Spectroscopy of the Ã(B21)-X̃(A11) transition of jet-cooled fluorobenzene: Laser-induced fluorescence, dispersed fluorescence, and pathological Fermi resonances

Abstract

A detailed study of the S1((1)B2)-S0((1)A1) electronic transition of jet-cooled fluorobenzene has been carried out using laser-induced fluorescence and dispersed fluorescence (DF) spectroscopies. Analysis of over 40 single vibronic level DF spectra resulted in the assignment of 16 fundamental frequencies in the excited electronic state. Progressions in totally symmetric modes, particularly in the ring-breathing mode nu9, feature in both types of fluorescence spectrum. There is also significant activity in non-totally-symmetric modes, with activity in Franck-Condon (FC)-allowed overtones, FC-forbidden combinations induced by Duschinsky mixing, and symmetry-forbidden transitions induced by the same Herzberg-Teller vibronic coupling mechanism that induces the benzene S1-S0 transition. Fermi resonances (FRs) are extensive throughout the spectrum, especially in the important FC-active a1 modes. A consequence of these extensive FRs is that several important previous assignments are shown to be incorrect and have been reassigned here. Ab initio and density functional theory calculations have also been performed to support the experimental assignments.

Assignment of the vibrations of the S, S1, and D+ states of perhydrogenated and perdeuterated isotopologues of chlorobenzene

“…28 Also seen is the b2 mode, M29, which was also observed, although much more weakly than in the DF spectra of FBz. 28 When dispersing from the M30 band, the v = 0 propensity rule is obeyed and an intense band corresponding to the ClBz ground state M30 vibration is also seen, confirming the assignment, and in agreement with the observations in the ZEKE spectrum. 24 DF spectra were also recorded via two bands at 384 and 380 cm -1 , which were extremely similar, and assignments were made to the 35 ClBz-h5 and 37 ClBz-h5 S1…”

“…These have been found to be "bright" in the case of toluene using time-resolved photoelectron spectroscopy in combination with ZEKE spectroscopy, 34 and have also been observed for FBz. 28,30 These are hence expected to be relatively intense here, but may be in Figure 7). The assignment of the major contributors to the two most intense bands in this region thus seems relatively secure; as will be noted later, these assignments are also in line with previous ZEKE and REMPI-PES work.…”

“…state of FBz 28 and also in toluene-h8. 34 These observations led to some uncertainty in deciding on the assignment of these ZEKE features, although the assignment of the S1 state features was consistent with that here; namely, the 1042 cm -1 band was M10M11 (with likely contributions from (M29 FR ) 2 components, and the 1066 cm feature was 7 (we recall that we have noted previously 9 and above, that the wavenumber ordering of the M6 and M7 modes in ClBz are switched compared to FBz; hence, the 7 designation in ref.…”

“…To gain more insight we compare with the DF spectra of FBz by Butler et al 28 who also dispersed the fluorescence from equivalent levels. Looking at the DF spectrum for FBz when exciting M9, strong bands corresponding to the M9, M9 2 and M9 M10 transitions were seen, as well as the M10 fundamental and many higher wavenumber features.…”

Intermolecular vibrations of fluorobenzene-Ar up to 130 cm−1 in the ground electronic state

“…The spectrum shown is a composite of three separately acquired segments. The parent FB spectrum is well known 17 and there are no FB vibrations with low enough energy to contribute in the region displayed in Fig. 1.…”