Infrared spectra of the solvated hydronium ion: Vibrational predissociation spectroscopy of mass-selected H3O+.cntdot.(H2O)n.cntdot.(H2)m

Infrared spectra of the mass-selected clusters H{sub 3}O{sup +}{sm bullet}(H{sub 2}O){sub n}{sm bullet}(H{sub 2}) and H{sub 3}O{sup +}{sm bullet}(H{sub 2}){sub n} (n = 1,2, and 3) were observed by vibrational predissociation spectroscopy. The clusters were mass-selected and then trapped in a radio frequency ion trap. Cluster dissociation by loss of H{sub 2} followed excitation of OH or H{sub 2} stretches. Spectra were recorded by detecting fragment ions as a function of laser frequency. From spectra of H{sub 3}O{sup +}{sm bullet}(H{sub 2}O){sub n}{sm bullet}(H{sub 2}), the authors were able to determine the spectrum of the hydrated hydronium ion H{sub 3}O{sup +}{sm bullet}(H{sub 2}O){sub n}, because the H{sub 2} formed weak complexes with the hydrates. Spectra in the OH stretching region (3,000-4,000 cm{sup {minus}1}) were observed at a resolution of 1.3 cm{sup {minus}1} for clusters n = 1,2, and 3. The structure of the clusters and the perturbing effect of the H{sub 2} were inferred from a comparison with recent unpublished ab initio vibrational frequencies calculated by Remington and Schaefer.