Vibrational characteristics of silver clusters in surface-enhanced Raman scattering

Evidence from a number of previous experiments is found for Ag-cluster vibrations in enhanced Raman scattering from a Ag surface. By the analysis of the conditions specific to such experiments, the most likely identity of these clusters has been revealed to be ${\mathrm{Ag}}_4$ ${\mathrm{}}^{+}$. To further confirm this assignment, the vibrational signature of the clusters has been tested in a simple calculation. The model includes the effects of (1) adsorption of the ${\mathrm{Ag}}_4$ ${\mathrm{}}^{+}$ cluster, (2) the image interaction with the metal, and (3) coadsorbed anions. The calculated (values in parentheses) modes correspond well to the experimentally observed features at 73 (75), 110 (100 and 118), and 161 ${\mathrm{cm}}^{\mathrm{-}1}$ (170 ${\mathrm{cm}}^{\mathrm{-}1}$).