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

Hydrogen molecules have two nuclear spin isomers: ortho-H-2 and para-H-2. 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 H-2 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 +/- 800 s at 10 K to 750 +/- 40 s at 18 K. Similar NSC time constants and temperature dependence were observed for a graphite surface, indicating that bonding motifs (sp(3) or sp(2) hybridization) have little effect on the NSC rates.

Automated summary

This study experimentally investigated the nuclear spin conversion processes of H-2 molecules on diamond-like carbon and graphite surfaces, revealing that bonding motifs have little effect on the NSC rates.