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 c hlorobenzene cation were recorded via the origin, 6b, and 16a16b vibrations of the cation ground state ((X) over tilde(+)). The resonance-enhanced mul ti photon dissociation spectroscopy (REMPD) spectrum of the (B) over tilde( +)<--(X) over tilde(+) transition of the chlorobenzene cation was also obta ined. 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 all owed this hypothesis to be tested, because the 16a vibration would be the p rimary 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 vi brational 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 multi configurational self-consistent field (CASSCF) calculations with 6-31G** ba sis sets were performed to ascertain the symmetry assignments of the excite d ionic states. These calculations resulted in the possibility that at leas t one excited state of the cation of B-2(1) symmetry lies below any state o f 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).