Vibrational spectra of small silicon monoxide cluster cations measured by infrared multiple photon dissociation spectroscopyw

The first gas-phase infrared spectra of silicon monoxide cations (SiO)n , n = 3–5, using multiple photon dissociation in the 550–1250 cm 1 frequency range, are reported. All clusters studied here fragment via loss of a neutral SiO unit. The experimental spectra are compared to simulated linear absorption spectra from calculated low energy isomers for each cluster. This analysis indicates that a ‘‘ring’’ isomer is the primary contributor to the (SiO)3 + spectrum, that the (SiO)4 + spectrum results from two close-lying bicyclic ring isomers, and that the (SiO)5 + spectrum is from a bicyclic ring with a central, fourfold-coordinated Si atom. Experiment and theory indicate that the energies and energetic orderings of (SiO)n + isomers differ from those for neutral (SiO)n clusters.