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