High Resolution Electron Energy Loss and Surface Enhanced Raman Studies of Pyridine and Benzene on Ag(111)
High resolution electron energy loss (EELS), UV photoemission (UPS), low energy electron diffraction (LEED), Auger electron, and thermal desorption spectroscopic techniques have been applied to characterize the bonding, molecular orientation and vibrations of pyridine and benzene on clean Ag(111). These results are used to interpret the surface enhanced Raman spectra of these molecules adsorbed on a clean, well-defined single crystal Ag(111) surface containing a smooth periodic modulation (1 micron wavelength,~ 500 Å amplitude) which permits optical coupling to surface plasmons-polaritons. We observe enhanced Raman scattering (RS) for chemisorbed species only when we are at appropriate incidence angles so as to excite surface plasmons. Coverage-dependent RS studies indicate a long range field enhancement of ~ 102 for physisorbed layers and a mode-selective, short-range enhancement of ~ 104 for the symmetric ring breathing mode of pyridine. This short-range enhancement only occurs for pyridine coverages slightly above ~ 3 × 1014 molecules/cm2 where changes in the UPS spectra and our UPS derived uptake-rates are also observed. Similar coverage-dependent effects are also observed in UPS and EELS for chemisorbed pyridine on the ideal Ag(111) surface. These as well as LEED results showing the modulated surface to be predominately (~ 90 %) single crystal Ag(111), encourage us to directly compare the results obtained for the two surfaces.
From EELS we determine that chemisorbed pyridine π-bonds and lies flat on the surface of Ag(111) to within ~ 5° but at higher chemisorption coverages (> 3 × 1014 molecules/cm2) becomes nitrogen-lone-pair-bonded, inclined ~ 55° to the surface and rotated ~ 30° about the C2 v symmetry axis. Benzene is found to π-bond and has vibrations characteristic of a C3 v(od) point group symmetry. We thereby relate the strong short-range enhancement observed in RS to the occurrence of nitrogen lone-pair-bonding and the inclination of chemisorbed pyridine to Ag(111). Both the mode selectivity and structure sensitivity for chemisorbed pyridine are shown to be consistent with the recent surface plasmon-polariton model of enhanced Raman scattering by Kirtley, Jha and Tsang. Also observed in RS are additional vibrations which we associate with pyridine bound to step sites and at or near graphitic carbon impurities.
KeywordsSurface Enhance Raman Scattering Point Group Symmetry Liquid Benzene Ring Breathing Mode Surface Enhance Raman Spectrum
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