Observation of predissociation and tunnelling processes in O-2(2+): A study using Doppler free kinetic energy release spectroscopy and ab initio CI calculations

A Doppler free kinetic energy release spectrum of O-2(2+) has been recorded at a line width (FWHM) of 40-60 meV using a coincidence time-of-flight technique. Vibrational structure has been resolved in the W (3) Delta(u), B (3) Sigma(u)(-), 1 (1) Sigma(u)(-) 1 (1) Delta(u), B (3) Pi(g) and 1 (1) Pi(g) electronic states. Complete active space self-consistent field (CASSCF) and multi-reference configuration interaction (MRCI) calculations have been performed to predict vibrational levels, Franck-Condon factors and dissociation channels. For the B (3) Pi(g) and 1 (1) Pi(g) states, spin-orbit interaction with the 1 (5) Sigma(g)(+) state leads to predissociation into the ground-state ionic pair O+(S-4) + O+ (S-4). Spin-orbit interaction with the repulsive 1 (5) Pi(u) and C (3) Pi(u) states accounts for the dissociation of the B (3) Sigma(u)(-), 1 (1) Sigma(u)(-) and 1 (1) Delta(u) electronic states into O+ (S-4) + O+ (D-2). The W (3) Delta(u) and 1 (5) Sigma(u)(-) states probably dissociate by tunnelling into the ion pair O+ (S-4) + O+ (D-2) through potential barriers which separate these states from the dissociation continuum.