Formation of H$_3^-$ by radiative association of H$_2$ and H$^-$ in the interstellar medium

TL;DR

This paper develops a theoretical framework to study the formation of H$_3^-$ ions via radiative association of H$_2$ and H$^-$, with implications for detecting H$^-$ in the interstellar medium.

Contribution

It introduces a close-coupling theory for atom-diatom radiative association and applies it to H$_3^-$ formation, providing new calculations of rovibrational levels and formation cross sections.

Findings

Calculated rovibrational levels of H$_3^-$ ground state.

Determined cross sections for H$_3^-$ formation via radiative association.

Discussed the potential observability of H$_3^-$ in interstellar environments.

Abstract

We develop the theory of radiative association of an atom and a diatomic molecule within a close-coupling framework. We apply it to the formation of H$_3^-$ after the low energy collision (below 0.5 eV) of H$_2$ with H$^-$. Using recently obtained potential energy and permanent dipole moment surfaces of H$_3^-$, we calculate the lowest rovibrational levels of the H$_3^-$ electronic ground state, and the cross section for the formation of H$_3^-$ by radiative association between H$^-$ and ortho- and para-H$_2$. We discuss the possibility for the H$_3^-$ ion to be formed and observed in the cold and dense interstellar medium in an environment with a high ionization rate. Such an observation would be a probe for the presence of H$^-$ in the interstellar medium.