Unravelling overlaps and torsion-facilitated coupling using two-dimensional laser-induced fluorescence

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.

Observation of the onset of torsion-induced, mode-specific dissipative intramolecular vibrational redistribution (IVR)

“…In this region we expect to see combinations involving levels on which we have recently reported, at ~400 cm -1 (Ref. 26) and ~800 cm -1 (Ref. 27); additionally, combinations involving the levels at ~400 cm -1 and those at ~845 cm -1 (Ref.…”

“…In other papers, we have provided correlations with the labels used in previous work to aid the reader in referring to earlier studies. 23,26 Note that we shall refer to previously calculated values (B3LYP/aug-cc-pVTZ) of the vibrational wavenumbers in the three electronic states pertinent to the present study, presented in our previous work -those for the S1 and D0 + states are taken from Ref. 23, while those for the S0 state are from Ref.…”

“…In Figure 1, we show a REMPI spectrum of pFT up to 1250 cm -1 . Indicated are a set of bands at ~400 cm -1 that are dominated by 14 2 , 29 1 and 11 1 transitions, 18,21,22,26 but where there are also additional, including vibtor, transitions. 21,22,26 At ~800 cm -1 there are a further set of transitions, dominated by a pair of levels that largely comprise a FR between the 9 1 and 29 2 levels, 23,27,46,47 but again, there are other transitions in this region, including those involving vibtor levels (see Refs.…”

“…Recently, we have examined the bands close to 400 cm -1 (Ref. 26) and those at ~800 cm -1 (Ref. 27) using 2D-LIF, and compared the results to our earlier ZEKE work.…”

“…The ability to describe the making and breaking of chemical bonds and the flow of energy through a molecule necessitates a knowledge of the internal energy level structure; in particular, vibrations and torsions. Building on work by Parmenter and coworkers, 1,2,3,4,5,6,7 and the group of Weisshaar, 8,9,10 recent work by the Lawrance group and ourselves has identified that vibration-torsional ("vibtor") coupling is of key importance in the following para-substituted molecules that contain methyl groups: toluene, 11,12,13,14,15,16,17 pFT 18,19,20,21,22,23,24,25,26,27 and para-xylene (pXyl). 25,28,29 Some of the most recent work has employed the technique of two-dimensional laser-induced fluorescence (2D-LIF), which has been reviewed recently.…”