High Resolution Electron Energy Loss and Surface Enhanced Raman Studies of Pyridine and Benzene on Ag(111).
IBM THOMAS J WATSON RESEARCH CENTER YORKTOWN HEIGHTS NY
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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 Ag111. These results are used to interpret the surface enhanced Raman spectra of these molecules adsorbed on a clean, well-defined single crystal Ag111 surface containing a smooth periodic modulation 1 micron wavelength, approx. 500 A 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 approx. 100 for physisorbed layers and a mode-selective, short-range enhancement of approx 10,000 for the symmetric ring breathing mode of pyridine. This short-range enhancement only occurs for pyridine coverages slightly above 3 x 10 to the 14th power moleculessq cm 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 Ag111 surface. These as well as LEED results showing the modulated surface to be predominately approx. 90 single crystal Ag111, encourage us to directly compare the results obtained for the two surfaces.
- Atomic and Molecular Physics and Spectroscopy