Progress in understanding the intramolecular vibrational redistribution dynamics in the S1 state of para-fluorotoluene

Abstract

We employ zero-kinetic-energy (ZEKE) photoelectron spectroscopy with nanosecond laser pulses to study intramolecular vibrational redistribution (IVR) in S(1) para-fluorotoluene. The frequency resolution of the probe step is superior to that obtained in any studies on this molecule to date. We focus on the behavior of the 13(1) (C-CH(3) stretch) and 7a(1) (C-F stretch) vibrational states whose dynamics have previously received significant attention, but with contradictory results. We show conclusively that, under our experimental conditions, the 7a(1) vibrational state undergoes significantly more efficient IVR than does the 13(1) state. Indeed, under the experimental conditions used here, the 13(1) state undergoes very little IVR. These two states are especially interesting because their energies are only 36 cm(-1) apart, and the two vibrational modes have the same symmetry. We discuss the role of experimental conditions in observations of IVR in some detail, and thereby suggest explanations for the discrepancies reported to date.

Torsion and vibration-torsion levels of the S1 and ground cation electronic states of para-fluorotoluene

“…The apparatus employed has been described previously in detail elsewhere, 51 incorporating small modifications in order to perform the two-colour ZEKE experiments, which have also been described, 52 and so only a brief description is given here. For all experiments, the ionization laser was a dye laser (Sirah Cobra-Stretch) operating with Pyrromethene 597 laser dye and was pumped with the second harmonic (532 nm) of a Surelite I Nd:YAG laser.…”

Vibrations of the low energy states of toluene ($\tilde X$X̃ 1A1 and $\tilde A$Ã 1B2) and the toluene cation ($\tilde X$X̃ 2B1)

“…45, incorporating small modifications in order to perform the ZEKE experiments, which have also been described, 46 and so only a brief description is given here. The second (532 nm) and third (355 nm) harmonics of a neodymium-doped yttrium aluminium garnet laser (Nd:YAG, Surelite III, 10 Hz) were each used to pump one of two tuneable dye lasers (Sirah Cobra Stretch).…”

Methyl rotor dependent vibrational interactions in toluene

“…In addition to the mode numbering for the methyl vibrations, the addition of the methyl group adds the complication that the molecule can no longer be classified strictly as C 2v , although much of its spectroscopy can be understood within the constraints of that group. Toluene is properly assigned to the permutationinversion group G 12 . In order to assign the methyl modes or, more generally, the modes of a poly-atom mono-substituent, we suggest that they be treated as a separate set and labeled according to the Mulliken convention 54 in the appropriate symmetry group.…”

“…This means labeling the modes in ascending order based on the vibrational frequencies arranged in descending order within each symmetry class, with the group's symmetry classes arranged in the usual order. In the toluene case, the methyl modes would be labeled in G 12 , with the symmetry classes arranged in the order A 1 , A 2 , A 1 , and A 2 . Gardner and Wright have introduced the prefix M to indicate their mode numbering scheme.…”

“…[1][2][3][4][5][6] In order to provide a detailed understanding of the mechanisms leading to this effect, Reid and co-workers have developed the technique of picosecond time resolved photoelectron spectroscopy as a probe of the behavior of relatively low-lying vibrational levels, with particular application to toluene and pFT. [7][8][9][10][11][12][13][14] The aim of the experiments has been to investigate couplings where there are comparatively few potential interactions, allowing the mechanism involved to be deduced.…”