EELS, Surface Plasmon and Adsorbate Vibrations Ao Teng 2010.10.11
EELS, Surface Plasmon and Adsorbate Vibrations
Ao Teng
2010.10.11
Outline
I. Electron Energy Loss Spectroscopy(EELS)and High Resolution EELS (HREELS)
II. Surface Plasmon
III. Adsorbate Vibrations
Surface Analytical Techniques
I.EELS and HREELS
Basic theory
Instrumentation
Basic theory
Classified by the geometry and by thekinetic energy of the incident electrons
t = λln(ITotal/I0),Or
ITotalexp(-t/λ)=I0
Interpretation of EELS spectra
a – zero loss
b – phonons
c – band transitions
d – surface plasmons
e – bulk plasmons
f – inner shell absorption edge
Phonon/adsorbate vibration(Eloss < 100 meV)
Valance electrons/plasmons(Eloss~1-20 eV)
Core electrons(Eloss ~ 50-1000 eV)
Core-level EELS
Plasmon Detection with Normal EELS
satellite peaks near elastic or core-level loss peaks
Multiple plasmon loss peaks
Surface plasmon
grazing emission to enhance sensitivity
Bulk plasmon
normal emission
High-resolution EELS (HREELS)
Phonon detection
Determine adsorbate configuration on surface (characteristic vibration modes of a particular bonding)
High energy resolution (≤ 5 meV or 40 cm-1)
Physics of EELS
Parallel to the surface:
Dipole Scattering
Dipole scattering can be applied when the scattered beam is very near to the specular direction.
The incident electron can scatter inelastically what means it excites vibrations in the dipole structure.
Impact Scattering
When the scattering plane is a plane of reflectionsymmetry then the scattering amplitude for everyks in the scattering plane vanishes.When the plane perpendicular to the surface andthe scattering plane is a plane of reflectionsymmetry and time reversal symmetry holds thenthe scattering amplitudes in specular directionvanishes for modes whose normal coordinates areodd under the reflection.When the axis normal to the surface is an axis oftwofold symmetry, and time reversal symmetryholds then the scattering amplitudes in speculardirection vanishes for modes whose normalmodes are odd under the twofold rotation.
Assume that the energy lost in the inelastic scattering process is negligible
Instrumentaion
Resolution at 5 meV (FWHM)
Primary beam energy 0-240 eV
Energy Scan -5 eV (gain) to+15 eV loss energy
Composed of •Double-pass monochromator
•Rotating Analizer
•25+ lenses in 4 groups
•Filament source
•Channeltron (e multiplier)
Electron Optics
)2(
)1(4
20
20
cmE
cmE
EREx
+
+∆=∆
II. Surface Plasmon
Surface plasmon polariton:
EM wave at metal-dielectric interface
EM wave is coupled to the plasma oscillations of the surface charges
( ) ( )tzkxkidd
zxeEtzxE ω−+= 0,,,
( )tzkxkimm
zxeEtzxE ω−−= 0,),,(
For propagating bound waves:- kx is real- kz is imaginary
x
z
Maxwell Equations
xx ikk "' +=Dispersion relation ω(kx)
2/1
"'
+
=+=dm
dmxxx c
ikkkεεεεω
Bound SP mode requirement:
kz imaginary: εm + εd < 0,
kx real: εm < 0
Therefore εm < -εd
2/12
,,, "'
+=+=
dm
mmzmzmz c
ikkkεε
εω
Complete Solution
0
2
εω
mne
p =
Recall bulk plasmon
Drude model: conduction electrons with damping
tieEdtdxm
dtxdm ωγ e02
2=+
( )
( )ωγωω
ωγωε
εεε
i
imne
Enex
EP
p
−−=
−−=
+=+=
2
2
20
200
1
1
11
γωω
εωω
ε 3
2
2
2
",1' pp =−=ωγ <<if
ω
ωp
d
p
ε
ω
+1
Re kx
real kx real kz
imaginary kx real kz
real kx imaginary kz
d
xckε
Bound modes
Radiative modes
Quasi-bound modes
Surface plasmon dispersion relation:
Dielectric: εd
Metal: εm = εm' + εm
"
x
z
(ε'm > 0)
(−εd < ε'm < 0)
(ε'm < −εd)
Eg.Plasmon shift as indicator of H adsorption
III. Adsorbate vibrations
Sites of adsorbate
Bond Stretching Bond Bending
symmetric
asymmetric
In-plane rocking
In-plane scissoring
Out-of-plane wagging
Out-of-plane twisting
Vibrations of adsorbate
Eg. CO adsorbed on Ni(111)&Pt(111)
Data (Mg plasmon)
SP dispersion (by eyes) Minimum SP energy (by fitting): at different angles
Dispersion Line Width
Angle Angle
Ener
gy (
eV)
Ener
gy (
eV)
BP: Bulk PlasmonMP: Multipole Plasmon
SP: Surface PlasmonEP: Extra Peak
Energies & Line Widths (by curve fitting)
Dispersion: Energy vs. momentum (by curve fitting)
Surface Plasmon Dispersion: Energy vs. momentum (by curve fitting)
Exemplary Spectra
Exemplary Spectra