Davies, Alexander R.
(2022)
Fluorescence and ZEKE spectroscopy of small, jet-cooled aromatic molecules.
PhD thesis, University of Nottingham.
Abstract
A detailed spectroscopic analysis is presented for the S1 ↔ S0 and D0 + ← S1 transitions of meta-fluorotoluene (mFT) and N-methylpyrrole (NMP) through resonance-enhanced multiphoton ionisation (REMPI), laser-induced fluorescence (LIF), dispersed fluorescence (DF), two-dimensional laser-induced fluorescence (2DLIF) and zero-electron-kinetic-energy (ZEKE) spectroscopy. Assignments have been made for various vibrations, torsions and vibration-torsion bands in these spectra, detailing the energy levels in the three electronic states. These studies comprise the most comprehensive spectroscopic studies of these two molecules to date.
For mFT, discussion has been made regarding how vibration-vibration and vibrationtorsion eigenstates couple together and how this evolves from a purely localised, restricted coupling mechanism at low internal energies to widespread, dissipative intramolecular vibrational energy redistribution (IVR) at only modest internal energy. Furthermore, an extreme case of Duschinsky mixing has been examined, which manifests itself in the spectra in a similar way to anharmonic coupling; this has been elucidated through a combination of emission and excitation spectroscopy, quantum chemical calculations and a comparison to meta-chlorotoluene (mClT), in which this mixing is also present, yet not so extreme. In the D0 + cationic state, the activity observed is discussed largely in terms of significant changes in torsional potential compared to the S1 state, leading to off-diagonal torsional transitions having considerable, and often dominant, intensities. Additionally, the ZEKE spectra appear less enlightening in ascertaining the assignment of the activity in S1 than do the 2DLIF spectra, although they are useful as a confirmatory/diagnostic tool.
The S1 ← S0 transition of NMP is electric dipole forbidden and, as such, the activity observed in the REMPI and LIF spectra is largely due to vibronic coupling with nearby, higher-lying electronic states. Routes for the observed vibronic coupling have been proposed and discussed, alongside commentary regarding the results of previous photodynamical studies. The ZEKE spectra of NMP have been investigated, which exhibit highly diagonal Franck-Condon factors, except for those via the b1 symmetry, ring-localised modes. Nevertheless, it was possible to derive a precise adiabatic ionisation energy (AIE) of 64250 ± 5 cm-1. The 2D-LIF spectra are significantly more complicated than one might expect.
Furthermore, a new labelling scheme for N-substituted pyrroles is proposed, which allows for the comparison of vibrations of different molecules of the same symmetry class, which are expected to be valid for almost all (reasonable) substituents. This scheme is similar to those proposed for mono- and disubstituted benzene molecules, and the corresponding scheme for meta-disubstituted benzenes is used in the analysis of mFT.
Finally, REMPI and ZEKE spectra have been recorded for the first time, of the NMP-RG (RG = Ar, Kr) complexes, noting that the presence of the rare gas is sufficient to perturb the electronic structure to allow the observance of the NMP-RG origin in the REMPI spectra. Furthermore, we deduce AIEs of 64077 ± 5 cm-1 and 64029 ± 5 cm-1 for NMP-Ar and NMP-Kr, respectively.
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