Infrared photodissociation spectroscopy of Ag{sub n}(C{sub 6}H{sub 6}){sub m} and Ag{sub n}(C{sub 6}D{sub 6}){sub m} clusters: Evidence of adsorption-induced symmetry reduction in benzene

Photodepletion spectroscopy has emerged in recent years as a powerful tool for probing the optical response of both neutral and charged metal clusters. Comparison of measured spectra with theoretical predictions has allowed for inferences about size-specific cluster structures. The vibrations of benzene-h{sub 6} and benzene-d{sub 6} on single crystal metal surfaces have been thoroughly investigated using a number surface science technique. On silver surfaces, it has been found that benzene adsorbs weakly, with vibrational frequencies that are shifted downward by approximately 5 cm{sup {minus}1} relative to those of gas-phase benzene. Furthermore, modes which are inactive for gas-phase benzene are seen when the benzene is adsorbed to the surface, showing that although the interaction is weak, it is sufficient to lower the symmetry of the benzene from D{sub 6h} to C{sub 3v} ({sigma}{sub d}). The orientation of the benzene relative to the surface remains a point of controversy. It seems likely that the orientation depends upon the coverage. The observation of the a{sub 1g} C-C breathing mode of benzene in infrared photodepletion measurements of Ag{sub n}Bz{sub m} clusters demonstrates that there is a reduction in the benzene symmetry when it is adsorbed on silver clusters. Furthermore, the absence of detection of the e{sub 1u} mode for most clusters investigated suggests that the surface selection rule may apply even in these very small systems. Experiments to further probe the infrared response of M{sub n}Bz{sub m} (M = coinage metal) clusters are ongoing