Measurements of Ortho-to-para Nuclear Spin Conversion of H2 on Low-temperature Carbonaceous Grain Analogues: Diamond-like Carbon and Graphite

TL;DR

This study investigates how hydrogen molecules change their nuclear spin states on carbonaceous surfaces at very low temperatures, providing insights into astrochemical processes.

Contribution

It experimentally measures the nuclear spin conversion rates of H2 on diamond-like carbon and graphite surfaces, revealing minimal influence of bonding motifs on conversion rates.

Findings

NSC time constants range from ~3900s at 10K to ~750s at 18K

Bonding motifs (sp3/sp2) have little effect on NSC rates

Similar temperature dependence observed for both surfaces

Abstract

Hydrogen molecules have two nuclear spin isomers: ortho-H2 and para-H2. The ortho-to-para ratio (OPR) is known to affect chemical evolution as well as gas dynamics in space. Therefore, understanding the mechanism of OPR variation in astrophysical environments is important. In this work, the nuclear spin conversion (NSC) processes of H2 molecules on diamond-like carbon and graphite surfaces are investigated experimentally by employing temperature-programmed desorption and resonance-enhanced multiphoton ionization methods. For the diamond-like carbon surface, the NSC time constants were determined at temperatures of 10-18 K and from 3900 plusminus 800 s at 10 K to 750 plusminus 40 s at 18 K. Similar NSC time constants and temperature dependence were observed for a graphite surface, indicating that bonding motifs (sp3 or sp2 hybridization) have little effect on the NSC rates.