Assignment of the B+ state of the chlorobenzene cation using photoinduced Rydberg ionization (PIRI) spectroscopy

The photoinduced Rydberg ionization (PIRI) spectra of the Bi state of the chlorobenzene cation were recorded via the origin, 6b, and 16a16b vibrations of the cation ground state ((X) over tilde(+)). The resonance-enhanced multi photon dissociation spectroscopy (REMPD) spectrum of the (B) over tilde(+)<--(X) over tilde(+) transition of the chlorobenzene cation was also obtained. To date it has been thought that (B) over tilde(+)<--(X) over tilde(+) is an electronically forbidden transition (C-2 upsilon, symmetry), taking place from the B-2(1) ground state to a B-2(2) 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 (B) over tilde(+)<--(X) over tilde(+) 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 B-2(1) symmetry lies below any state of B-2(2) symmetry. Hence, we propose that the ionic transition observed in the acquired PLRI/REMPD spectra of the cation is an allowed transition to a B-2(1) state, thus giving rise to the observation of the origin of the (B) over tilde(+) state at 18 219 cm(-1).