Vibration-Rotation Spectroscopy of the Hydrated Hydronium Ions H 5O +2 and H 9O +4

Abstract

High-resolution vibration-rotation spectra in the OH antisymmetric stretching region near 3700 cm −1 are reported for H 5O + 2 and H 9O + 4. The clusters are produced in a corona discharge ion source, cooled by supersonic expansion, mass-selected, and trapped in an RF octopole ion trap. Spectroscopic interrogation using a two-color laser scheme leads to rovibrational excitation of the trapped ions followed by preferential multiphoton dissociation of the vibrationally excited ions and detection of the resultant fragment ions. Many more lines appear in the partially resolved vibration-rotation spectrum of H 5O + 2 than can be explained if the molecule is rigid, and we have assumed that these additional lines arise from tunneling splittings caused by large-amplitude internal motions in this ion. Despite the low signal-to-noise ratio, all the observed spectral features can be grouped into roughly 12 R branches with a line spacing only 14% less than the B + C value calculated from the ab initio structure. Theoretically expected splitting patterns were calculated using a formalism developed earlier for tunneling motions in hydrazine, since H 2N-NH 2 and H 2O-H +-OH 2 are group-theoretically similar if the central proton of the ion is located symmetrically between the two water molecules. We tentatively conclude that the 12 branches represent the overlapping of six tunneling-split components for the in-phase and six for the out of-phase OH antisymmetric stretching vibrations expected in this region, but the low signal-to-noise ratio in the present measurements prevented unambiguous comparison of theory and experiment.