The Optics Laboratory
Group of Hans Hallen, Physics Department, North Carolina State University
GFR should be observable in a Raman experiment when the molecule studied is near a metal surface. Most of the Raman work (except NSOM) performed near metal surfaces has been concerned with SERS. We do not comment on SERS models here, but note which peaks not normally seen in Raman are expressed.
In particular, when the strong new (not normally Raman allowed) lines involve strong IR vibrations, it suggests that GFR is important. Surface effects, for the rough surfaces typically used for SERS, must also be included in the analysis of the selection rules. When the spectra is minimally impacted by the surface, indicated by the normally allowed Raman energies unperturbed from the bulk material, new active modes probably result from GFR or the quadrupole term rather than symmetry changes or bonding. The presence of GFR induced lines cannot be proved, since particular scenarios could cause these 'GFR preferred' lines to be strong even if another mechanism altered the selection rules. However, strong circumstantial evidence for GFR results from the presence of normally forbidden modes and their relative strength of compared to bulk IR strength, corrected for orientation with respect to the surface.
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More info is in the papers.
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