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