Vibration and vibration-torsion levels of the S 1 state of para-fluorotoluene in the 580-830 cm -1 range: Interactions and coincidences

A study of the vibration and vibration-torsion levels of para-fluorotoluene in the 580-830 cm region is presented, where a number of features are located whose identity is complicated by interactions and overlap. We examine this region with a view to ascertaining the assignments of the bands; in particular, identifying those that arise from interactions involving various zero-order states (ZOSs) involving both vibrations and torsions. Resonance-enhanced multiphoton ionization (REMPI) is employed to identify the wavenumbers of the relevant transitions, and subsequently zero-kinetic-energy (ZEKE) spectra are recorded to assign the various eigenstates. In some cases, a set of ZEKE spectra are recorded across the wavenumber range of a REMPI feature, and we construct what we term a two-dimensional ZEKE (2D-ZEKE) spectrum, which allows the changing ZOS contributions to the eigenstates to be ascertained. Assignment of the observed bands is aided by quantum chemical calculations and all b and a symmetry vibrational wavenumbers are now determined in the S state and cation, as well as those of the D vibration. We also compare to the activity seen in the corresponding S ← S spectrum of para-difluorobenzene.