Zero electron kinetic energy spectroscopy of the para-fluorotoluene cation

Abstract

Zero electron kinetic energy (ZEKE) spectroscopy is employed to gain information on the vibrational energy levels of the para-fluorotoluene (pFT) cation. Vibrationally resolved spectra are obtained following excitation through a range of intermediate vibrational energy levels in the S1 state. These spectra allow the observation of different cationic vibrational modes, whose assignment is achieved both from a knowledge of the S1 vibrational states and also by comparison with density functional calculations. In one notable case, clean ZEKE spectra were obtained from two overlapped S1 features. From the authors' data, the adiabatic ionization energy of pFT was derived as 70,946+/-4 cm(-1). The information on the cationic energy levels obtained will be useful in untangling the intramolecular vibrational redistribution dynamics of pFT in the S1 state.

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)

“…In the present experiments, it was confirmed that the photoionization cross-section for toluene was about an order of magnitude lower than that of the para-fluorotoluene (pFT) molecule we have studied previously; 47 this is in line with conclusions reached by Bacon and Hollas 48 with regard to the oscillator strength of the S 1 ← S 0 transition for toluene compared to other monosubstituted benzenes. As such, there was a continual compromise between resolution and signal-to-noise ratios in the ZEKE spectra and this limited the range of S 1 resonances we could employ as intermediates.…”

“…The excitation laser has been calibrated by comparison of the bands observed in the (1 + 1) REMPI spectrum in the present work and the fluorescence excitation spectrum reported by Hickman et al, 3 with the origin position calibrated to that reported by Borst and Pratt; 31 use was also made of the known positions of the S 1 origin of pFT, 47 observed in a recent series of experiments with an absolute error of ±1 cm −1 ; we estimate our relative error to be ∼0.5 cm -1 . Calibration of the ionization laser has been performed through comparison of the ZEKE bands observed in the present work and those seen for toluene by Lu et al 42 …”

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

“…At 441 cm -1 is the intense, symmetryallowed D11 ZEKE band and combination band with this vibration are also present in most spectra. The assignments of these higher-wavenumber features are largely the same as in our previous publication, 37 albeit with a different nomenclature; we shall address this higherwavenumber region in more detail in a future publication. 55…”

“…The recorded spectrum is very similar to that reported by us in ref. 37, and so we only show a vertically-expanded view of the lowwavenumber region of the new spectrum in Figure 3, with an expanded view of the first 150 cm -1 presented in Figure 4(b). All ZEKE spectra have been calibrated to an adiabatic ionization energy of 70946 cm -1 corresponding to the origin transition, in line with our earlier value.…”

“…All ZEKE spectra have been calibrated to an adiabatic ionization energy of 70946 cm -1 corresponding to the origin transition, in line with our earlier value. 37 Close to the very strong v = 0 origin band, there are a number of significantly weaker bands, with the first of these appearing as a barely-discernible shoulder on the blue edge of the origin band; however, it can be seen more clearly in the ZEKE spectrum recorded via the S1 m originating at m = 1 in the S0 state will appear in the REMPI spectrum at a transition wavenumber that is lower than the actual S1 relative wavenumber by the S0 m = 1 -m = 0 spacing; this shift will also be present for such bands in the ZEKE spectra.) The second band at 48 cm -1 may be assigned to one of the m = 3 levels, but it is not immediately clear whether this should be to the m = 3(+) or m = 3(-) level and we shall discuss this further below.…”