EELS, Surface Plasmon and Adsorbate Vibrations

Transcription

1 EELS, Surface Plasmon and Adsorbate Vibrations Ao Teng

2 Outline I. Electron Energy Loss Spectroscopy(EELS) and High Resolution EELS (HREELS) II. Surface Plasmon III. Adsorbate Vibrations

3 Surface Analytical Techniques

4 I.EELS and HREELS Basic theory Instrumentation

5 Basic theory t = λln(itotal/i0), Or ITotalexp(-t/λ)=I0 Classified by the geometry and by the kinetic energy of the incident electrons

6 Interpretation of EELS spectra Phonon/adsorbate vibration (E loss < 100 mev) Valance electrons/plasmons (E loss ~1-20 ev) Core electrons (E loss ~ ev) a zero loss b phonons c band transitions d surface plasmons e bulk plasmons f inner shell absorption edge

7 Core-level EELS

8 Plasmon Detection with Normal EELS satellite peaks near elastic or core-level loss peaks Multiple plasmon loss peaks Bulk plasmon normal emission Surface plasmon grazing emission to enhance sensitivity

9 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 )

10 Physics of EELS Parallel to the surface:

11 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.

12 Impact Scattering When the scattering plane is a plane of reflection symmetry then the scattering amplitude for every k s in the scattering plane vanishes. When the plane perpendicular to the surface and the scattering plane is a plane of reflection symmetry and time reversal symmetry holds then the scattering amplitudes in specular direction vanishes for modes whose normal coordinates are odd under the reflection. When the axis normal to the surface is an axis of twofold symmetry, and time reversal symmetry holds then the scattering amplitudes in specular direction vanishes for modes whose normal modes are odd under the twofold rotation. Assume that the energy lost in the inelastic scattering process is negligible

13 Instrumentaion Resolution at 5 mev (FWHM) Primary beam energy ev Energy Scan -5 ev (gain) to +15 ev loss energy

14 Composed of Double-pass monochromator Rotating Analizer 25+ lenses in 4 groups Filament source Channeltron (e multiplier)

15 Electron Optics

16

17 (1 + E 2) 4R m0c x = E E (2 + E 2) m c 0

18 II. Surface Plasmon

19 Surface plasmon polariton: EM wave at metal-dielectric interface z Maxwell Equations x E E d m ( ) i( kxx+ kz z ωt ) x, z, t E e ( x, z, t) = d, 0 = Em, 0 e Dispersion relation ω(kx) i ( k x k z ωt ) x z = k ' + ik" For propagating bound waves: - k x is real - k z is imaginary EM wave is coupled to the plasma oscillations of the surface charges x x

20 2 1/ " ' + = + = d m d m x x x c ik k k ε ε ε ε ω Bound SP mode requirement: k z imaginary: ε m + ε d < 0, k x real: ε m < 0 Therefore ε m < -ε d 2 1/ 2,,, " ' + = + = d m m m z m z m z c ik k k ε ε ε ω Complete Solution 0 2 ε ω m ne p = Recall bulk plasmon Drude model: conduction electrons with damping t i ee dt dx m dt x d m ω γ e = + ( ) ( ) ωγ ω ω ωγ ω ε ε ε ε i i m ne E nex E P p = = + = + = γ ω ω ε ω ω ε ", 1 ' p p = = ifγ << ω

21 Surface plasmon dispersion relation: ω ck x ε d Radiative modes (ε' m > 0) real k x real k z ω p ω p Quasi-bound modes ( ε d < ε' m < 0) imaginary k x real k z 1+ ε d z x Dielectric: ε d Metal: ε m = ε m ' + ε m " Bound modes (ε' m < ε d ) real k x imaginary k z Re k x

22 Eg.Plasmon shift as indicator of H adsorption

23 III. Adsorbate vibrations

24 Sites of adsorbate

25 Vibrations of adsorbate Bond Stretching Bond Bending symmetric In-plane rocking asymmetric In-plane scissoring Out-of-plane wagging Out-of-plane twisting

26 Eg. CO adsorbed on Ni(111)&Pt(111)

27

28 Data (Mg plasmon)

29 SP dispersion (by eyes) Minimum SP energy (by fitting) : at different angles

30 Angle Angle nergies & Line Widths (by curve fitting) Dispersion Line Width BP: Bulk Plasmon MP: Multipole Plasmon SP: Surface Plasmon EP: Extra Peak Energy (ev) Energy (ev)

31 Dispersion: Energy vs. momentum (by curve fitting)

32 Surface Plasmon Dispersion: Energy vs. momentum (by curve fitting)

33 Exemplary Spectra

34 Exemplary Spectra

35