Autodetachment from vibrational levels of the O−2 <i>A</i> 2Π<i>u</i> resonance across its dissociation limit by photoexcitation from O−2 <i>X</i> 2Π<i>g</i>

Abstract

We report the observation of resonance structure in the photodetachment spectrum of O−2 in the 4 eV range, which results from the excitation of autodetaching vibrational levels of the O−2 A–X transition near the dissociation limit. The evolution of the resonances with increasing vibration is simply explained using continuity of the inner part of the vibrational wave functions across the dissociation threshold. This affords the possibility of investigating the DA process at the half-collision, in a kind of ‘‘correspondence limit’’ where the outer turning point slowly recedes and the vibrational wave function incrementally adopts the character of the dissociation continuum. Photoexcitation near one of the resonances results in the population of significantly higher vibrational levels in the O2 a1Δg state (which are cleanly resolved) than the typical ‘‘Franck–Condon’’ pattern observed for nonresonant photodetachment. Finally, hot-band structure is also observed in the detachment spectrum, allowing us to extract a more accurate value of the O−2 vibrational quantum (ΔG=134.4±0.8 meV) by about an order of magnitude over previous determinations.