Surface and Interface Science Physics 627; Chemistry 542

Lectures 19 Apr. 5 2013

Vibrational Spectroscopy

1

References

Overview

•High Resolution Electron Energy Loss Spectroscopy •Molecular Symmetry at Surfaces •Adsorbate-adsorbate Interactions •IR Spectroscopy

U(r) r

...!2

1)()(

2

2

2

0

00

r

r

Ur

r

UrUrU

rr

U(ro)

ro

...2

1)()(

2

0 rkrUrU

Why Measure Vibrations at Surfaces?

• Molecular Identification • Gas phase spectra well known • Most retained at surfaces

• Adsorption site identification • symmetry • Molecular analogs and shifted wo

• Bonding identification

• Energy Transfer

• Adsorbate-Adsorbate Interaction

• Phonons, surface stress

High Resolution Electron Energy Loss Spectroscopy

+

-

+

-

Eo ~ 5 eV

Eo

Dipole

Image

E ~ 4 meV

Eo – hw1 Eo – hw1

ti

o pepp w c.c. 2

)(3

tie

r

pzrV w

c.c. )( ||||||2||

ziQrQidQtieeperV o

w

Time varying dipole: Potential seen by electron:

zzrr ˆ||

Let: ||Q

Wave vector || to surface

Component with wave vector Q|| reaches into vacuum a distance l ~ Q||

-1

Consider incident electron: scatters into state: where ii Ek ,

ss Ek ,

ois EE w

Born Approximation: Done by components of the potential with: is kkQ ||||||

ik

Sk

||Q

s

Suppose ; small s0i |||| Qkk i

s

s

Interaction time: sis

i

ooo EkvQvv

lt

~

2~

2~

2

||

If two << 1 then too fast no excitation

If two >> 1 then adiabatic no excitation

i

os

si

oo

EEt

w

ww

11

eV 5meV 100

i

o

Ew 6.002.0 s

s

Scattering Kinematics

l

HREELS Spectrometer

Monochromator Analyzer

HEELS Spectrometer

(H. Ibach, Jülich)

Examples: Molecular Adsorbates

545K

1000K

CO/Ni(111)

O2/Ru(001) O

C

O

C

H

W W

H

W W

H

W W Sv ASv Wv

Sv

ASvASv

Sv

Impact Scattering Non-dipole

Molecular Bonding Geometry

Group Symmetry Selection Rules

Surface Phonons

Negative Ion Resonance (vibrational overtones)

Infrared Reflection Absorption Spectroscopy

• Spectral Range: ~ 65 – 500 meV ~ 500 – 4000 cm-1 • Resolution 0.125 – 0.625 meV 1 – 5 cm-1 • Sensitivity: ~ 10-3 ML - Selection Rules: - Strict dipole • Versatility: - photon in-photon out:

• Only p-component of light survives at the surface

• Only m^ is observed

• High substrate reflectivity (at grazing angle)

• Versatility: photon in-photon out:

2

2

1

1Im

cos

sin8

d

R

R

p

p

IRAS Instrumentation

O

C

O

C

IRAS Examples

O

C

O

C

• Observation of dipole forbidden mode at 285 cm-1

• Coupling of discrete vibrational mode of molecule with continuum excitation of substrate

•Broadband absorption as function of coverage electron resistivity