Vibrational spectroscopy

• Chemical composition: finger print• Bonding orientation: adsorption structure on surfaces

Infrared Spectroscopy (IR)

High Resolution Electron Energy Loss Spectroscopy (HREELS)

Surface Enhanced Raman Spectroscopy (SERS)

Second Harmonic Generation (SHG)

Photo-acoustic Spectroscopy (PAS)

Inelastic electron tunneling Spectroscopy (IETS)

Inelastic Neutron Scattering (INS)

Surface Infrared spectroscopy

Refs: Y.J. Chabal, Surf. Sci. Rep. 8, 211 (1988)

F.M. Hoffman, Surf. Sci. Rep. 3, 107 (1983)

Transmission IR Spectroscopy

-supported metal cataysts

- IR transparent samples (Si)

Diffuse Reflectance Infrared Fourier Transform

Spectroscopy (DRIFTS)

-refocus the diffusively scattered IR beam

-high surface area catalytic samples

-low surface area single crystals

Reflection-Absorption IR Spectroscopy ( RAIRS )

-specular reflected IR beam

-single crystal samples

Multiple Internal Reflection Spectroscopy ( MIR ) or

Attenuated Total Reflection (ATR)

-total internal reflection

-SAM , polymer films

Background

Transmission and absorption mode

Transmittance T = I/I0 = exp(kcl)

Absorbance A = cl

k: absorption coefficient; : absorptivity

c : concentration; l : cell thickness

Imaginary part of refractive index n = k

n = n + ik for absorbing medium

n = n for dielectric non-absorbing medium

-needs to take reference and sample spectra

-not popular for surface studies due to the large bulk contribution

-

I0

I-+

Reflection

The reflection angles

Snell’s law

n1/n2 = sini/sint

Crtical angle: c= sin-1(n2/n1)

Intenstiy of the reflected light

- Depend on polarizations

Fresnel’s equations

n = n + ik

s-polarized light : || the plane of incidence

Rs = [(n-sec)2+k2]/ [(n+sec)2+k2]

p-polarized light : ㅗ the plane of incidence

Rp = [(n-cos)2+k2]/ [(n+cos)2+k2]

- i must be large: grazing incidence for thin fi

lms on reflective surface

Ep

xEs

i r

t

x

the plane of incidence

Phase shift , electric field, intensity of p-polarized light as a function of incidence angle from a metal surface

0 incidence angle 90

Ph

ase shift o

n refelctio

ns

0

p-pol||

s-pol ㅗ

0 incidence angle 90

Su

rface inten

sity fu

nctio

n

(E/E

o)2sec

n =3, k=30

20

40

60

Su

rface electric field

E/E

0

s-polarized light at the surface - uniform phase shift - vanishing E field at the surfacep-polarized light at the surface - dependent on incidence angle - strong E field at large incidence angle, ie, grazing incidence

Absorbance is proportional to

E2 and area of surface as 1/cos I ~ E2/cos

Adsorbate covered surface

Dielectric constant e = (n+ik)2

Vcauum 1n1

Adsorbate 2(n2, k2)

Metal 3(n3, k3)

Ro R

RRo

Absorption function A = (R- R0) /R0 = R/R

3 >> 2~1, d<<

Rs/Rs = (8dcos/)Im((2– 3)/(1-2))

Rp/Rp = (8dcos/ )Im([(2– 3)(1-(1/ 2 3)(2+ 3)sin2

(1-2)(1/3) )(+ 3)sin2

Reflectivity change of s-polarized light is negligibly small

Assuming 3 >> 2 and cos > 3-1

Rp/Rp = (8dsin2/lcos)Im(-1/2)a large reflectivity change at high incidence angle

d

Surface selection rule

-The electric field of light and the molecule interact with surface electrons-The incident light must be p-polarized-Only vibrations with a dipole moment perpendicular to the surface-The incident light should be reflected at grazing incidence

+ -

- +

+

-

-

+

M M

imageimage

IR inactive IR active

fi = <f||i> 0, d/dr 0

- for lying down molecules, molecular and image dipoles are cancelled out- for upright molecules, molecular and image dipoles are enhanced

Surface IR spectra of adsorbed moleculesIdentification of adsorbate: high resolution : 2-4 cm-1

Orientation of adsorbed molecule by surface dipole selection rule

How to confirm the metal-adsorbate bond ?

- frequency shift of internal modes compared to gas-phase spectra

- additional metal-molecule vibration: <800 cm-1

Frequency shift of internal and external modes for adsorbed layers

- weakening of metal-molecule bond: decreases as coordination of surface atoms

increases

- formation of adsorbate islands

- compression structures

R/R: 0.1~10-3 often small: sufficient for submonolayer sensitivity for

molecule with strong dynamic dipole moment

R/R roughly linear with coverage, but not a good indicator of population

Peak width and intensity

homogeneous broadening

- coupling to phonon

- electron-hole creation

inhomogeneous broadening

- inhomogeneous distribution of harmonic oscillator

- intermolecular interaction

energy transport between molecule and surface

dipole-surface interaction: dynamic dipole interaction

Instrumentation: RAIRS

J.E. Reutt-Robey et al, JCP 93, 9113 (1990)

Instrumentation: MIR IR

IR finger print

Modes of vibration

IR spectra of CO on Pd(100)

Lower frequency shift compared to that of gas phase ? - Interaction with the vibrating dipole with the image dipole\ - Chemical effect due to backdonation, which change the CO bond strengthHigher frequency shift as coverage - vibrational coupling : dipole-dipole, dipole-metal electrons - chemical effect: reduced backdonation into antibonding orbitals - electrostatic effect due to charge transfer between the metal and moelcule - intermolecular repulsion

-threefold:site : 1800~1900 cm-1

-bridge site: 1900~2000 cm-1

-on top site: 2000~2100 cm-1

IRRAS spectra

CO on Pd(111)

Diffuse reflectance IR spectra

High Resolution Electron Energy Loss Spectroscopy

- Inelastic scattering of low energy electron beam- Energy loss due to the vibrational excitation - observe vib. modes parallel and perpendicular to the surface- Lower resolution 3meV (=24 cm-1 )(compare with IR ~2 cm-1) - Submonolayer sensitivity- can observe surface-atom vib. freq. <800 cm-1

+

-

EoE

Eo E

Eo-E = hvI

v

Scattering mechanism Dipole scatteringImpact scatteringResonance scatteringDipole scattering

- electrons interact with the long range field at surface- electron momentum perpendicular to the surface normal is condserved- forward scattering by adsorbate- peaked in the specular position elastic electrons: specular inelastic electrons: near specular- vibration perpendicular to the surface normal can be excited- larger cross section for smaller Eo(~5 eV)

EoE

g||

ki kf

||

ㅗ

g ||

ㅗki

+ -

- +

+

-

-

+

M M

imageimage

Impact scattering

- short range interaction(~ a few A) of electron with atomic core potential of surface- strong multiple scattering- Isotropic angular distributions- scattering probability depends on surface dipole amplitude and electron energy- favored by high incident electron energy > 50 eV- off specular angle- lower scattering cross section the the dipole scattering

Negative ion resonance scattering

- short range interaction- electron trapped in empty Rydberg state of adsorbate- temporary negative ion- enhancement of vib. Intensity over relatively narrow range of Ei- very small cross- section off resonance- molecular orientation on surface

Peak positions for different adsorption states

Instrumentation

Examples: CO on W(100)

565 cm-1 ; W-C stretching

630 cm-1 : W-O stretching

363 cm-1 W-CO (on top)

2081 cm-1 CO stretching

CO(g): 2140 cm-1

Interaction ions with solid

Evac

EF

Ei

Auger neutralization

Resonance ionization

Resonance neutralization

Quasi-resonance neutralization

- Charge transfer: neutralization of ion and electronic excitation- Kinetic energy transfer: sputtering, scattering

e

Atomic and nuclear collision

Impact parameter (b) scattering process energy transfer (Tc)

~1 A inelastic excitation 10eV of valence electrons

~10-1 A inelastic excitation 100eV of L-shell electrons

~10-2 A inelastic excitation 1 keV of K-shell electrons

~ 10-4 A elastic scattering ~100keV from nuclei

Ion scattering spectroscopy

Low energy ion scattering (LEIS): 0.5 ~ 3 keVMedium energy ion scattering (MEIS): 10~500 keVHigh energy ion scattering (HEIS) orRutherford backscattering spectroscopy (RBS): 0.5 ~5 MeV

Binary elastic collisionKinematic factor K= E1/Eo

E1/Eo = [((M22

– M12)sin)1/2+M1cos) /(M2+M1)]2

M1,M2 : mass of incident atom and target= scattering angle

Ion trajectroy

Blocking, shadowing, and channeling effect

- scattering cross section at a certain angle depend on atomic potentials of incident and substrate atoms-scattering depend on incident angle and impact parameter-lower ion energy, larger shadow cone

Scattering cross section

2bdb = s() 2sind() = b(db/d )/sin = # of scattered paricles into d/total # of incident particles

Rutherford formulad /d = [Z1 Z2e2/4Ecsinc/2]2 Ec = [M2/(M1+M2)]Eo

db

b

d

Quantitative analysis

Total # of particles of impurity mas M3, atomic number Z3, surfacedensity N3(atoms/cm2)The measured yield Y3

Y3 = N3 (d /d) QQ: measured # of incident particles : solid angle accepted by detector- N3 can be determined typically with an accuracy better than 10%

Stopping power and depth resolution

Electronic stoppingduring going in

Elastic scattering

Electronic stopping during going out

Final Energy of a particle at normal incidenceE1 = Eo – Ein – Es - Eout

-the rate of energy loss dE/dx depends on mass of projectiles, traget, and incident energy-for 0.5~2.0 MeV, dE/dx is independent of energy-Depth resolution: 30~100 Å

Energy spectrum

Channeling and blocking

Surface peaks

Energy distribution of sputtered species

Sputtering yield: ion energy dependence

Sputtering yield: dependence on element

Sputtering yield: angle dependence

-varies 1/cos-Drop at grazing incidence angle

Secondary Ion Mass Spectrometry (SIMS)

Ion beam

S+

-Sensitive to top most layers-Chemical composition-Structural informations-Very high sensitivity-Imaging: 100~1000nm-Depth profiling: 5nm-Ion yield depends on surface concentration and sputtering yield-Organic anlaysis: m/z = 5000~40,000-Matrix effect: secondary ionization mechanism-Destructive: implantation, mixing, sputtering, ion beam induced surface chemistry, radiation induced atomic redistribution

Mass

detect sputtered species (neutrals, ions)from the sample

S

SIMS modes

-Static SIMS - low sputter rate ~1nA/cm2

<10 Å/hr - nondestructive- Submonolayer analysis

-Dynamic SIMS - high sputter rate ~10 mA/cm2

~100 m/hr - destructive- Depth profiling

1nA/cm2

=10-9A/cm2/1.6x10-19 C= 6.3x109 ions/sec-cm2

= 6.3x109 ions/sec-cm2

1015atoms/cm2

= 1.6x10-5 ML

InstrumentationIonization methods: -electron impact - microwave-field ionization-laser ablation

Ion sourceAr+ ionO2

+: for electropositive elementsCs+: for electronegative elementsLiquid metal: Ga+, In+

- small beam size

Mass spectrometer

Quadrupole-inexpensive, compactDouble focusing electrostatic/magnetic sector-high transmission-High mass resolutionTime of flight-high molecular weight

From Jeol

Example

Imaging SIMS

-scan ion beam or ion detector-Beam size <10nm-Resolution ~100mm

Thermal desorption spectroscopyTemperature programmed desorpion

-measure desorbing molecules by heating the surface using mass spectrometer

Quadrupole mass spectrometer

heater

Adsorbed molecules

-Heat of adsorption if Eads =Edes-Surface coverage: peak area-Adsorption sites: peak position-Intermolecular interaction-Kinetics of desorption : peak shape

Analysis of TPD

Redhead, Vacuum 12, 203 (1963)The rate of desorptionrd = -d/dt = kon exp(-Ed/kT) n: order of reaction ko : prexponential factor : coverage Ed: activation barrier for desorptionThe sample temperature varies linearly T(t) = T0 + t = dT/dt : heating rate[K/s] rd = -d/dT = (1/)kon exp(-Ed/kT)

coverage kd=k0eEa/kT

TPD spectra

TemperatureIn

ten

sit

y

Ea = 24kcal/mol= 10 K/secn=1ko=1013 1/sec

Ed,ko’ Desorption temperatureko’ n: peak shapePeak area:

Zero-order desorption kinetics

-independen of coverage-exponential increase with T-common leading edge-Rapid drop-Tmax move to higher T with coverage-Pseudo zerp-order for strong intermolecular interactions between adsorbates

Inte

nsit

y

T/K

rd = -d/dt = ko exp(-Ed/kT)

First-order desorption kinetics n =1

Inten

sit

yT/K

rd = -d/dt = koexp(-Ed/kT)

exp(-Ed/kT)

-rate proportional to coverage-balance between and exp(-Ed/kT)-Tpeak independent of -Asymmetric line shape-Tpeak as Ed -Molecular desorption

Second order desorption kinetics n=2

T/K

rd = -d/dt = ko2 exp(-Ed/kT)

-rate proportional to coverage-balance between and exp(-Ed/kT)-Tpeak varies with -symmetric line shape-Common trail of peaks-Recomnative desorption-Pseudo-2nd order for strong intermolecular interactions

Inte

nsit

y

Fractional order desorption kinetics

- indica

Indicate cluster formation on the surfaceDesorption from edge of clusters

Effect of activation barrier Ed=50~400kJ/mol

Inte

nsit

y

Ed 1020

3040

50

Ed Tpeak peak width At saturation coverageEd/RTp= 30kJ/mol

Effect of pre-exponential factor k0 =1011 ~1015 1/sec

Inte

nsit

y

k0 =1015

k0 =1011

T/K

-oscillation frequency for adsorbate particles

Effect of heating rate

Inte

nsit

y

T/K

= dT/dt =18.5

= 18.5= 17.5

CO/Ni(110)

Determination of activation barrier Ed

The maximum rate in the desorption ratedrd/dt =0, konn-1 exp(-Ed/kT) = Ed/kTp

2

-Ed/kT = ln (kTp2 ln(Ed/ konn-1 )

Plot of ln vs 1/T at constant initial coverage: Ed

Tp

Ed

ko/=1014/K

ko/=1010/K

Other methods:Chan, Aris, Weinberg, Appl. Surf. Sci. 1, 360 (1978)Habenschaden, Kuppers, Surf. Sci., 138 L148 (1984)D.A. King, Surf. Sci. 47, 384 (1975)