Intramolecular energy transfer in highly vibrationally excited methanol. III. Rotational and torsional analysis

We report here torsional analysis of rotationally resolved spectra of the 3 nu(1), 5 nu(1), and 6 nu(1) (OH stretch) bands of jet-cooled methanol. The upper states are reached by a double resonance excitation scheme involving the selection of single rotational states in the n1 fundamental band. Dete ction of the overtone transitions (n nu(1)<--nu(1)) is by infrared laser as sisted photofragment spectroscopy (IRLAPS). The torsional tunneling frequen cy declines monotonically from 9.1 cm(-1) in the vibrational ground state t o 1.6 cm(-1) at 6 nu(1). For the available rotational levels at 3 nu(1) (K = 0-3) and 6 nu(1) (K = 0,1), the pattern of torsional energies is approxim ately regular. To obtain the vibrational dependence of the torsional barrie r V-3, it was necessary to use the OH radical and HOOH as models for the vi brational dependence of the torsional inertial constant F. The assumed line ar dependence of V-3 on nu(1) accounts for the torsional tunneling splittin gs at nu(1) = 0, 3, and 6 and for the pattern of the torsional energies. V- 3 increases by 40-45 cm(-1) per quantum of OH excitation. (C) 1999 American Institute of Physics. [S0021-9606(99)02417-4].