Photodissociation and radiative association of HeH+ in the metastable triplet state

TL;DR

This study explores the photodissociation and formation of HeH+ in its metastable triplet state, revealing significant vibrational dependence and the importance of excited states, with implications for astrophysical plasma models.

Contribution

It provides the first detailed calculations of cross sections and rate coefficients for HeH+ triplet state photodissociation and radiative association, including vibrational and electronic effects.

Findings

Photodissociation cross sections vary strongly with vibrational state.

Excited electronic states and non-adiabatic couplings are significant.

Calculated rate coefficients inform astrophysical plasma models.

Abstract

We investigate the photodissociation of the metastable triplet state of HeH+ as well as its formation through the inverse process, radiative association. In models of astrophysical plasmas, HeH+ is assumed to be present only in the ground state, and the influence of the triplet state has not been explored. It may be formed by radiative association during collisions between a proton and metastable helium, which are present in significant concentrations in nebulae. The triplet state can also be formed by association of He+ and H, although this process is less likely to occur. We compute the cross sections and rate coefficients corresponding to the photodissociation of the triplet state by UV photons from a central star using a wave packet method. We show that the photodissociation cross sections depend strongly on the initial vibrational state and that the effects of excited electronic states and non-adiabatic couplings cannot be neglected. We then calculate the cross section and rate coefficient for the radiative association of HeH+ in the metastable triplet state.