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Hydrogen molecules have two nuclear spin isomers: ortho-H2 and para-H2. Theortho-to-para ratio (OPR) is known to affect chemical evolution as well as gasdynamics in space. Therefore, understanding the mechanism of OPR variation inastrophysical environments is important. In this work, the nuclear spinconversion (NSC) processes of H2 molecules on diamond-like carbon and graphitesurfaces are investigated experimentally by employing temperature-programmeddesorption and resonance-enhanced multiphoton ionization methods. For thediamond-like carbon surface, the NSC time constants were determined attemperatures of 10-18 K and from 3900 plusminus 800 s at 10 K to 750 plusminus40 s at 18 K. Similar NSC time constants and temperature dependence wereobserved for a graphite surface, indicating that bonding motifs (sp3 or sp2hybridization) have little effect on the NSC rates.

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