Ab initio studies on the electronic excited states and photodissociation of O3 anion

Abstract

Extensive ab initio calculations have been performed for the low-lying electronic states of O3− to elucidate the mechanism of photodissociation processes. The identity of the mysterious state implied by the recent experiment of Continetti et al. has been discussed based on the current calculations. Calculations reveal that B22 is a truly bound state favoring a strongly bent geometry with ∠O–O–O∼90.0°, and crosses with the X 2B1 at a similar angle. Therefore, if O3− is produced in a highly bent geometry, B22 might be preferentially populated. The large transition dipole moment and the vertical excitation energy for B22→2A1 also suggest that B22 may be electronically excited efficiently to A12 at the wavelength of 523 nm. The computed energetics of B22 and B12 and the corresponding dissociation limits may explain the larger maximum kinetic energy release (KER) observed in the second experiment of Continetti and the smaller O2–O− bond energy derived from the experiment of Hiller, if we assume that B22 is the parent state in both cases. Furthermore, meta-IRC (intrinsic reaction coordinate) calculations suggest rather different final state distribution of the photofragments from B12→2A2 and B22→2A1 processes, in qualitative agreement with the experimental observations. Although the vibrationally excited ground state O3− might also produce rotational hot, vibrational cold photofragments through the angular dependence of the seam between the two diabatic excited A″2 states, the exact effect of parent vibrational excitation requires future dynamics calculations. At the current stage, our calculations strongly support that the B22 electronic state has been accessed in the second experiment of Continetti et al.