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

David J Kemp,Timothy G Wright,Laura E Whalley,William D Tuttle,Lewis G Warner,Adrian M Gardner

doi:10.1063/1.5083682

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, 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 the recently reported zero-electron-kinetic-energy 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.

Highlights

Understanding the internal energy level structure in molecules is a key quest of chemical and molecular physicists
We have studied the lower-wavenumber regions of the S1 S0 transition in parafluorotoluene using resonance-enhanced multiphoton ionization (REMPI) and zero-kineticenergy (ZEKE) spectroscopy;[12,13,17,20] while both we[13,20] and the Lawrance group[16] have employed the technique of two-dimensional laser-induced fluorescence (2D-LIF).[21]
That there are a fair number of zero-order states (ZOSs) contributing to the 790–825 cm-1 region, the present results and those of Ref. 27 indicate that we are still in the restricted intramolecular vibrational redistribution (IVR) regime, but with the situation being far more complicated than a simple Fermi resonance

Summary

Introduction

Understanding the internal energy level structure in molecules is a key quest of chemical and molecular physicists. Chemistry can be further affected by the rotation of the molecule and the torsional motion of hindered rotor groups. All of these motions can couple, this coupling can usually be treated as a perturbation on the overall vibrational energy structure. Building on work by Parmenter and coworkers,[1,2,3,4] recent work by the Lawrance group and ourselves has identified that vibration-torsional (“vibtor”) coupling is of key importance in molecules that contain methyl groups: toluene, 5,6,7,8,9,10,11, para-fluorotoluene (pFT) 12,13,14,15,16,17 and para-xylene (pXyl).[17,18,19] Both a combination of an increasing density of states (DOS) and symmetry-allowed vibtor coupling have been invoked to rationalize the rapid increase in interactions that occur in such molecules,[17] which drives energy dispersal through a molecule

Results

Discussion

Conclusion