Metal Surface Raman Spectroscopy: Theory.
MINNESOTA UNIV MINNEAPOLIS DEPT OF CHEMISTRY
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Studies of the Raman and infrared spectra of several molecules adsorbed to metal surfaces indicate that special selection rules govern these spectra. In the case of infrared reflection adsorption spectroscopy IRS as in the related techniques of electron energy loss spectroscopy ELS and inelastic electron tunneling spectroscopy IETS, only modes which develop dipole moment perpendicular to the metal surface can be excited. Both infrared and Raman spectra of oriented metal adsorbates can be used to probe molecular orientation. Depolarization ratios for specific orientations and scattering geometries have been derived and used to analyze the results of several experimental examples. The adsorbed molecule whose Raman spectrum has been most thoroughly studied is pyridine on silver. Using secondary ion mass spectroscopy, we have been able to show that the silver surfaces are atomically quite clean. The case of pyridine adsorbed on Ag is also of interest because of the relative intensity of some of its Raman lines. By comparison with the same lines in liquid pyridine, the intensity is apparently enhanced by a factor of 10000. Several theories of this enhancement are analyzed.
- Atomic and Molecular Physics and Spectroscopy