Abstract
Zero-kinetic-energy (ZEKE) spectra are presented for m-fluorotoluene, employing different low-lying (<350 cm-1) intermediate torsional and vibration-torsional ("vibtor") levels of the S1 state. The adiabatic ionization energy (AIE) is found to be 71 997 ± 5 cm-1 (8.9265 ± 0.0006 eV). It is found that the activity in the ZEKE spectra varies greatly for different levels and is consistent with the assignments of the S1 levels deduced in the recent fluorescence study of Stewart et al. [J. Chem. Phys. 150, 174303 (2019)]. For cation torsional levels, the most intense band corresponds to changes in the torsional quantum number, in line with the known change in the phase of the torsional potential upon ionization. This leads to the observation of an unprecedented number of torsions and vibtor levels, with the pronounced vibtor activity involving out-of-plane vibrations. Interactions between levels involving torsions are discussed, with evidence presented, for the first time it is believed, for modification of a torsional potential induced by a vibration. Also, we discuss the possibility of distortion of the methyl group leading to a change from G6 molecular symmetry to Cs point group symmetry.
Highlights
The coupling of methyl torsion and vibrational motions has been the subject of a series of studies on toluene,1–7 parafluorotoluene,8–18 and para-xylene15,19,20 using a combination of fluorescence and photoionization spectroscopies
These studies have shown that such coupling can be examined and understood in detail in the low-wavenumber region and is likely to be prevalent to higher wavenumbers, driving the transition to statistical (“dissipative”) intramolecular vibrational redistribution (IVR)
Since neither Wilson30/Varsányi31 nor Mulliken32/Herzberg33 notations are appropriate for the vibrations of mFT,34,35 we shall employ the Di labels from Ref. 35, as used in the recent work by Stewart et al.22. This Cs point group labeling scheme is based on the vibrations of the meta-difluorobenzene molecule, for which the S1 ← S0 transition has been investigated using laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectroscopy.36 (While mDFB has C2v point group symmetry, the labeling scheme in Ref. 35 was developed to be applicable to both symmetric and asymmetric substitutions.) not pursued in the present work, we find that the vibrational activity in the corresponding electronic transition of mDFB is similar to that in mFT, consistent with comparisons made for the corresponding para-substituted molecules
Summary
The coupling of methyl torsion and vibrational motions has been the subject of a series of studies on toluene, parafluorotoluene (pFT), and para-xylene (pXyl) using a combination of fluorescence and photoionization spectroscopies. These studies have shown that such coupling can be examined and understood in detail in the low-wavenumber region and is likely to be prevalent to higher wavenumbers, driving the transition to statistical (“dissipative”) intramolecular vibrational redistribution (IVR). The work of Stewart et al. and the present study build on that of Ito and coworkers who have recorded laser-induced fluorescence (LIF), dispersed fluorescence (DF), resonance-enhanced multiphoton ionization (REMPI), and ZEKE spectra of the low-wavenumber region of mFT. It will be seen that mFT is an excellent molecule for investigating torsions
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