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 (C 2 v 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.

CASPT2 study on electronic states of the C6H5Cl+ ion

“…Our assignment of the B state of the C 6 H 5 Cl + ion to 1 2 B 2 (and assignment of the C state to 2 2 B 1 ) is in line with the assignment presented in all the published papers concerning excited states of the C 6 H 5 Cl + ion, except the paper of Anand et al [6]. In 1999 Anand et al [6] suggested assignment of the B state of C 6 H 5 Cl + to 2 B 1 based on their analyses on the vibrationally resolved PIRI spectra.…”

“…In 1999 Anand et al [6] suggested assignment of the B state of C 6 H 5 Cl + to 2 B 1 based on their analyses on the vibrationally resolved PIRI spectra. However, 3 years later Kwon et al [8] again assigned the B state of C 6 H 5 Cl + to 2 B 2 based on their analyses on the vibrational spectra measured by mass-analyzed threshold ionization (MATI), and in their paper [8] they argued that in the PIRI experiments the C 6 H 5 Cl neutral prepared in a Rydberg state correlating with the cation ground state by two-photon absorption had been further excited and autoionzation or fragmentation in the B or higher electronic states had been observed, and in contrast the one-photo MATI technique did not suffer from the complication related to multiphoton absorption.…”

“…Electronic states of the chlorobenzene ion, C 6 H 5 Cl + , have been extensively investigated experimentally and computationally [1][2][3][4][5][6][7][8][9][10]. However, the assignment of the B state of the C 6 H 5 Cl + ion is still controversy.…”

“…The B state had been assigned to 2 B 2 in all the papers on electronic states of this ion published before 1999. Anand et al [6] assigned the B state to 2 B 1 based on their analyses on the vibrationally resolved PIRI spectra in 1999. However, Kwon et al [8] assigned the B state to 2 B 2 again based on their analyses on the vibrational spectra measured by mass-analyzed threshold ionization (MATI) in 2002.…”

“…The T 0 values for excited states of the C 6 H 5 Cl + ion could be considered to be equal to the differences between the adiabatic ionization potential (AIP) values for excited states and the AIP value for the ground state (1 2 B 1 ) of the ion, and therefore the experimental T 0 values for the X, A, B, and C states of the C 6 H 5 Cl + ion were evaluated to be 0.0, 0.64, 2.26, and 2.63 eV, respectively, using the experimental AIP values reported by Potts et al [7] in 2000. Based on the CASPT2//CASSCF T 0 calculations, the X, A, B, C, and D states of the C 6 H 5 Cl + ion were assigned to the 1 2 B 1 , 1 2 A 2 , 1 2 B 2 , 2 2 B 1 , and 1 2 A 1 states, respectively [10], and the assignment of the B state to 1 2 B 2 (C to 2 2 B 1 ) was in contrast to the assignment (B to 2 B 1 ) of Anand et al [6]. Difficulties in assignments of the B and C states of the C 6 H 5 Cl + ion are caused by the fact that the two states are closely lying.…”