Ortho-para-H$_2$ conversion processes in astrophysical media

TL;DR

This review summarizes recent quantum calculations of ortho-para H$_2$ conversion rates in astrophysical media, highlighting the mechanisms, temperature dependence, and astrophysical implications of these processes.

Contribution

It presents new quantum calculations of H$_2$ conversion rates using accurate potential energy surfaces, providing insights into the mechanisms across a wide temperature range.

Findings

Proton exchange dominates at low temperatures (≤100 K).

Hydrogen atom contribution becomes significant above 300 K.

Resonance structures observed in H$^+$+H$_2$ reactions due to H$_3^+$ intermediate.

Abstract

We report in this review recent fully-quantum time-independent calculations of cross sections and rate constants for the gas phase ortho-to-para conversion of H$_2$ by H and H$^+$. Such processes are of crucial interest and importance in various astrophysical environments. The investigated temperature ranges was 10$-$1500 K for H+H$_2$ and 10$-$100 K for H$^+$+H$_2$. Calculations were based on highly accurate H$_3$ and H$_3^+$ global potential energy surfaces. Comparisons with previous calculations and with available measurements are presented and discussed. It is shown that the existence of a long-lived intermediate complex H$_3^+$ in the (barrierless) H$^+$+H$_2$ reaction give rise to a pronounced resonance structure and a statistical behaviour, in contrast to H+H$_2$ which proceeds through a barrier of $\sim 5000$ K. In the cold interstellar medium ($T\leq 100$ K), the ortho-to-para conversion is thus driven by proton exchange while above $\sim$300 K, the contribution of hydrogen atoms become significant or even dominant. Astrophysical applications are briefly discussed by comparing, in particular, the relative role of the conversion processes in the gas phase ({\it via} H, H$^+$, H$_2$ and H$_3^+$) and on the surface of dust particles. Perspectives concerning future calculations at higher temperatures are outlined.