Abstract
Two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy is employed to identify contributions to fluorescence excitation spectra that arise from both overlapping bands and coupling between zero-order states (ZOSs). Evidence is found for the role of torsional motion in facilitating the coupling between vibrations that particularly involves the lowest-wavenumber out-of-plane vibrational modes. The experiments are carried out on jet-cooled p-fluorotoluene, where the molecules are initially in the lowest two torsional levels. Here we concentrate on the 390–420 cm−1 features in the S1 ← S0 excitation spectrum, assigning the features seen in the 2D-LIF spectrum, aided by separate dispersed fluorescence spectra. The 2D-LIF spectra allow the overlapping contributions to be cleanly separated, including some that arise from vibrational-torsional coupling. Various coupling routes open up because of the different symmetries of the lowest two torsional modes; these combine with the vibrational symmetry to provide new symmetry-allowed vibration-torsion (‘vibtor’) interactions, and the role of the excited m = 1 torsional level is found to be significant.
Highlights
There has been a concentrated effort at understanding the interactions between vibrations and torsions in substituted benzenes, which form vibration-torsional (‘vibtor’) levels
Further work on higher-lying S1 levels has been undertaken using tr-photoelectron spectroscopy (PES),27 ZEKE28,29 and 2D-LIF spectroscopy.29 (A reassignment of the levels discussed in Ref. 27 has been presented in Ref. 29, and a reassignment of some of the levels discussed in Refs. 14 and 19 is presented in Refs. 15, 16 and 30.) The work of Gascooke, Lawrance and coworkers (GL) on toluene and pFT has provided the most-detailed description of the vibtor interactions occurring in both the S0 and S1 states and discussed the necessity of including such interactions in any detailed understanding of the spectroscopy of these molecules
We have recorded separate dispersed fluorescence (DF) spectra with higher averaging to get better signal to noise than taking a vertical slice through the 2D-LIF image. These DF spectra were recorded with the same spectrometer as for the 2D-LIF spectra, and were recorded three times accumulating over 5000 shots each time, and an average taken
Summary
There has been a concentrated effort at understanding the interactions between vibrations and torsions in substituted benzenes, which form vibration-torsional (‘vibtor’) levels. We tackled the p-xylene (pXyl) molecule recently and showed that the vibtor interactions, involving a two-rotor system, were very similar to those of toluene and pFT. We showed that the vibtor levels at ~845 cm-1 associated with the methyl torsion and the first overtone of an out-of-plane vibration in pFT were opening up new routes to interactions involving vibrations of different symmetry. This involved interactions between an excited torsional level of an overtone with vibtor levels formed by combinations with a set of levels at ~ 400 cm-1. The present work is an extension to recently-reported work by GL, which focused on a detailed understanding of the S1 region < 400 cm-1, highlighting interactions occurring for vibtor levels involving the three lowest-wavenumber vibrations
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