The Low-Temperature Nuclear Spin Equilibrium of H3+ in Collisions with H2

TL;DR

This study measures the nuclear spin ratios of H3+ colliding with H2 at low temperatures, showing results consistent with thermal equilibrium and informing interstellar chemistry models.

Contribution

It provides the first experimental measurements of H3+ / H2 nuclear spin ratios at low temperatures, confirming theoretical predictions.

Findings

Results align with thermal equilibrium expectations.

Experimental data agree with previous micro-canonical models.

Implications for diffuse interstellar medium chemistry.

Abstract

Recent observations of H2 and H3+ in diffuse interstellar sightlines revealed a difference in the nuclear spin excitation temperatures of the two species. This discrepancy comes as a surprise, as H3+ and H2 should undergo frequent thermalizing collisions in molecular clouds. Non-thermal behavior of the fundamental H3+ / H2 collision system at low temperatures was considered as a possible cause for the observed irregular populations. Here, we present measurements of the steady-state ortho/para ratio of H3+ in collisions with H2 molecules in a temperature-variable radiofrequency ion trap between 45-100 K. The experimental results are close to the expected thermal outcome and they agree very well with a previous micro-canonical model. We briefly discuss the implications of the experimental results for the chemistry of the diffuse interstellar medium.