VIBRATION-ROTATION SPECTROSCOPY OF THE HYDRATED HYDRONIUM IONS H5O2+ AND H9O4+

High-resolution vibration-rotation spectra in the OH antisymmetric str etching region near 3700 cm(-1) are reported for H5O2+ and H9O4+. The clusters are produced in a corona discharge ion source, cooled by supe rsonic expansion, mass-selected, and trapped in an RF octopole ion tra p. 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 detect ion of the resultant fragment ions. Many more lines appear in the part ially resolved vibration-rotation spectrum of H5O2+ than can be explai ned if the molecule is rigid, and we have assumed that these additiona l lines arise from tunneling splittings caused by large-amplitude inte rnal motions in this ion. Despite the low signal-to-noise ratio, all t he observed spectral features can be grouped into roughly 12 R branche s with a line spacing only 14% less than the B + C value calculated fr om the ab initio structure. Theoretically expected splitting patterns were calculated using a formalism developed earlier for tunneling moti ons in hydrazine, since H2N-NH2 and H2O-H+-OH2 are group-theoretically similar if the central proton of the ion is located symmetrically bet ween the two water molecules. We tentatively conclude that the 12 bran ches represent the overlapping of six tunneling-split components for t he in-phase and six for the out of-phase OH antisymmetric stretching v ibrations expected in this region, but the low signal-to-noise ratio i n the present measurements prevented unambiguous comparison of theory and experiment. (C) 1994 Academic Press, Inc.