Torsion-rotation analysis of OH stretch overtone-torsion combination bands in methanol

We report rotationally resolved spectra of jet-cooled methanol for the OH stretch overtones, 2v(1) and 3v(1), and for the torsional combinations, 2v(1)+v(12), 2v(1)+2v(12), 3v(1)+v(12), and 3v(1)+2v(12). The spectra are obtained by direct excitation from the vibrational ground state with an infrared laser pulse. Population in the resulting upper state levels is detected by infrared laser assisted photofragment spectroscopy (IRLAPS). Global fits of the spectra to the Herbst Hamiltonian yield the torsional and rotational parameters, including F, rho, V-3, and V-6, for each OH stretch excited state. For each quantum of OH stretch excitation, we find that the torsional barrier height V-3 increases by 40.9 +/-1.9 cm(-1) and the torsional inertial F decreases by 0.89 +/-0.02 cm(-1). With reference to ab initio calculations, we explain the increase in V-3 in terms of changes in the electronic structure of methanol as the OH bond is elongated. For Deltav(12)=1 we observe only transitions with DeltaK=+/-1, and for Deltav(12)=2 we observe only DeltaK=0. We present a Franck-Condon model to explain these apparent selection rules and the overall pattern of intensity. (C) 2002 American Institute of Physics.