Timothy G. Wright
University of Nottingham, UK
Title: Unpicking vibrational and vibrational torsional couplings in substituted benzenes
Biography
Biography: Timothy G. Wright
Abstract
We initially present vibrationally-resolved electronic spectra obtained using resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The spectra are obtained from jet-cooled seeded expansion using lasers. The spectra exhibit many bands, identifying the energetic positions of vibrational levels in the S1 electronic state; a number of these are found to arise from overlapped and/or interacting vibrational levels. By fixing one laser at the energy of one of those levels, we then ionize the electronically-excited molecule and record zero-kinetic-energy (ZEKE) spectra, whose assignment allows the deduction of the make-up of the intermediate S1 vibrational levels. In many cases we can identify the so-called zero-order states (ZOSs) which have coupled to give the resultant eigenstate; this coupling occurs as a result of Fermi resonance. As well as “pure” vibrations, we find that these ZOSs may be torsional levels or vibration-torsion (vibtor) levels. The coupling of the ZOSs leads to levels whose motions are more delocalized across the molecule. This has implications for photostability and chemical control. Assignment of the spectra is aided by recording ZEKE spectra at different energies through a REMPI feature that corresponds to coupled ZOSs. In this way, we can see activity move in and out of resonance through the feature. By plotting these spectra together we obtain a “two-dimensional” ZEKE spectrum. Quantum chemical calculations are used to aid in the assignments. The treatment of the torsional levels requires the use of molecular symmetry groups: G12 for toluene and para-fluorotoluene; G72 for para-xylene.
