Vibrational excitation and dissociative recombination of the LiH + ion

Abstract

LiH+ is one of the remaining enigmas among small, simple molecules, in its unexpectedly high dissociative recombination (DR) rate. The dissociative recombination process, in which a colliding electron is captured, results in dissociation and destruction of the molecule. The conventional mechanism characteristic of most species with a high DR rate, which normally involves direct capture into a dissociative resonance potential curve, is absent here. For this reason, this is a prototype for indirect DR that must instead proceed through Rydberg capture pathways. We apply ab initio multichannel quantum defect theory (MQDT), in combination with a rovibrational frame transformation based on Siegert pseudostates, to treat indirect DR for this fundamental diatomic ion. The starting point is a set of ab initio quantum defects determined using the R-matrix approach, functions of the internuclear distance. The calculated DR rate coefficient agrees with recent experimental data [S. Krohn, et al., Phys. Rev. Lett. 86, 4005 (2001)]. Found to play a key role are complex resonance manifolds that couple closed channels having both high and low principal quantum numbers, of the same type studied in molecular Rydberg spectroscopy by Jungen and co-workers over the years.