RAMAN-SPECTROSCOPY OF CARBON-MONOXIDE ADSORBED ON SILVER ISLAND FILMS

CO was adsorbed at 17 K on clean silver island films prepared in UHV. The surface enhanced Raman scattering (SERS) spectrum shows besides the stretch band at ∼ 2112 cm-1 of "chemisorbed" CO an unshifted band at ∼ 2139 cm-1 whose intensity saturates at monolayer coverage. It is assigned to " physisorbed" CO. The Raman signal of condensed multilayer CO is below the detection threshold. With respect to the integrated CO stretch band of thick condensed CO films, the integrated broad CO stretch band of "chemisorbed" CO is ∼ 2 × 105 fold enhanced, the narrow band of "physisorbed" CO about 1 × 104 fold, exceeding the chemically unspecific classical enhancement for an adsorbed monolayer of ∼ 3 × 103. We propose that "chemisorption" sites of atomic scale roughness also induce SERS by species physisorbed near these sites via photoinduced charge transfer (so-called proximity effect). © 1990.