The observation of strong pseudo-Jahn-Teller activity in the benzene cation (B)over-tilde(2)E(2g) state

Photoinduced Rydberg ionization (PIRI) spectroscopy. has been applied to the problem of recording vibrationally resolved spectra of the dipole-forbidden (B) over tilde E-2(2g) <-- (X) over tilde E-2(1g) transition of C6H6+ and C6D6+. PIRI spectra of the (B) over tilde state have been recorded via a number of vibrational slates of the ionic ground state. A combination of Herzberg-Teller and pseudo-Jahn-Teller vibronic coupling between the (B) over tilde state and a close lying (C) over tilde state were used to explain the complex (B) over tilde state vibrational structure. The two pseudo-Jahn-Teller vibrational modes, nu(16) and nu(17), were both found to be active in the PIRI spectra. In addition, evidence for a strong pseudo-Jahn-Teller interaction within each of these modes was found. The perturbed vibronic bands resulting from this pseudo-Jahn-Teller activity were successfully modeled using a two-mode pseudo-Jahn-Teller vibronic coupling model. Using a combination of the spectral results and the pseudo-Jahn-Teller calculations, we were able to generate absolute vibronic symmetry assignments for many of the lower (B) over tilde state vibrational levels, from which vibrational assignments were made. The calculations also produced unperturbed vibrational frequencies for both pseudo-Jahn-Teller active modes along with values for their respective linear coupling constants. (C) 1997 American Institute of Physics.