Temperature dependence of ortho-to-para conversion of H2 on amorphous solid waterat around 10 K
H. Ueta,1,2 N. Watanabe,1 T. Hama,1 A. Kouchi1
11Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan
2
National Institute for Material Science, Tsukuba, Ibaraki, Japanwatanabe@lowtem.hokudai.ac.jp

The ortho-to-para abundance ratio (OPR) of H2 is crucial for chemical evolution anddeuterium fractionation of molecules in molecular clouds, because H2 in the ortho-groundstate (J=1) is more energetic (and thus reactive) than that in the para-ground state (J=0) byapproximately 14.6 meV corresponding to 170 K, which is significantly higher than typicaltemperatures of molecular clouds. Since the radiative transformation of molecular nuclear
spins is forbidden in the gas phase, the OPRs of hydrogen molecule (H2) observed towardvarious astronomical objects have been often considered as tracers of chemical history of themolecule.
In the gas phase, spin-exchange reaction with either ionic or neutral hydrogens can alter theOPR of H2. In contrast, little is known about how the nuclear spins behave on cosmic ice dust.
It was often assumed without the experimental evidences that the OPR of H2 formed on thecosmic dust surface is statistical value of 3. Recently, our group has tackled this issue
experimentally and found that the OPR of nascent H2 is 3 at the formation by H-H
recombination on cosmic ice dust [1]. In the preset study, using experimental techniques ofmolecular beam, photostimulated-desorption, and resonance-enhanced multiphoton ionization,
we measured the OPRs of H2 photodesorbed from amorphous solid water (ASW) attemperatures around 10 K, which is an ice dust analogue. We demonstrated that the OPR ofH2 easily varies on ASW and that the rate of spin conversion from ortho to para steeply
increases from 2.4×10-4 to 1.6×10-3 s-1 within the very narrow temperature window of 9.5
to12 K and reach a plateau at temperatures above 12 K [2]. This temperature dependencecannot be explained solely by state-mixing models ever proposed [3]. The temperaturedependence of the conversion rate was very well represented by the power law of T7 where T
is the surface temperature. This indicates that the conversion rate at lower temperatures isdominated by the two-phonon energy dissipation process (Raman process) for the excessenergy (~14.6 meV) arising from the conversion, and at higher temperatures the rate may belimited by a spin-flip transition rate.
From the present and our previous results, astrochemical history of interstellar H2-OPR isdepicted as follows. When H2 molecule is produced by H-H recombination on cosmic ice dust,
the OPR of nascent H2 is 3 [1]. If the H2 is immediately desorbed at the formation, the OPR ofH2 released should be near 3. However, if H2 is trapped even for a short period on the ice dust
surface before desorption, the OPR at desorption strongly depends on the surface temperatureand duration of trapped.

References[1] Watanabe, N., Kimura, Y., Kouchi, A., Chigai, T., Hama, T., Pirronello, V., Astrophys. J.
Lett. 714, L233 (2010).
[2] Ueta, H., Watanabe, N., Hama, T., Kouchi, A., Phys. Rev. Lett. 116, 253201 (2016).
[3] Sugimoto, T., Fukutani, K., Nature Physics 7 307 (2011).