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The surface-enhanced Raman spectrum of CO adsorbed on colloidal silver particles of about 100 Å diameter is reported. The spectrum resembles that observed for CO adsorbed on vapor-deposited silver. Only three strong Raman lines appear, at 2113, 160, and 64 cm−1. These are assigned, respectively, to the CO stretching, Ag–C stretching, and AgCO bending vibrations. The excitation profile of the 2113 cm−1 line follows closely the plasmon absorption spectrum of the silver colloid, increasing as one goes from the red to the violet; that of the 160 cm−1 line shows a more gradual increase in the same direction. The two types of behavior are accounted for by means of a model which considers the enhancement of the Raman signal to be due to coupling of the adsorbate vibration with the conduction–electron surface plasma mode through a charge transfer from the adsorbate to the metal, synchronous with the vibration. The two types of excitation profile are generated by assuming that for the AgC vibration, positive displacement of the normal coordinate produces charge injection into the metal while for the CO vibration, charge is withdrawn as the CO bond is elongated.

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AbstractThe laser Raman spectrum of matrix-isolated As4-tetrahodra is reported. The frequencies are different to former data obtained by gas-phase measurements.