Participation of Methyl Internal Rotation in the Vibrational Predissociation of the Aromatic van der Waals Complex p-Fluorotoluene-Ar

摘要

S_1-S_0 fluorescence excitation and dispersed fluorescence spectra from p-fluorotoluene-Ar (pFT-Ar) formed in a supersonic expansion are used to characterize the vibrational predissociation (VP) from S_1 complex levels with 800, 1200 and 2000 cm~(-1) of vibrational energy. VP from the lower level occurs in at least 12 vibrational channels that involve only the three lowest energy pFT ring modes. VP from the higher levels is different, but those two levels are a matched pair with respect to their VP characteristics. They have almost identical channels and branching ratios, including a dominant channel producing free pFT in its zero point level. That channel requires conversion of as much as 1800 cm~(-1) of ring mode vibrational energy into translational and rotational energy. These behaviors appear to be unique among the aromatic van der Waals complexes so far studied, particularly with respect to the non-selective nature of the VP channels. Evidence pointing to the active participation of the methyl internal rotation in the VP dynamics is presented. Conjecture is offered about how this internal rotation contributes to the highly unusual VP characteristics.