Torsional vibrational levels combined with higher frequency modes of the jet-cooled p-methoxybenzyl alcohol in the S 1 excited state

Abstract

Resonant two-photon ionization (R2PI) spectrum of p-methoxybenzyl alcohol prepared in the supersonic jet has been obtained. Long-progression bands associated with torsional mode of the CH 2OH moiety with respect to the rest of the molecule in the S 1 state are observed in the wide spectral range of 35,500–37,500 cm −1. The torsional barrier height is found to decrease when the torsional mode is combined with other low-frequency bands at 84 ( ν 1=1) and 163 (ν 1=2) cm −1, indicating that this low-frequency mode is modestly coupled to the torsional mode. On the other hand, when the torsional mode is built on the higher-frequency band at 798 ( ν 2=1) and 1595 cm −1 ( ν 2=2), torsional frequencies are found to be little changed from those located at the origin. Ab initio calculation suggests that this relatively high-frequency mode may be associated with ring-deformation motions, which does not significantly modify the distance and angle between the CH 2OH moiety and aromatic ring. Rotational contour analysis suggests that the transition dipole moment is oriented along the molecular b-axis. The torsional progression bands in the excitation energies higher than ∼800 cm −1 are found to be much broader than those located at the origin, indicating that intramolecular vibrational redistribution (IVR) becomes quite efficient at such excitation energies.