Far-infrared spectroscopy of small neutral silver clusters

The vibrational spectra of Ag-3 and Ag-4 are recorded in the far-infrared between 100 and 220 cm(-1) using multiple photon dissociation spectroscopy of their complexes with Ar atoms. For Ag-3-Ar two IR active bands are found at 113 and 183 cm(-1), for Ag-4-Ar one band at 163 cm(-1) and very weak IR activity at 193 cm(-1) are observed. This, together with recent theoretical studies, allows for a reassignment of the controversial vibrational data reported earlier for the bare Ag-3 cluster. The influence of the number of Ar atoms in the complexes on the frequency of the IR active modes is found to be minor. However, the low-frequency IR-active band of Ag3 shifts with increasing Ar coverage from 113 cm(-1) for Ag-3-Ar to about 120 cm(-1) for Ag-3-Ar-4, the value known for Ag-3 embedded in rare gas matrices.