Identification of isotopomers in natural abundance using two dimensional laser induced fluorescence: S1–S0 spectral shifts for the four 13C isotopomers of fluorobenzene

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We have examined the S1←S0 transition of toluene in the region from the 0(0)(0) band to ∼210 cm(-1) above it. The spectrum reveals methyl rotor levels of 0(0) toluene up to m = 6 and of the lowest frequency vibration, 20(1), up to m = 4. The rotor levels of both 20(1) and 0(0) are perturbed by torsion-vibration coupling. The inclusion of torsion-vibration coupling leads to the S1 torsional barrier, V6, being revised from -26.376 cm(-1) to -5.59 cm(-1). The torsion-vibration coupling constant is determined to be 21.1 cm(-1). This situation is the S1 analogue of that recently reported for S0 toluene [Gascooke et al., J. Chem. Phys. 142, 024315 (2015)]. Torsion-vibration coupling alters both the rotor band positions and the rotational contours, which particularly affects the rotational constants associated with motion around the a-axis, about which the methyl group rotates. Every vibrational state (indicated generically by X) will be involved in the corresponding X - X20(1) torsion-vibration coupling; so, this interaction permeates the vib-rotor manifold, providing a mechanism to enhance intramolecular vibrational energy redistribution.

Section: Introductionmentioning

confidence: 61%

Torsion-vibration coupling in S1 toluene: Implications for IVR, the torsional barrier height, and rotational constants

Gascooke

Virgo

Lawrance

2015

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“…One possibility is in terms of a coincident band, and among others, Lawrance and co-workers have shown that 13 C isotopologues of substituted benzenes give rise to features in the excitation spectrum that generally appear ∼4 cm −1 to higher wavenumbers than the 12 C bands. 49 As such, by reference to Fig. 2, it may be seen that the 13 C-mFT m 1 band is expected to lie underneath the 12 C-mFT band, and as can be seen from Fig.…”

Section: (D) and Table Ii] The Fact That Only Low-frequencymentioning

confidence: 85%

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

Kemp

Fryer

Davies

et al. 2019

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.

“…Our investigation is based on the technique of two-dimensional laser induced fluorescence (2D-LIF). [16][17][18][19][20][21][22][23][24][25] This technique involves measuring a region of the dispersed fluorescence spectrum as the laser is scanned over absorption features. The dispersed fluorescence effectively provides a filter that allows different species to be separated.…”

Section: Introductionmentioning

confidence: 99%

Methyl rotor dependent vibrational interactions in toluene

Gascooke

Lawrance

2013

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The methyl rotor dependence of a three state Fermi resonance in S 1 toluene at ∼460 cm −1 has been investigated using two-dimensional laser induced fluorescence. An earlier time-resolved study has shown the Fermi resonance levels to have different energy spacings at the two lowest methyl rotor states, m = 0 and 1 [J. A. Davies, A. M. Green, and K. L. Reid, Phys. Chem. Chem. Phys. 12, 9872 (2010)]. The overlapped m = 0 and 1 spectral features have been separated to provide direct spectral evidence for the m dependence of the resonance. The resonance has been probed at m = 3a 1 for the first time and found to be absent, providing further evidence for a large change in the interaction with m. Deperturbing the resonance at m = 0 and 1 reveals that the m dependence arises through differences in the separations of the "zero-order," locally coupled states. It is shown that this is the result of the local "zero-order" states being perturbed by long-range torsion-vibration coupling that shifts their energy by small amounts. The m dependence of the shifts arises from the m = ±3n (n = 1, 2, . . . ) coupling selection rule associated with torsion-rotation coupling in combination with the m 2 scaling of the rotor energies, which changes the E for the interaction for each m. There is also an increase in the number of states that can couple to m = 1 compared with m = 0. Consideration of the magnitude of reported torsion-rotation coupling constants suggests that this effect is likely to be pervasive in molecules with methyl rotors.