Multiple-resonance spectroscopy of high vibrational levels in water and methanol

We have measured the rovibrational levels in the electronic ground state of the water molecule at the previously inaccessible energies above 26000 cm-1. The use of laser double-resonance overtone excitation combined with laser-induced fluorescence (LIF) photofragment detection extends this limit to 34200 cm-1, which corresponds to 83% of the water dissociation energy. These data have allowed the theoretical group of Oleg Polyansky to generate a semiempirical potential energy surface on the basis of their ab initio surface [O. L. Polyansky, A. G. Csaszar, S. V. Shirin et al., Science 299 (5606), 539 (2003)] that now allows prediction of water levels with sub-cm-1 accuracy at any energy up to the new limit. A new ab initio potential energy surface is being constructed by Polyansky et al. that is intended to reproduce the rovibrational levels of water up to the dissociation threshold. We have calculated the electronic energy of ca. 2500 nuclear geometries at multireference configuration interaction level with 6Z basis set. We have performed a direct measurement of one of the most fundamental thermochemical values: the O-H bond energy in water. Using a triple-resonance laser excitation scheme, we excite the molecule through a series of vibrational overtone transitions to access directly the onset of the dissociative continuum. The dissociation energy obtained from our experiments, 41145.94 ± 0.15 cm-1, is ca. 30 times more accurate than the currently accepted value and has important implications for other thermochemical quantities linked to the bond energy of water. We have studied the conformational dependence of intramolecular vibrational redistribution in the 5v1 OH stretch overtone region of methanol. Previous state-selected spectra in the 5ν1 region [O. V. Boyarkin, T. R. Rizzo, and D. S. Perry, J. Chem. Phys. 110, 11346 (1999)] revealed a structure indicating an intramolecular vibrational redistribution on three time scales. Whereas in that work, methanol in the 5 v1 bright state was prepared close to the staggered conformation, methanol in the "partially eclipsed" conformation is prepared in our experiments by double resonance excitation through a torsionally excited intermediate state. We detect the excited molecules by infrared laser assisted photofragment spectroscopy (IRLAPS). In partially eclipsed methanol, the strong coupling of the v1 OH stretch to the v2 CH stretch becomes weaker, but the coupling responsible for the widths of the narrowest features becomes stronger.