Characterization of phonon modes by XPD on a (2×2) s -V 2 O 3 layer on Pd(111)

1. Characterization of phonon modes by XPD on a (2×2) s-V2O3 layer on Pd(111) Mauro SambiDipartimento di Scienze ChimicheUniversità di PadovaVia Marzolo 135131 Padovamauro.sambi@unipd.it

2. “despised and rejected by men” (Isaiah 53, 3)

3. (2×2) surface-V2O3/Pd(111) - structural determination M. Sambi, M. Petukhov, B. Domenichini, G. A. Rizzi, S. Surnev, G. Kresse, F. P. Netzer and G. Granozzi Surf. Sci. 534 (2003) L234. XPD FS STM DFT model V 2p, expt. KE=972 eV V 2p, SSC-SW based on DFT zV-O(expt.)=0.72±0.07 Å zV-O(DFT)=0.723 Å

4. O-C FS Prehistory: Formate adsorption on Cu (100) M. Sambi, G. Granozzi, M. Casarin, G. A. Rizzi, A. Vittadini, L. S. Caputi and G. Chiarello: Surf. Sci., 315 (1994) 309. Cross Bridge (CB) Diagonal Atop (DA) Short Bridge (SB)

6. Quantitative determination: SSC-SW simulations d(Cu-O) = 1.95±0.05 Å <(OCO) = 129°±5° ω ≈ 70 cm-1

7. Back to (2×2) s-V2O3/Pd(111) – FS peaks azimuthal broadening M. Sambi, M. Petukhov, B. Domenichini, G. A. Rizzi, S. Surnev, G. Kresse, F. P. Netzer and G. Granozzi Surf. Sci. 534 (2003) L234. V 2p, expt. KE=972 eV V 2p, SSC-SW based on DFT, simple DW attenuation

8. (2×2) s-V2O3/Pd(111) – a vibrational study M. Sambi, S. Surnev, G. Kresse, F. P. Netzer and G. Granozzi, Phys. Rev. B68 (2003) 155417 DFT prediction: ω=14.7 cm-1! Soft phonon mode involving in-plane displacements of O scatterers with respect to V emitters? θ = 68°

9. HARMONIC OSCILLATOR MODEL γ - angular displ. from equilibrium ω - frequency Nν - normalisation constant Hν - νth order Hermite polynomial γ Boltzmann-weighted sum

10. The convolution procedure γ

11. The smallest cluster: 4 atoms ω=50±10 cm-1

12. The effect of correlated atomic displacements - the largest cluster: 73 atoms NNN scatterers de-phasing ω=45±30 cm-1

13. The optimal cluster: 34 atoms ω=40±25 cm-1

14. The linear oscillator model

15. Conclusions • Anisotropic deformation of FS features in XPD scans are indicative of the presence of soft phonon modes involving the emitters and/or the scatterers of the photoelectron wave; • Such modes cannot be accounted for by isotropic DW attenuation; • A simple model based on a harmonic torsional/linear oscillator has been adopted to account for the anisotropic deformation (azimuthal broadening vs polar narrowing) of V – O FS features in a s-V2O3 monolayer film; • The frequency of oscillation obtained by fitting the XPD curves with the model is of the same order of magnitude of the DFT computed one; • In a periodically ordered overlayer, the extent of anisotropic deformation of FS peaks depends on the coherence length of the vibrational mode as seen by the photoelectron emitter, which is limited to 2 – 4 coordination circles around the emitter.

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17. XPD – simulations: checking non-structural parameters • SS C-SW simulations (Fadley code). MS was checked to be negligible (Van Hove MSCD code) at the outgoing photoelectron KE (952 eV); • Simulations are substrate-insensitive and barely sensitive to the presence of V scatterers coplanar with the V emitters: FS dominates; first order features arise from V -> NNN O atoms. • Simulations are largely insensitive to the choice of the inelastic attenuation length in a broad range (5 – 100 Å), due to the essentially non-attenuating nature of the oxide monolayer. • The same for the choice of the inner potential, due to the high photoelectron KE. • Scattering phase shifts were calculated by MUFPOT. MT radii were varied in a reasonable interval to check their effect of FS peaks FWHM – negligible.