Complexity surrounding an apparently simple Fermi resonance in p-fluorotoluene revealed using two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy

Abstract

Two-dimensional laser-induced fluorescence (2D-LIF) spectroscopy is a powerful tool allowing overlapped features in an electronic spectrum to be separated, and interactions between vibrations and torsions to be identified. Here the technique is employed to assign the 790-825 cm -1 region above the origin of the S1  S0 transition in para-fluorotoluene (pFT), which provides insight into the unusual time-resolved results of Davies and Reid [Phys. Rev. Lett. 109, 193004 (2012)]. The region is dominated by a pair of bands that arise from a Fermi resonance; however, the assignment is complicated by contributions from a number of overtones and combinations, including vibration-torsion ("vibtor") levels. The activity in the 2D-LIF spectra is compared to recently reported zero-electron-kinetic-energy (ZEKE) spectra [Tuttle et al. J. Chem. Phys. 146, 244310 (2017)] to arrive at a consistent picture of the energy levels in this region of the spectrum.

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

“…26) and those at ~800 cm -1 (Ref. 27) using 2D-LIF, and compared the results to our earlier ZEKE work. 18,21,23 The assignments of the 2D-LIF spectra indicated that both vibrational interactions and vibration-torsional coupling were occurring.…”

“…As well as 111 (see Figure 5), there are also other emissions, with those in the range ~840-855 cm -1 being assigned as 142, 91 and 292. (Note that there was some indication that, as well as 9 1 and 29 2 being in Fermi resonance in the S1 state, 27,47 the 91 and 292 levels may be in Fermi resonance in S0, and the same was true of + 9 1 and + 29 2 in D0 + . 27 )…”

“…(Note that there was some indication that, as well as 9 1 and 29 2 being in Fermi resonance in the S1 state, 27,47 the 91 and 292 levels may be in Fermi resonance in S0, and the same was true of + 9 1 and + 29 2 in D0 + . 27 )…”

“…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. 23 and 27).…”

“…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.…”