Abstract

Explicit evidence for the role of methyl rotor levels in promoting energy dispersal is reported. A set of coupled zero-order vibration/vibration-torsion (vibtor) levels in the S1 state of para-fluorotoluene (pFT) are investigated. Two-dimensional laser-induced fluorescence (2D-LIF) and two-dimensional zero-kinetic-energy (2D-ZEKE) spectra are reported, and the assignment of the main features in both sets of spectra reveals that the methyl torsion is instrumental in providing a route for coupling between vibrational levels of different symmetry classes. We find that there is very localized, and selective, dissipation of energy via doorway states, and that, in addition to an increase in the density of states, a critical role of the methyl group is a relaxation of symmetry constraints compared to direct vibrational coupling.

Highlights

  • We find that there is very localized, and selective, dissipation of energy via doorway states, and that, in addition to an increase in the density of states, a critical role of the methyl group is a relaxation of symmetry constraints compared to direct vibrational coupling

  • The process of energy dispersal is o en termed intramolecular vibrational redistribution (IVR) as studies are usually focused on vibrational coupling,[4,5,6,7,8,9,10,11,12] but, following on from earlier work,[13] recent studies have generalized this to include vibration-torsional coupling in cases where an internal rotor is present in the molecule.[14,15,16,17]

  • The symmetries of levels corresponding to vibrational combinations and overtones, as well as vibtor level symmetries, can be found in the G12 molecular symmetry group using a direct product table for the D3h point group, to which it is isomorphic

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Summary

Introduction

The dispersal of energy through a molecule can be a valuable aid to increasing its stability following the input of localized energy, for example following photoexcitation or as the result of the formation of a chemical bond.[1,2,3] The process of energy dispersal is o en termed intramolecular vibrational redistribution (IVR) as studies are usually focused on vibrational coupling,[4,5,6,7,8,9,10,11,12] but, following on from earlier work,[13] recent studies have generalized this to include vibration-torsional coupling in cases where an internal rotor is present in the molecule.[14,15,16,17] If energy cannot rapidly be dispersed, the molecule may dissociate following photoexcitation, or a nascent chemical bond may break again in a biomolecular reaction. It has been used to analyze mixtures by Neij et al.[45] and Kable and coworkers,[46] and as a detailed probe of rovibronic structure.[44,47,48] More recently, it has given highly informative information on the role of internal rotation in vibrational energy dispersal by Lawrance, Gascooke and coworkers.[14,17,23,34,44] DF and LIF spectra can be recorded separately, each can be obtained from a 2D-LIF spectrum; and there is much more information in a 2D-LIF spectrum than in the separate cases.[44]

Experimental
G12 Symmetry a1
Summary
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