Vibrational spectra and structures of bare and Xe-tagged cationic Si_nO_m⁺ clusters

Vibrational spectra of Xe-tagged cationic silicon oxide clusters Si_nO_m⁺ with n = 3-5 and m = n, n ± 1 in the gas phase are obtained by resonant infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory calculations. The Si_nO_m⁺ clusters are produced in a laser vaporization ion source and Xe complexes are formed after thermalization to 100 K. The clusters are subsequently irradiated with tunable light from an IR free electron laser and changes in the mass distribution yield size-specific IR spectra. The measured IRMPD spectra are compared to calculated linear IR absorption spectra leading to structural assignments. For several clusters, Xe complexation alters the energetic order of the Si_nO_m⁺ isomers. Common structural motifs include the Si₂O₂ rhombus, the Si₃O₂ pentagon, and the Si₃O₃ hexagon.