Novel effects in the laser induced fluorescence spectrum of C6F+6

Abstract

The B̃ ²A_2u←X̃ ²E_1g(0) laser induced fluorescence excitation spectrum of the hexafluorobenzene cation in a free jet expansion has been measured from 21 500 to 22 580 cm⁻¹. The spectrum displays a number of strong transitions involving the totally symmetric and the Jahn–Teller active vibrations. In addition, many weak transitions involving other vibrational modes are observed using high laser powers. These weak, interspersed transitions are readily detected because the ion is cooled in a supersonic expansion which results in sharp, well resolved lines. These weak transitions derive their intensities by a variety of mechanisms, the most novel of which depends on the presence of mixed cross‐quadratic nuclear potential terms in the ground (²E_1g) state. It is shown that these terms permit the observation of transitions from the vibrationless level of the ground state to combination levels of the excited state involving two modes of different symmetries. The identification of transitions of this type provides the first experimental demonstration of the occurrence of mixed cross‐quadratic nuclear potential terms in spatially degenerate electronic states. The analysis of the spectrum has allowed overall the determination of fundamental frequencies for 16 of the 20 normal modes of C₆F⁺₆ in its B̃ ²A_2u electronic state.