda with the square of donor + acceptor concentration in wt%.

185

Fig.4.14: The graph between log (I0/Iob-1) and log C/C*

186

Fig. 4.15: Absorption spectra Er-3+ (1 wt %) in zinc phosphate glass.

187

CHAPTER 5

STUDY OF SENSITIZE LUMINESCENCE AND ENERGY TRANSFER PROCESS IN DY-PR

AND TM-ER SYSTEMS IN ZINC PHOSPHATE

GLASS

188

CHAPTER 5

STUDY OF SENSITIZE LUMINESCENCE AND ENERGY TRANSFER PROCESS IN Dy-Pr AND Tm-Er

SYSTEMS IN ZINC PHOSPHATE GLASS

5.1 INTRODUCTION

Optical properties of dysprosium doped glasses are now attracting a

practical interest because the 1.3 µm emission can be utilized for the optical

amplification in telecommunication systems [1-3]. Also the intense yellow

emission due to the 4F9/ 2 → 6H13/ 2 can be utilized as visible solid state laser

acting at novel wavelength [4].

The dysprosium ion in the trivalent state has a 4f9 electronic

configuration in it outermost electronic shell [5]. Generally, the luminescence

of Dy3+ ion is observed due to the transition from 6P7/2, 4I13/2 and 4F9/2 levels

to the ground 6H multiplets [6]. In glasses due to high phonon energy the

luminescence is observed only from 4F9/2 level.

The dysprosium (Dy3+) ion has been used as an energy donor in

transfer processes by many workers. Uitert & Dearborn [7] have observed a

non-radiative energy transfer from Dy3+ to Tb3+ in tungstates. Taking Dy3+ as

energy donor Joshi et. al. [8,9] have reported non-radiative energy transfer

from Dy3+ to Ho3+ and from Dy3+ to Er3+ in zinc phosphate glass. Joshi &

189

Joshi [10] have also studied diffusion limited energy transfer from Dy3+ to

Nd3+ in calibo glass.

Joshi et. al. [11] sensitized the praseodymium ion (Pr3+) by Mn2+ in

phosphate glass and reported that the energy transfer from Mn2+ to Pr3+

takes place non-radiatively and the mechanism of energy transfer observed

by them was electric dipole-dipole in nature. R.Lohani [12] has reported non-

radiative energy transfer from Sm3+ to Pr3+ in zinc phosphate glass. Some of

the workers studying properties of Pr3+ are cited in references [13-15].

A thulium ion with 4f12 is generally found in trivalent state. Reisfeld

and Eckstein [16, 17] have made the detailed study of thulium ion, alone as

well as codoped with erbium ions in many glassy matrices. Eyal et.al.[18]

found energy transfer from Mn2+ to low laying levels of Tm3+ in transition

metal fluoride glass. Joshi and Joshi [19] sensitize Pr3+ ions by Tm3+ ions in

sodium phosphate glasses.

In recent years, the erbium is the most studied RE ions in different

host materials [20-24]. Lohani [12] have reported the transfer of energy from

Mn2+ to Er3+ and energy transfer from Dy3+ to Er3+ in zinc phosphate glass.

Keeping above facts in mind, we have chosen the Dy3+ - Pr3+ and

Tm3+ - Er3+ systems in zinc phosphate glass to investigate the following

points:

(a) Nature of the energy transfer from Dy3+ to Pr3+ and Tm3+ to

Er3+.

190

(b) The mechanism of energy transfer between the ions in each of

the systems mentioned above.

(c) The levels between which the energy transfer takes place.

(d) Calculation of parameters related to energy transfer as a

function of concentration (e.g. average donor –acceptor

distance (DD-A), transfer probabilities (PDA), transfer efficiencies

(η) etc..

5.2 EXPERIMENTAL MATERIALS AND METHODS:

Sodium dihydrogen phosphate 2-hydrate (NaH2PO4 .2H2O) and

zinc oxide (ZnO), both of reagent grade, were used in a proportion of 3:1 by

weight, were used as the constituents of the glass matrix .The method for

preparing the glass pallets has already been discussed in chapter 2.

The following series of glasses were prepared by doping the above

mentioned rare earth ions for studying the energy transfer.

Series 3.I: This series consists of glasses doped with 1 wt % (fixed)

of Dy3+ codoped with 0.25 wt %, 0.50 wt% ,0.75

wt%,1.0 wt % ,1.25 wt% of Pr3+.

Series 3.II: This series consists of glasses doped with 1 wt % (fixed)

of Tm3+ codoped with 0.2 wt %, 0.4 wt% ,0.6 wt%,0.8 wt

% ,1.0 wt% and 1.2 wt% of Er3+.

Glass samples doped with 1 wt% of Er3+ and doped with 1 wt% of

Pr3+ are prepared to study the absorption spectra.

191

Emission & absorption spectra were taken according to the method

described in chapter 2.

192

5.3 THEORY:

In glasses, the energy levels of rare earth ions get broadened due to

the crystal field perturbation. It decreases the possibility of resonant energy

transfer from sensitizer to an activator. However, small mismatch of energy

levels of interacting ions can be compensated by the emission or absorption

of the phonons. Such a phonon-assisted transfer theory was first given by

Orbach [25]. The multiphonon emission depends on temperature, energy

gap and host. When the energy levels of a donor and an acceptor mismatch

of energy of the order of several thousand cm-1, the energy transfer by

multiphonon must be considered. The phonon energy of various glasses is

as

Glass BOND Stretching frequency

Phosphate P-O 1140-1300 cm-1

Borate B-O 1310-1388 cm-1

Silicate Si-O 1010-1115 cm-1

Germinate Ge-O 840-930 cm-1

Miyakawa and Dexter [26] in their theoretical analysis of multiphonon

processes derived a comparative relaxation analogue of the multiphonon

gap dependence. According to their theory the probability of phonon

assisted energy transfer (WPAT) is expressed as:

( ) ( ) )exp(0 EWEW PATPAT ∆−=∆ β (5.1)

193

where ∆E represents the energy gap between the electronic levels of

donor and acceptor ions and β is the parameter determined by the strength

of electron lattice coupling as well as by the nature of phonons involved and

is given by:

( ) ⎥⎦

⎤⎢⎣

⎡−

⎭⎬⎫

⎩⎨⎧

+= 11ln1 ngN

hωβ (5.2)

Where g is known as electron-lattice coupling constant is the number

of phonons excited at the temperature of the systems , hω is the energy of

phonons which contributes dominantly to these multiphonon processes and

N is the number phonons emitted in these processes such that

N= ∆E/ hω (5.3)

Non-resonant phonon-assisted energy transfer between various

trivalent rare earth ions in yttrium oxide crystals were thoroughly studied by

Yamada et. al. [27]. On increasing the temperature phonon assisted energy

transfer rate also increases since stimulated emission of phonons becomes

operative. The temperature dependence of phonon assisted transfer rate, if

it is assumed that phonons involved in energy transfer are of equal energy is

given by:

( ) ( ) NPATPAT nWEW )1(0 +=∆ (5.4)

194

5.4 RESULTS AND DISCUSSION

5.4.A) The Dy - Pr system in zinc phosphate glass

Fluorescent spectra:

Emission spectra of Dy3+ (1wt %) and Dy3+ (1wt %) + Pr3+ (1wt %) are

shown in part ‘A’ and part ‘B’ of Fig. 5.1 respectively. Two peaks in Dy3+

emission spectra arise due to transition 4F9/2-6H15/2 and 4F9/2-6H13/2 lying at

482nm and 475nm respectively. When glass matrix is excited by 365 nm

group of mercury lines, Dysprosium ions rapidly depopulates to luminescent

4F9/2 level by non radiative decay and remains there for 570µs in zinc

phosphate glass.

Nature of energy transfer:

Fig.5.2 shows the variation of Dy3+ emission intensity with the varied

concentration of Pr3+ ions and in Fig. 5.2(B) shows the emission spectra of

Dy3+ (1wt%) + Pr3+ (1wt%). Both the observations indicate that there is

overall decrease of emission intensity of Dy3+. The overall decrease of the

emission intensity of Dy3+ ion suggests that there is non-radiative energy

transfer from Dy3+ to Pr3+ ion.

Energy transfer by exchange process is negligible in our case

because it needs acceptor-donor separation of about 0.3-0.4nm with overlap

of wavefunction, while in our case donor-acceptor separation varies from

1.36 nm to 1.68 nm. Fig.5.5 shows absorption spectra of Pr3+. Possibility of

195

radiative energy transfer from Dy3+ to Pr3+ is ruled out as absorption peaks

(Fig.5.5) of acceptor (Pr3+) ions do not falls at either emission peaks of donor

(Dy3+) ions.

Mechanism of energy transfer:

In order to find out the mechanism of energy transfer, we proceed

as follows:

Energy level diagrams of Dy3+ and Pr3+ are presented in Fig.5.3.

When glass matrix is excited by 365 nm group of mercury lines, Dysprosium

ions rapidly depopulate to luminescent 4F9/2 level by non radiative decay and

remain there for 570µs in zinc phosphate glass. A close examination of

energy level diagrams of Dy3+ and Pr3+ shows that 4F9/2 level of Dy3+

matches with 1P1 level of Pr3+. Hence sEnergy can non-radiatively transfer

from Dy3+ to Pr3+. The small mismatch of energy level is compensated by

some low energy phonons.

Multipolar term responsible for energy transfer:

To find out multipolar term responsible for energy transfer from Dy3+

to Pr3+, we plotted graph (Fig.5.4) between energy transfer probabilities Pda

and square of donor + acceptor concentration. It comes out as a straight

line. The linearity of the graph shows electric dipole-dipole interaction is

responsible for the energy transfer supporting the Fong-Diesler theory .

The critical donor-acceptor distance at which the probability of energy

transfer rate is equal to radiative decay is 1.37 nm in Dy3+-Pr3+ system which

196

can be compared with R0=1.88 nm in Dy3+-Er3+ in zinc phosphate Eyal

et.al. , Joshi et.al. R0=1.91 nm in Eu3+-Tm3+ system in zinc

phosphate glass.

Other parameters involved in the energy transfer:

In this series, the average donor acceptor distance along with the

energy transfer probabilities & transfer efficiencies are presented in table 3.1

which are calculated by using the following formulae:

DD→A = 1 / (Cd + Ca) 1/3

where Cd & Ca are donor and acceptor ion concentration per cm3 in

the host matrix.

Pda =1/τ0 (Id0/Id - 1),

η= 1 – Id/Id0.

5.4.B) The Tm - Er system in zinc phosphate glass

Fluorescent spectra:

Emission spectra of Tm3+ (1wt %) and Tm3+ (1wt %) + Er3+ (1wt %)

are shown in part ‘A’ and part ‘B’ of Fig. 5.6 respectively. The Tm3+ ions

when excited by 365 nm radiation in zinc phosphate glass gives emission in

blue-violet region of spectrum. The fluorescence spectrum of Tm3+ in zinc

phosphate glass shows two peaks at 454 nm and 475 nm. The peak at 454

nm corresponds to the transition 1D2-3H4 and peak at 475 nm to 1G4-3H6.

197

Emission spectrum shows that the peak due to the transition 1D2-3H4 is more

intense that of the transition 1G4-3H6.

Nature of energy transfer:

Fig.5.8 shows the variation of Tm3+ emission intensity with the

varied concentration of Er3+ ions and in Fig. 5.6(B) shows the emission

spectra of Tm3+ (1wt%) + Er3+ (1wt%). Both the observations indicate that

the incorporation of Er3+ ions to the Tm3+ ions decrease the Tm3+ emission.

The emissions of both the peaks decrease with the same proportion. From

the overall decrease of the emission intensity of Tm3+ ion we suggest that

there is non-radiative energy transfer from Tm3+ to Er3+ ion.

Energy transfer by exchange process is negligible in our case

because it needs acceptor-donor separation of about 0.3-0.4nm with overlap

of wavefunction, while in our case donor-acceptor separation varies from

1.42nm to 1.74nm.

Possibility of radiative energy transfer from Tm3+ to Er3+ is ruled out

as no absorption peaks (Fig.5.10) of acceptor (Er3+) ions falls at emission

peaks of donor (Tm3+) ions.

Mechanism of energy transfer:

In order to find out the mechanism of energy transfer, we proceed

as follows:

Tm3+ and Er3+ ions are randomly distributed in the glass matrix.

When the glass matrix is excited by the 365 nm radiation, the Tm3+ ions

198

excite to the 1D2 level and the Er3+ ions to ground state 4I15/2. For finding out

the levels of donor and acceptor between which the transfer of energy takes

place, we have plotted the energy level diagrams of Tm3+ and Er3+. Careful

observation to the energy level shows that the 4G9/2 level of Er3+ is close to

the emitting 1D2 level of Tm3+.Hence the energy easily can be transferred to

the Er3+ ions from Tm3+ ions.

Multipolar term responsible for energy transfer:

The graph is plotted between energy transfer probabilities Pda and

square of donor + acceptor concentration in order to find out the multipolar

term responsible for energy transfer from Tm3+ to Er3. It comes out straight

line. The linearity of the graph shows electric dipole-dipole interaction is

responsible for the energy transfer supporting the Fong-Diesler theory.

Dipole-dipole interaction is further corroborated by using Van-Uitert

theory [32]. From Fig. 5.8 we have obtained the value of C* as 0.67 wt% of

Er3+. The curve plotted between log (I0/Iob-1) and log C/C* has been

presented in Fig. 5.11, which comes a straight line indicating a multipolar

interaction between Tm3+ and Er3+. The slope of the line i.e. θ/3 gives a value

around 6 to θ, suggesting dipole-dipole interaction between donor and

acceptor ions.

The critical donor-acceptor distance at which the probability of energy

transfer rate is equal to radiative decay is 1.56 nm in Tm3+-Er3+ system

which can be compared with those obtained by Joshi et.al.[33] R0=1.65 nm

199

in Tm-Ho system in zinc phosphate glass and by Joshi et.al. [34] 1.29 nm in

Tb -Ho system in calibo glass.

Other parameters involved in the energy transfer:

In this series, the average donor acceptor distance along with the

energy transfer probabilities & transfer efficiencies are presented in table 3.1

which are calculated by using the following formulae:

DD→A = 1 / (Cd + Ca) 1/3

where Cd & Ca are donor and acceptor ion concentration per cm3 in

the host matrix.

Pda =1/τ0 (Id0/Id - 1),

η= 1 – Id/Id0.

5.5 CONCLUDING REMARKS:

In this chapter, we study the process of energy transfer in zinc

phosphate glass in Dy-Pr and Tm-Er system. Various terms necessary for

the non-radiative energy transfer such as donor-acceptor distance, energy

transfer efficiency and energy transfer probability are calculated for the

above mention series.

In the Dy-Pr system, the donor (Dy3+) concentration is taken fixed

1.0 wt% and the acceptor (Pr3+) concentration is varied from 0.25 to 1.25wt%

(in the step of 0.25 wt%). The emission intensity of Dy3+ ions are found

decreasing with the increasing concentration of Pr3+. Non-radiative energy

200

transfer from Dy3+ to Pr3+ions is mainly responsible for decreasing the Dy3+

emission. Fong-Diestler theory is used to find out the multipolar term

responsible for the transfer, it explain the dipole-dipole interaction is

responsible for the transfer which is further supported by Forster and Dexter

theory.

In Tm-Er system, the Tm3+ concentration is taken fixed 1.0 wt% and

the Er3+ concentration is varied from 0.2 to 1.2wt% (in the step of 0.2 wt %).

Decreases in Tm3+ emission with increasing concentration suggest that

there is non-radiative energy transfer from Tm3+ ions to Er3+ ions. Careful

observation to the energy level shows that the 4G9/2 level of Er3+ is close to

the emitting 1D2 level of Tm3+.Hence the energy easily can be transferred to

the Er3+ ions from Tm3+ ions. The graph is plotted between energy transfer

probabilities Pda and square of donor + acceptor concentration in order to

find out the multipolar term responsible for energy transfer from Tm3+ to Er3.

It comes out straight line. The linearity of the graph shows electric dipole-

dipole interaction is responsible for the energy transfer supporting the Fong-

Diesler theory.

201

Table 5.1: Energy transfer and energy transfer probabilities and donor-

acceptor distances between donor (Dy3+) and acceptor (Pr3+)

Cdonor Cacceptor DD-A Ido Id η Pda x 103 S-1

(wt%) (wt%) (nm) (±1) (±1) (±0.05) (±0.05)

1.0 0.25 1.68 91 80 0.12 0.24

0.50 1.57 73 0.19 0.43

0.75 1.48 65 0.28 0.70

1.00 1.42 50 0.45 1.41

1.25 1.36 43 0.52 1.96

Where Cdonor is the donor concentration, Cacceptor is the acceptor

concentration, DD-A is the average donor-acceptor distance, Ido is the donor

intensity in the absence of acceptor, Id is the donor intensity in presence of

acceptor, η is the energy transfer efficiency (=do

d

II

−1 ) and Pda is the energy

transfer probability (= ⎟⎟⎠

⎞⎜⎜⎝

⎛−11 0

d

d

II

τ )

202

Table 5.2: Energy transfer and energy transfer probabilities and donor-

acceptor distances between donor (Tm3+) and acceptor (Er3+)

Cdonor Cacceptor DD-A Ido Id η Pda x 104 S-1

(wt%) (wt%) (nm) (±1) (±1) (±0.05) (±0.05)

1.0 0.2 1.74 94 81 0.14 1.18

0.4 1.65 65 0.31 3.28

0.6 1.58 44 0.53 8.35

0.8 1.52 39 0.59 10.36

1.0 1.46 37 0.61 11.32

1.2 1.42 27 0.71 18.24

Where Cdonor is the donor concentration, Cacceptor is the acceptor

concentration, DD-A is the average donor-acceptor distance, Ido is the donor

intensity in the absence of acceptor, Id is the donor intensity in presence of

acceptor, η is the energy transfer efficiency (=do

d

II

−1 ) and Pda is the energy

transfer probability (= ⎟⎟⎠

⎞⎜⎜⎝

⎛−11 0

d

d

II

τ )

203

BIBLIOGRAPHY

1. K.Wei,D.P.Machewirt J.Wenzel, E.Snitzer, G.H.sergen:

J.Opt.Lett., 19(12)(1994)904.

2. D.H.Hewak, B.N.Samson, J.A.Mederion Neto, R.T.Laming,

D.N.Payne: Electron Lett., 30(1994)968.

3. .Tanabe,T.Hanada,M.wattnabe,T.Hayshi,N.Sogo:

J.Am.Ceram.Soc., 78(1995) 2917.

4. S.Tanabe, J.Kang, T.Hanada:J.Non Cryst.Solids, 239(1998)170.

5. B.R.Judd: Phys.Rev., 127(1962)750.

6. G.H.Dieke: Spectra and energy levels of rare earths (Wiley

Interscience, New York) 1965.

7. L.G.VanUitert, E.F.Dearborn and J.T.Rubin: J chem. Phys.,

46(1967)3551.

8. B.C.Joshi & R. Lohani, Bimal Pandey: J. non-crystalline Solids,

337(2004) 97.

9. B.C.Joshi & R. Lohani: J. Non-crystalline Solids, 189(1995)242.

10. J.C.Joshi and B.C.Joshi: J Solid state chem., 26(1978)179.

11. B.C.Joshi and M.C.Joshi & B.D.Joshi:J. Phy. Chem. Solids, 52

(1991) 939.

12. R.K. Lohani: Unpublished Ph.D. thesis submitted to Kumaun

University, (2000).

13. Y.K. Sharma, R.K. Singh and S.S.L.Surana, International

Conference on disordered systems, Goa (India) Sept. 24 (2004).

14. N.Krasutsky and H.W.Moos: Phys.Rev., B8 (1973)1010.

204

15. W.T.Carnall, P.R.Fields and K.Rajanak: J.Chem.Phys.,

49(1968)4424.

16. R.Reisfeld & Y.Eckstein: J. Non-crystalline solids, 12(1973) 357.

17. R.Reisfeld & Y.Eckstein: J.Solid state Chem., 11(1973) 261.

18. M. Eyal, R. Reisfeld, A.Schiller, C.Jacobani &

C.K.Jorgenson:Chem. Phys. Lett. , 140 (1987)595.

19. B.C.Joshi & M.C.Joshi: J. Non-crystalline Solids, 142 (1992) 171.

20. M. Shojiya, M. Takahashi, R. Kanno, Appl. Phys. Lett., 65(1994)

1874.

21. K. Kojima, S. Yoshida, H. Shiraishi: Appl. Phys. Lett. , 67(1995)

3423.

22. J. J. Ju, T. Y. Kwon, S. I. Yun: Appl. Phys. Lett., 69(1996) 1358.

23. T.Gregorkiewicz, D.T.X.Yhao, J.M.Langer: Physica Status Solidi,

210(1999)735.

24. S.M.Karstriskii, Yu N.Kortisko, V.A.Fedorov, C.Sada: Physica

status solidi, 1(11) (2004)3158.

25. R.Orbach: Proc. Royal Soc. Am., 264 (1961)458.

26. T.M.Miyakawa and D.L.Dexter: Phys.Rev., B1(1970)2961.

27. N.S.Yamada, S.Shionoya and T.Kushida: J.Phys.Soc.Japan,

32(1972)1577.

28. K.K. Pandey : Indian Journal of Pure and Applied Physics,

29(1991) 362.

29. F.K. Fong Fong & D.J.Diestler: J Chem Phys 56(1972) 2875.

30. M. Eyal, R. Reisfeld: Chem Phys Lett 40(1987) 595.

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31. B.C.Joshi & C.C.Dhondiyal: Indian J of pure & applied Phys

43(2005)918.

32. L.G. Van Uitert and L.F. Johnson: J.Chem.Phys., 44 (1966) 3514.

33. B.C.Joshi, R.Lohani: J. Non Cryst.Solids, 215(1997)103.

34. J.C.Joshi, N.C.Pandey, B.C.Joshi and Janardan Joshi: J. Non-

Crystalline Solids, 27(1978)173.

206

Figure Caption

Fig.5.1: Emission spectra of (A) Dy3+ (1.0 wt % fixed).

(B) Pr3+ (0.8 wt% fixed).

Fig.5.2: Variation of Dy3+ emission with the Pr3+ concentration.

Fig.5.3: Energy level diagram of Dy3+ and Pr3+.

Fig.5.4: Variation of Pda with the square of donor + acceptor concentration in wt%.

Fig.5.5: Absorption spectra Pr-3+ (1 wt %) in zinc phosphate glass.

Fig.5.6: Emission spectra of (A) Tm3+ (1.0 wt % fixed).

(B) Tm3+ (1.0 wt %) + Er3+ (0.8 wt% ).

Fig.5.7: Energy level diagram of Tm3+ and Er3+.

Fig.5.8: Variation of Tm3+ emission with the Er3+ concentration.

Fig.5.9: Variation of Pda with the square of donor + acceptor concentration in wt%.

Fig.5.10: Absorption spectra Er-3+ (1 wt %) in zinc phosphate glass.

Fig.5.11: The graph between log (I0/Iob-1) and log C/C*

207

Fig. 5.1 Emission spectra of (A) Dy3+ (1.0 wt %) and (B) Dy3+ (1.0wt %) + Pr3+ (1.0 wt%) in zinc phosphate glass.

208

Fig. 5.2 Emission spectra of Dy3+ with varying concentration of Pr3+ in zinc phosphate glass.

209

Dy3+ Pr3+

Fig. 5.3 Energy level diagram of Dy3+ and Pr3+.

210

Fig. 5.4 Variation of energy transfer probability, Pda, with square of (donor + acceptor) concentration.

211

Fig.5.5 Absorption spectra Pr3+ (1 wt%) in zinc phosphate glass.

212

Fig.5.6: Emission spectra of (A) Tm3+ (1.0 wt % fixed).

(B) Er3+ (0.8 wt% fixed).

213

Fig.5.7: Energy level diagram of Tm3+ and Er3+.

214

Fig.5.8: Variation of Tm3+ emission with the Er3+

concentration.

215

Fig.5.9: Variation of Pda with the square of donor + acceptor concentration in wt%.

216

Fig. 5.10 Absorption spectra of Er-3+ (1 wt %) in zinc phosphate

glass.

217

Fig.5.11: The graph between log (I0/Iob-1) and log C/C*

218

CHAPTER 6

STUDY OF EMISSION AND ABSORPTION SPECTRA OF SOME

DYES DOPED IN POLYMER AND

STUDY OF PROCESS OF ENERGY

TRANSFER FROM FLUORESCEIN TO ERYTHROSIN-B IN

POLY VINYL ALCOHOL

219

CHAPTER 6

STUDY OF EMISSION AND ABSORPTION SPECTRA OF SOME DYES DOPED IN POLYMER AND STUDY OF

PROCESS OF ENERGY TRANSFER FROM FLUORESCEIN TO ERYTHROSIN-B IN POLY VINYL

ALCOHAL

6.1 INTRODUCTION:

Luminescence is the emission of photons from electronically excited

states. Luminescence is divided into two types, depending upon the nature of the

ground state and excited states. In single excited state, the electron in the higher-

energy orbital has opposite spin orientation as the second electron in lower

orbital. These two electrons are said to be paired. In a triplet state these electrons

are unpaired i.e. there spins have the same orientation. Fluorescence is the

emission which results from the return to the lower orbital of paired electron. Such

transitions are quantum mechanically allowed and these having high emissive

rate with fluorescence lifetimes near 10-8sec or 10nsec. Phosphorescence is the

emission which results from transition between the states of different multiplicity,

generally a triple excited state returning to ground single state. Typical

phosphorescence lifetimes range from millisecond to second.

Recently, fluorescence sensors or optrodes have been found to gain

considerable interest in the direction of various analytes [1-8]. Optrodes have

advantages over conventional device because of their small size (micro sensor

220

can be fabricated), freedom from electrical interference, remote sensing etc.

Basically an optrodes consists of a fluorescent molecule embedded in a support

sensing matrix (polymers, porous glass etc.) deposited at the end of a bifurcated

optical fiber to carry the exciting and emitted radiation. For getting good response,

the support matrix should be such a nature that the fluorescence comes in

contact with the analyte in a very short time.

The xanthenes dyes are probably the most intensely studied class of

luminescent dyes, Interest has been spurred both by the special spectral

characteristics of the dyes and by their wide range of applications. Polymers

appropriately doped with dye molecule, emitting in the visible spectrum provide

stable sources of light for displays and illumination sources at a significantly lower

cost than semiconductor. Organic light emitting diodes may indeed evolve as the

most inexpensive alternatives to fluorescent light sources. Polymer fibers doped

with organic dyes have proved to be potential candidate for use in fiber lasers and

fiber amplifiers media of holographic recording and permanent optical memory,

solar energy converters etc. [9, 10].

One of the most known polymeric matrices for holographic recording is

poly vinyl alcohol (PVA), which is water soluble polymer that undergoes

crosslinking. The polymer is found with different molecular weights, which

determine its optical properties.

6.2 EXPERIMENTAL DETAILS AND MATERIALS:

221

Thin films of polyvinyl alcohol (PVA) soluble in cold water with or without

doping dyes were prepared according to the method already discussed in chapter

2.

The following samples of dye-doped polymer were prepared by doping

different dyes in polyvinyl alcohol (PVA)

1. Eosine of molar concentration 0.63 x 10-5M, 1.25 x 10-5M, 2.5 x

10-5M and 5 x 10-5M doped in PVA.

2. Saffranine T of molar concentration 1.25 x 10-5M and 5 x 10-5M

doped in PVA.

3. Erythrosin B of molar concentration 0.63 x 10-4M, 0.80 x 10-4M

and 2.5 x 10-4M in PVA.

4. Malachite green of molar concentration 2.5 x 10-5M, 1.25 x 10-5M

in PVA.

5. Fluorescein of molar concentration of 10-4M and Erythrosin B of

molar 0.31 x 10-5M, 0.63 x 10-5M, 0.80 x 10-5M, 1.0 x 10-5M and

2.5 x 10-5M doped in PVA concentration.

In addition to this reference sample of PVA was prepared without doping

any dye to study the absorption spectra of above mentioned dye-doped polymers.

The emission and absorption spectra of these samples were taken

according to the method already described in chapter 2.

6.3 THEORY:

222

The absorption and emission spectra of dye molecules show broad bands

in UV/VIS region. The molar absorptivity at a given energy is computed from the

Beer-Lambert law

ε=1/cl (log I0/I) (6.1)

where c is the molar concentration of the absorbing ion per unit volume, l is the

path length and log (I0/I) is the absorptivity or optical density .

The intensities of absorption transitions are measured in terms of Pexp

which represents the number of classical oscillators present in one ion, more

commonly referred to as the probability for absorption of radiant energy or

oscillator strength. In the case of absorption band, it is given [11] by

Pexp=fnHK

nn max

2217

)2(1031.1

+× (6.2)

where ε max is oscillator strength, n is refractive index of medium (for calculation;

n is taken as 1.50 for PVA [12], fn is conc. of ion / cm3, maxK is absorption

coefficient in cm-1 at the peak and H is half width of the band in eV.

Excitation by light may initiate both intramolecular and intermolecular

transformation of a molecule. The most conspicuous intermolecular events are

electron, proton and energy transfer [13]. Fluorescence resonance energy

transfer (FRET) is a photophysical process and occur via intermolecular energy

transfer mechanism where energy that is absorbed by fluorescent molecule

(donor) is transferred non-radiatively to second fluorescent molecule (acceptor).

Forster energy transfer occurs for the very weak range of dipole-dipole interaction

energies (10-1-101 cm-1) and has a rate range of 106-1011 S-1. Forster [13] was the

223

first to consider theoretically the long range dipole-dipole interaction between two

dissimilar molecules leading to non-radiative transfer from initially excited (donor)

to an unexcited (acceptor). According to Forster’s theory, the rate of energy

transfer depends upon the following factors [1, 14-15]

1. The extent of spectral overlap between donor emission and acceptor

absorption.

2. The quantum yield of the donor ( Dφ ).

3. The relative orientation of the donor and acceptor transition dipoles. And

4. The distance between the donor and acceptor transition dipole.

Forster showed that the rate of energy transfer between donor (D) and

acceptor (A) is

6

12

23

⎟⎠⎞

⎜⎝⎛= −

RR

Kk OADDA τ (6.3)

where R is the distance between donor and acceptor, K2 is the orientation factor

(for random orientation K2 is 2/3), Dτ is the donor decay time in the absence of

the acceptor and ROA is the critical transfer distance.

The critical transfer distance ROA [16-18]is given by

4

256

)()1086.5(

nR DDA

OAΦΩ×

=−

(6.4)

where DAΩ is the overlap integral for overlapping fluorescence spectra

of donor, DΦ is quantum yield of standard donor, n is refractive index of medium.

224

DAΩ has been calculated by using the formula [16]

νν

νννεν

df

df

d

ad

DA

)(

/)()(

0

4

0

∫

∫∞

∞

=Ω (6.5)

where ννεν df ad )()(0∫∞

is area of overlapping region. ν is average

wavenumber of the overlapping region.

Reduced concentration for donor-acceptor overlapping is calculated by

using [19]

OA

ADA C

C=γ (6.6)

where AC is acceptor concentration and OAC is critical acceptor

concentration. Critical acceptor concentration can be calculated by the relation

[19]

32/3 )(23000

OAOA RN

Cπ

= (6.7)

where N is Avagadro’s number.

Similarly critical transfer distance, overlap integral and reduced

concentration for self overlap have been calculated as:

4

256

)()1086.5(

nR DDD

ODΦΩ×

=−

(6.8)

225

νν

νννεν

df

df

d

Dd

DD

)(

/)()(

0

4

0

∫

∫∞

∞

=Ω (6.9)

OD

DDD C

C=γ (6.10)

6.4 MOLECULAR STRUCTURE OF DYES:

Fluorescein

Erythrosin B

226

Eosine

Malachite green

227

228

6.5 RESULTS AND DISCUSSION:

6.5.1 Emission and absorption spectra of different dyes doped in

polymers.

6.5.1(A) Saffranine T doped in polyvinyl alcohol: The emission and absorption

spectra of saffranine T (molar conc.= 5.0 x 10-5 M) is shown in Fig. 6.1 and Fig.

6.2. When excited by 500 nm wavelength saffranine T shows the peak at 562 nm

while the same sample shows absorption peak at 537 nm. Fig. 6.3 and Fig. 6.4

shows the emission and absorption spectra of saffranine T with two different

molar conc.. It shows a slight shift in maximum emission peak with concentration.

6.5.1(B) Erythrosin B doped in polyvinyl alcohol: The emission and absorption

spectra of erythrosin B (molar conc. = 2.5 x 10-4 M) is shown in Fig. 6.5 and Fig.

6.6. When excited by 535 nm wavelength erythrosin B shows the peak at 566 nm

while the same sample shows absorption peak at 531 nm. Fig. 6.7 and Fig. 6.8

show the emission and absorption spectra of erythrosin B with two different molar

concentrations. It shows a slight shift in maximum emission peak with

concentration.

6.5.1(C) Eosine doped in polyvinyl alcohol: The emission and absorption

spectra of eosine (molar conc. = 5 x 10-4 M) is shown in Fig. 6.9 and Fig. 6.10.

Fig. 6.11 shows the emission spectrum for different excitation, which gives

maximum emission intensity for the 470 nm excitation. When the eosine (molar

conc. = 5 x 10-4 M) is excited by 470nm wavelength of radiation, it shows the

emission peak at 546 nm. Fig. 6.12 and Fig. 6.13 show the emission and

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absorption spectra of eosine with different molar concentration. It shows a slight

shift in maximum emission peak with concentration. The sample shows

absorption peak at 519 nm.

6.5.1(D) Malachite green doped in polyvinyl alcohol: The excitation spectra of

malachite green (molar conc. = 2.5x10-5 M) is shown in Fig.6.14. The emission

spectrum of the malachite green (molar conc. = 2.5x10-5 M) for different excitation

is shown in Fig.6.15, which shows that the emission intensity is maximum for the

excitation wavelength 590 nm. The sample shows the emission peak at 656 nm

when excited by wavelength 590 nm and the absorption peak comes out at

630nm. The emission spectra of malachite green for two different molar

concentrations are shown in Fig. 6.16.

6.5.2 Study of energy transfer from fluorescein to erythrosin B in Poly

vinyl alcohol (PVA).

The fluorescein is taken as donor and erythrosin B as acceptor. These

two are taken as so because the extent of overlap between the fluorescence

spectra of fluorescein and absorption spectra of erythrosin B have large zone.

The quantum yield ( Dφ ) of fluorescein is 0.91[20]. The orientation of donor and

acceptor transitions dipole in PVA is assumed random. So orientation factor is

taken 2/3.

Fig.6.24 represents the emission spectra of fluorescein and erythrosin B

in which the fluorescein concentration (10-4 M) is kept fixed while the erythrosin

concentration is varied from 0.31 x 10-5 M to 2.50 x 10-5M. Fig.6.24 (A) represent

the fluorescein (10-4M) emission, it shows emission peaks at 522 nm when

230

excited by 450 nm of visible radiation. Erythrosin B is also excited by 450 nm of

radiation and it shows emission peaks at 570 nm which is shown in Fig.6.24 (G).

Fig.6.24 [B, C, D, E, F] shows emission spectra of fluorescein in presence of

erythrosin B concentration varied from 0.31 x 10-5 M to 2.50 x 10-5M. From the

emission spectrum it is observed that the fluorescein emission intensity is

continuously decreases as erythrosin B concentration increases. So there is non-

radiative energy transfer energy transfer from fluorescein to erythrosin B. The

shift in emission peak of fluorescein is observed from 522 nm to 512 nm, while

the shift in emission peak of erythrosin B is observed from 566 nm to 550 nm. The

relative emission of fluorescein-erythrosin B series in PVA, when excited by 450

nm of visible radiation is presented in table 6.3. There is shift of fluorescein

(donor) emission peak to the shorter wavelength with increase concentration of

erythrosin B (acceptor), so the energy transfer appears as radiative also.

Different parameters such as overlap integral, critical concentration and

reduce concentration are calculated for the fluorescein- erythrosin B and are

presented in table 6.4.

6.6 CONCLUDING REMARKS:

In this chapter the study of emission and absorption spectra and energy

transfer in dye-dye system have been made. Saffranin T, Erythrosin B, Eosine

and malachite green doped in PVA have been used for the study of absorption

and emission spectra. Various parameters from emission and absorption spectra

have been computed. The energy transfer from fluorescein to erythrosin B have

been studied from the measurement of steady state emission. Overlap integral,

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reduce concentration and critical transfer distances is calculated for the

fluorescein- erythrosin B series in PVA.

Table 6.1: Spectroscopic parameters of investigated dyes in PVA from

absorption spectra of single dye doped in PVA.

Dye Molar con. (λmax) ( ν∆ )1/2 maxε

(M) (nm) (nm) (lM-1cm-1)

Saffranine T 5 x 10-5 539 58 1.18x106

1.25 x 10-5 539 52 1.18 x106

Erythrosin B 2.5 x 10-5 531 32 1.03 x105

0.8 x 10-5 538 30 0.84 x105

0.63 x 10-5 538 29 0.57 x105

Eosine 5 x 10-4 519 34 1.29 x105

2.5 x 10-4 519 34 1.75 x105

1.25 x 10-4 519 30 1.94 x105

Malachite green 5 x 10-5 630 54 0.08 x106

1.25 x 10-5 630 62 0.23 x106

232

233

Table 6.2: Spectroscopic parameters of investigated dyes in PVA from

emission spectra of single dye doped in PVA.

Dye Molar con. (λexc.) (λmax) ν ( ν∆ )1/2 stoke shift

(M) (nm) (nm) (cm-1) (nm) (cm-1)

Saffrine T 5 x 10-5M 500 562 17790 33 759.28

Erythrosin B 2.5 x 10-4M 535 566 17670 32 1164.55

0.8 x 10-4M 535 560 17860 15 730.22

0.63 x 10-4M 535 560 17860 25 730.22

Eosine 5 x 10-4M 470 542 18450 47 817.64

2.5 x 10-4M 470 548 18590 47 1019.65

1.25 x 10-4M 470 532 18800 44 470.83

Malachite green

5 x 10-5M 590 660 15150 50 721.50

1.25 x 10-5M 590 656 15240 54 629.11

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Table6.3: Relative emission intensity of fluorescein and erythrosin B in

fluorescein- erythrosin series in PVA, when excited by 450 nm of

visible radiation.

Donor Conc. (M/l) Acceptor Conc. (M/l) Emission Peaks Emission

Intensity (Fluorescein) (Erythrosin B) (nm) (arb.units)

10-4 ------------ 522, ---- 3876, -----

10-4 0.31 x 10-5 516, 550 3201, 2511

10-4 0.63 x 10-5 514, 554 2698, 2550

10-4 0.80 x 10-5 514, 556 2494, 2573

10-4 1.00 x 10-5 514, 562 2404, 2628

10-4 1.25 x 10-5 512, 566 2264, 2686

----- 2.50 x 10-5 ---, 566 ------, 2194

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Table6.4: Calculated values of spectroscopic parameter for fluorescein

-erythrocene B system in PVA.

Host Polymers Molar Conc. DAΩ OAR OAγ

(M/l) (10-13 cm6 M-1)(nm) (10-3)

PVA 0.31 x 10-4 27.97 8.15 37.59

0.63 x 10-4 38.38 8.59 89.45

0.80 x 10-4 50.26 8.99 130.04

1.00 x 10-4 58.73 9.23 175.72

2.50 x 10-4 66.72 9.42 468.24

236

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238

FIGURE CAPTION

Fig 6.1: Emission spectra of Saffranine T (5 x 10-5M) in PVA.

Fig 6.2: Absorption spectra of Saffranine T (5 x 10-5M) in PVA.

Fig 6.3: Emission spectra of A)Saffranine T (5 x 10-5M) ,B)Saffranine T

(1.25 x 10-5M) in PVA.

Fig 6.4: Absorption spectra of A)Saffranine T (5 x 10-5M), B)Saffranine

T (1.25 x 10-5M) in PVA.

Fig 6.5: Emission spectra of Erythrosin B (2.5 x 10-4M) in PVA.

Fig 6.6: Absorption spectra of Erythrosin B (2.5 x 10-4M) in PVA.

Fig 6.7: Emission spectra of Erythrosin B A) 2.5 x 10-4M,

B) 0.80 x 10-4M, C) 0.63 x 10-4M in PVA.

Fig 6.8: Absorption spectra of Erythrosin B A) 2.5 x 10-4M,

B) 0.80 x 10-4M,C) 0.63 x 10-4M in PVA.

Fig 6.9: Emission spectra of Eosine (5 x 10-4M) in PVA.

Fig 6.10: Absorption spectra of Eosine (5 x 10-4M) in PVA.

Fig 6.11: Emission spectra of Eosine (5 x 10-4M) for different excitation

wavelength in PVA.

Fig 6.12: Emission spectra of Eosine A) 5 x 10-4M, B) 2. 5 x 10-4M,

C) 1.25 x 10-4M in PVA.

Fig 6.13: Absorption spectra of Eosine A) 5 x 10-4M, B) 2. 5 x 10-4M,

C) 1.25 x 10-4M in PVA.

Fig 6.14: Excitation spectra of Malachite green (1. 2 5 x 10-4M) in PVA.

Fig.6.15: Emission spectra of Malachite green (1.25 x 10-4M) in PVA.

239

(A) λ excitation-590 nm

(B) λ excitation-580 nm

(C) λ excitation-560 nm

Fig 6.16: Emission spectra of Malachite green A) 5 x 10-4M, B) 1.25 x

10-4M in PVA.

Fig 6.17: Emission spectra of Fluorescein (5 x 10-4M) in PVA.

Fig 6.18: Absorption spectra of Fluorescein (5 x 10-4M) in PVA.

Fig 6.19: Overlap between Fluorescein (10-4M) and Erythrosin B (2.5 x

10-5M) in PVA.

Fig 6.20: Overlap between Fluorescein (10-4M) and Erythrosin B (1.0 x

10-5M) in PVA.

Fig 6.21: Overlap between Fluorescein (10-4M) and Erythrosin B (0.80

x 10-4M) in PVA.

Fig 6.22: Overlap between Fluorescein (10-4M) and Erythrosin B

(0.63 x 10-5M) in PVA.

Fig 6.23: Overlap between Fluorescein (10-4M) and Erythrosin B

(0.31 x 10-5M) in PVA.

Fig 6.24: Variation of fluorescence intensities of fluorescein (donor)-

erythrosin B (acceptor) system in PVA at fluorescein

concentration (A) 10 different erythrosin B concentration (B) 0.31

x 10-5M (C) 0.63 x 10-5M (D) 0.80 x 10-5M (E) 1.00 x 10-5M (F)

1.25 x 10-5M (G) 2.5 x 10-5M (Alone)

240

Fig 6.1: Emission spectra of Saffranine T (5 x 10-5M).

Fig 6.2: Absorption spectra of Saffranine T (5 x 10-5M).

241

Fig 6.3: Emission spectra of A)Saffranine T (5 x 10-5M) ,

B)Saffranine T (1.25 x 10-5).

Fig 6.4: Absorption spectra of A)Saffranine T (5 x 10-5M),

B)Saffranine T (1.25 x 10-5M).

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Fig 6.5: Emission spectra of Erythrosin B (2.5 x 10-4M).

Fig 6.6: Absorption spectra of Erythrosin B (2.5 x 10-4M).

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Fig 6.7: Emission spectra of Erythrosin B A) 2.5 x 10-4M, B) 0.80 x 10-4M,

C) 0.63 x 10-4M

Fig 6.8: Absorption spectra of Erythrosin B A) 2.5 x 10-4M, B) 0.80 x 10-4M,

C) 0.63 x 10-4M

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Fig 6.9: Emission spectra of Eosine (5 x 10-4M).

Fig 6.10: Absorption spectra of Eosine (5 x 10-4M).

245

Fig 6.11: Emission spectra of Eosine (5 x 10-4M) for different excitation

wavelength.

246

Fig 6.12: Emission spectra of Eosine A) 5 x 10-4M, B) 2. 5 x 10-4M

C) 1.25 x 10-4M.

Fig 6.13: Absorption spectra of Eosine A) 5 x 10-4M, B) 2. 5 x 10-4M,

C) 1.25 x 10-4M.

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Fig 6.14: Excitation spectra of Malachite green (1. 2 5 x 10-4M) for λemission=500nm.

248

Fig.6.15: Emission spectra of Malachite green (1.25 x 10-4M)

(D) λ excitation-590 nm (E) λ excitation-580 nm (F) λ excitation-560 nm

249

Fig 6.16: Emission spectra of Malachite green A) 5 x 10-4M, B) 1.25 x 10-4M.

250

Fig 6.17: Emission spectra of Fluorescein (10-4M).

Fig 6.18: Absorption spectra of Fluorescein (10-4M).

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Fig 6.19: Overlap between Fluorescein (10-4M) and Erythrosin B (1.25 x 10-5M).

Fig 6.20: Overlap between Fluorescein (10-4M) and Erythrosin B (1.0 x 10-5M).

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Fig 6.21: Overlap between Fluorescein (10-4M) and Erythrosin

B (0.80 x 10-5M).

Fig 6.22: Overlap between Fluorescein (10-4M) and Erythrosin

B (0.63 x 10-5M).

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Fig 6.23: Overlap between Fluorescein (10-4M) and Erythrosin B (0.31 x 10-5M).

254

Fig 6.24: Variation of fluorescence intensities of fluorescein (donor)-erythrosin B (acceptor) system in PVA at fluorescein concentration (A) 10 different erythrosin B concentration (B) 0.31 x 10-5M (C) 0.63 x 10-5M (D) 0.80 x 10-5M (E) 1.00 x 10-5M (F) 1.25 x 10-5M (G) 2.5 x 10-5M (Erythrosin B alone)

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