Infrared spectra of the cluster ions H7O+3ṡH2 and H9O+4ṡH2

Infrared spectra of hydrated hydronium ions weakly bound to an H₂ molecule, specifically H₇O⁺₃ ṡH₂ and H₉O⁺₄ ṡH₂, have been observed. Mass-selected parent ions, trapped in a radio frequency ion trap, are excited by a tunable infrared laser; following absorption, the complex predissociates with loss of the H₂, and the resulting fragment ions are detected. Spectra have been taken from 3000 to 4000 cm^-1, with a resolution of 1.2 cm^-1. They are compared to recent theoretical and experimental spectra of the hydronium ion hydrates alone. Binding an H₂ molecule to these clusters should only weakly perturb their vibrations; if so, our spectra should be similar to spectra of the hydrated hydronium ions H₇O⁺₃ and H₉O⁺₄.