VIBRATIONAL SPECTROSCOPY: REFLECTION,

ABSORPTION INFRA-RED SPECTROSCOPY (RAIRS). AN

OVERVIEW

By

Abubakar Yakubu (Research Group Leader)

Ammar Abd Ali, Ali Mohammed, Aini Zafar

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What is Spectroscopy

Historically, spectroscopy originated through the study of

visible light dispersed according to its wavelength, by a

prism

Spectroscopy is probing the unknown so as to understand

phenomena that are not visible to the ordinary eyes or any

interaction with radiative energy as a function of its

wavelength.

Probing in most prominent characterisation techniques are

achieved by using photons, electrons, and neutrons.

The probing energy range for photons is 100-500Mev,

neutrons is 1-300MeV and electrons is 1-10eV

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What is RAIRS

The acronym RAIRS means; Reflection Absorption Infra-red Spectroscopy.

Provides the most definitive means of identifying the surfacespecies generated upon molecular adsorption and the speciesgenerated by surface reactions.

Used to obtain vibrational data from solid state or gas phasesamples

• Excellent energy resolution (<2 cm-1) - useful for separating

multiple peaks, phase transitions, lateral interactions, and

dynamics of coupling.

• Straight forward instrumentation

• Not restricted to surfaces in vacuum - can be used in "real

world" conditions12/23/2014 3

Best sensitivity for observing an adsorption

feature in RAIRS

• - p-polarized light

• - grazing incidence

• - molecule with transition dipole arranged along

surface normal (Selection rule)

• - molecule with large transition moment

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Principle of RAIRS Technique

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Infra-Red

Radiation

Frequency

1011 - 3.8x1014 Hz

Wavelength

3x10-3 - 8x10-7 m

How it works

Infrared light induces vibrational transitions in molecular bonds, and by measuring

the frequency and intensity of the absorbed infrared light, information such as

chemical environment, structure, and functional group identity can be elucidated.

Infrared radiation is directed from the source through a series of gold coated mirrors

towards the surface of material under test.

The impinging photons interact with the surface molecules and reflect from the

crystal substrate.

The reflected beam are directed and focused into the detector through series of gold

coated mirrors.

The raw signal is then Fourier transformed into the familiar frequency domain.

The mathematical basis of reflection infrared spectroscopy was developed by

Greenler in the 1960’s.

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Typical RAIRS Components

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Spectrometer

IR filter

Reflecting

mirrors

Polarizer

Chamber

Detector

Application

To study type of bonds present

To study surface-bound species and reactions

Adsorption

Reflection

Transmittance

Phase transition

Lateral interactions

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Theory

In transmission Mode

𝐼 = 𝐼𝑜𝑒𝑘𝑐𝑙

𝑇 = 𝑒𝑘𝑐𝑙 = Transmittance

Ln(T) = kcl

A= 𝜀𝑐𝑙 = 𝐴𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒

where k = absorption coefficient

𝜀 = absorptivity

c = surface concentration

l = path length

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n1 n2

For Reflectivity

Maxwell’s equation is used for analyzing the interface of the

different refractive index (n).

From Snell’s law, with reference to the diagram, if ∅𝐼 = ∅𝑅 ,𝑛1𝑛2

=𝑠𝑖𝑛∅𝐼

𝑠𝑖𝑛∅𝑇

If ∅𝐼 < ∅𝑐𝑟𝑖𝑡𝑖𝑐𝑎𝑙 ,

𝑠𝑖𝑛−1𝑛2𝑛1

= 𝑐𝑟𝑖𝑡𝑎𝑙 𝑎𝑛𝑔𝑙𝑒

The intensity of the reflected wave are given by Fresnel's Equation

If 𝑛1 = 1 𝑎𝑛𝑑 𝑛2 = 𝑛 + 𝑖𝑘

Then for P-polarized light,

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Then for P-polarized light, (//)

𝑅𝑝 =𝑐𝑜𝑠2∅𝑇 − 2𝑛𝑐𝑜𝑠∅𝐼𝑐𝑜𝑠∅𝑇 + 𝑛2 + 𝑘2 𝐶𝑜𝑠2∅𝐼

𝑐𝑜𝑠2∅𝑇 + 2𝑛𝑐𝑜𝑠∅𝐼𝑐𝑜𝑠∅𝑇 + 𝑛2 + 𝑘2 𝐶𝑜𝑠2∅𝐼

For S- polarized light, (┴)

𝑅𝑠 =𝑐𝑜𝑠2∅𝐼 − 2𝑛𝑐𝑜𝑠∅𝐼𝑐𝑜𝑠∅𝑇 + 𝑛2 + 𝑘2 𝐶𝑜𝑠2∅𝑇

𝑐𝑜𝑠2∅𝐼 + 2𝑛𝑐𝑜𝑠∅𝐼𝑐𝑜𝑠∅𝑇 + 𝑛2 + 𝑘2 𝐶𝑜𝑠2∅𝑇

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The Selection Rule

The observation of vibrational modes of

adsorbates on metallic substrates is subject to

the surface dipole selection rule.

The rule states that only those vibrational

modes which give rise to an oscillating dipole

perpendicular to the surface are IR active and

give rise to an observable absorption band.

Grazing Incidence

The best sensitivity for IR measurements on

metallic surfaces is obtained using a grazing-

incidence reflection of the IR light

only those vibrations giving rise to a dipole

change normal to the surface can be observed.

Major Principles of RAIRS

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P-polarized Light

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Light in a well-defined polarization state, either

parallel (p-polarized) or perpendicular (s-

polarized) to the plane of incidence, impinges onto

the surface at a well defined and controlled angle

of incidence.

The reflected light is detected at an angle equal to

the angle of incidence.

The polarization state of the beam and the surface

pressure sample are computer-controlled and

adjusted as desired.

s-polarized light almost cancelled by reflection at

grazing incidence

p-polarized light almost doubled by reflection at

grazing incidence

Only p-polarized component light can be reflected

from surface at high incidence angles.

s and p|| polarized light almost

canceled

p┴ polarized light almost

doubled

This means that reflected

intensity is proportional to Ep┴2

and 1/cosθ

Examples of RAIRS Spectral

n(N-O) spectra obtained

from a Pt surface

subjected to a fixed

exposure of NO at various

temperatures

The surface coverage of

adsorbed NO molecules

decreases as the

temperature is raised and

little NO remains

adsorbed at temperatures

of 450 K and above.

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Examples

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(A) Adsorption at 23 Kshowing both monolayerchemisorbed CO peak andmultilayered physisorbedCO.

(B) Effect of warming to 26 Kto desorb multilayers,leaving a singlephysisorbed monolayer.

(C) Effect of warming to 35 Kand recooling to 23 K.

(D) Surface described in Cfollowing furtheradsorption of CO at 23 K.128 co-added scans.

RAIRS spectra of CO/Cu(100)

Limitations

Successful finger printing

techniques for complex

functional groups at surfaces

Molecular state, orientation and

binding site accessible

Excellent RAIRS resolution

reveals even small shifts due to

phase transitions, lateral

interactions or dynamics of

coupling

RAIRS may operate at ambient

pressure (catalysis!).

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the signal is usually veryweak owing to the smallnumber of absorbingmolecules

Only dipole active modesvisible in RAIRS/dipole-scattering

Coverage quantitationdifficult

Low-energy frustratedmodes not accessible toRAIRS

RAIRS requires strongdynamic dipole moment(almost exclusively appliedto CO adsorption)

Advantages

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Conclusion

Photons is the source of probe in RAIRS analysis and the probe

energy is low in RAIRS instrumentation.

RAIRS is primarly used to study type of bonds, surface-bound

species and reactions, adsorption, reflection, transmittance, phase

transition, lateral interactions.

RAIRS have good energy resolution (<2 cm-1) which is useful for

separating multiple peaks.

RAIRS is a straight forward instrumentation which is not restricted

to surfaces in vacuum.

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Bibliography

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spectroscopy: principles and applications to lipid–protein interaction in Langmuir films.

Biochimica et Biophysica Acta (BBA)-Biomembranes, 1798(4), 788-800.

2. http://www-jenkins.ch.cam.ac.uk/, Reflection Absorption Infra-Red Spectroscopy

(RAIRS), Chemistry Department University of Cambridge, Uk

3. Mendelsohn, R., & Flach, C. R. (2002). Infrared Reflection–Absorption

Spectrometry of Monolayer Films at the Air–Water Interface. Handbook of vibrational

spectroscopy.

4. Greenler, R. G. (1975). Design of a reflection–absorption experiment for studying the

ir spectrum of molecules adsorbed on a metal surface. Journal of Vacuum Science and

Technology, 12(6), 1410-1417.

5. Hoffmann, F. M. (1983). Infrared reflection-absorption spectroscopy of adsorbed

molecules. Surface Science Reports, 3(2), 107-192.

6. Primera-Pedrozo, O. M., Soto-Feliciano, Y. M., Pacheco-Londoño, L. C., &

Hernández-Rivera, S. P. (2009). Detection of high explosives using reflection

absorption infrared spectroscopy with fiber coupled grazing angle probe/FTIR.Sensing

and Imaging: An International Journal, 10(1-2), 1-13.

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