Vibration-modified torsional potentials and vibration-torsion (“vibtor”) levels in the m-fluorotoluene cation

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 S 1 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 S 1 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 G 6 molecular symmetry to Cs point group symmetry.

Variations in Duschinsky rotations in m-fluorotoluene and m-chlorotoluene during excitation and ionization

“…In addition, the Franck-Condon factors between the vibrations and associated vibtor levels will differ for the S1  S0 and D0 +  S1 transitions, depending on the main geometry changes between the respective pair of electronic states. These have been discussed previously 6,7 for mFT and mClT, where the changes are very similar.…”

“…In Figure 1 we show the REMPI spectra of the first 500 cm -1 above the origin of the S1  S0 transition in mFT and mClT. The 0-350 cm -1 region of the mFT spectrum has been discussed in detail previously, in terms of 2D-LIF and ZEKE spectroscopy, 6,8 while the corresponding region of the mClT spectrum has also been discussed relating to ZEKE spectroscopy. 7 Because of the consistent vibrational labelling used for both molecules, 15 it can be seen that the activity in both spectra is similar.…”

“…We shall also refer to the methyl torsional motion for mFT and mClT, for which the G6 molecular symmetry group (MSG) is appropriate, and we shall use those symmetry labels throughout. The torsional levels will be labelled via their m quantum number, 6,8 and the correspondence between the Cs point group labels and the G6 MSG ones is given in Table I. To calculate the overall symmetry of a vibtor level, it is necessary to use the corresponding G6 label for the vibration, and then find the direct product with the symmetry of the torsion (Table I), noting that a C3v point group direct product table can be used, since the G6 MSG and the C3v point group are isomorphic.…”

“…In contrast, owing to nuclear-spin and rotational symmetry, the molecules can be in one of the m = 0 or m = 1 torsional levels, 24 with residual population in the m = 2 levels also seen. 6,7,8…”

“…Very recently, we published resonance-enhanced multiphoton ionization (REMPI) and zero-electronkinetic-energy (ZEKE) studies of the low-wavenumber regions of m-fluorotoluene (mFT) 6 and mchlorotoluene (mClT), 7 which mainly focused on torsions and vibration-torsion (vibtor) levels. The mFT study complemented the two-dimensional laser-induced fluorescence (2D-LIF) study of Stewart et al, who examined the first 350 cm -1 of the S1  S0 transition.…”