Vibrational spectroscopy of the hydrated hydronium cluster ions H3O+⋅(H2O)n (n=1, 2, 3)

Abstract

The gas phase infrared spectra of the hydrated hydronium cluster ions H₃O⁺⋅(H₂O)_n(n=1, 2, 3) have been observed from 3550 to 3800 cm⁻¹. The new spectroscopic method developed for this study is a two color laser scheme consisting of a tunable cw infrared laser with 0.5 cm⁻¹ resolution used to excite the O–H stretching vibrations and a cw CO₂ laser that dissociates the vibrationally excited cluster ion through a multiphoton process. The apparatus is a tandem mass spectrometer with a radio frequency ion trap that utilizes the following scheme: the cluster ion to be studied is first mass selected; spectroscopic interrogation then occurs in the radio frequency ion trap; finally, a fragment ion is selected and detected using ion counting techniques. The vibrational spectra obtained in this manner are compared with that taken previously using a weakly bound H₂ ‘‘messenger.’’ A spectrum of H₇ O⁺₃ taken using a neon messenger is also presented. Ab initio structure and frequency predictions by Remington and Schaefer are compared with the experimental results.