surface and interface science physics 627; chemistry 542 ...bart/627/p627_s13_l19_05... · why...
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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