Assignment of the B̃+ State of the Chlorobenzene Cation Using Photoinduced Rydberg Ionization (PIRI) Spectroscopy

Abstract

The photoinduced Rydberg ionization (PIRI) spectra of the B̃+ state of the chlorobenzene cation were recorded via the origin, 6b, and 16a16b vibrations of the cation ground state (X̃+). The resonance-enhanced multi photon dissociation spectroscopy (REMPD) spectrum of the B̃+←X̃+ transition of the chlorobenzene cation was also obtained. To date it has been thought that B̃+←X̃+ is an electronically forbidden transition (C2v symmetry), taking place from the 2B1 ground state to a 2B2 excited state. The ability of PIRI to provide spectra from specific lower-state vibrational levels allowed this hypothesis to be tested, because the 16a vibration would be the primary inducing mode in the transition. Assuming a forbidden transition, a comparison between the spectrum from the ground-state origin and that from the 16a16b vibration would necessitate an assignment that gives unlikely vibrational frequencies. It is therefore concluded that the B̃+←X̃+ transition of chlorobenzene is electronically allowed. Configuration interaction of singles (CIS) and complete active space multiconfigurational self-consistent field (CASSCF) calculations with 6-31G** basis sets were performed to ascertain the symmetry assignments of the excited ionic states. These calculations resulted in the possibility that at least one excited state of the cation of 2B1 symmetry lies below any state of 2B2 symmetry. Hence, we propose that the ionic transition observed in the acquired PIRI/REMPD spectra of the cation is an allowed transition to a 2B1 state, thus giving rise to the observation of the origin of the B̃+ state at 18 219 cm-1.