The ortho-to-para ratios (OPRs) of molecules observed toward various targets have been often considered as tracers of chemical history of those molecules because the radiative transformation of molecular nuclear spins are forbidden in the gas phase. In molecular clouds, the OPR of H2 is crucial for chemical evolution and deuterium fraction of molecules through reaction, H3+ + HD → H2D+ + H2, because H2 in the ortho-ground state (J=1) is more energetic and thus reactive than that in the para-ground state (J=0) by approximately 14.6 meV corresponding to 170 K which is significantly higher than the typical cloud temperatures. In contrast to the gas phase, although it was not obvious how the nuclear spins behave on ice dust because of lack of experiments, it was often assumed that the OPR of H2 formed on the dust is statistical value of 3. However, recently, the OPR was found to change significantly on the ice surface. The mechanism of ortho-to-para conversion is not very clear. Recently, our group has tackled this issue experimentally. To understand what controls the conversion rate, the ice-temperature-dependence of the rate is measured with the experimental techniques of molecular beam, photostimulated-desorption, and resonance-enhanced multiphoton ionization, we measured the OPRs of H2 photodesorbed from ice at around 10 K. We obtained the clear evidence that the conversion rate of H2 strongly depends on the surface temperature. This result indicates that the rates is limited by energy dissipation processes via 2 phonons. I will review our recent experiments regarding this.

Ortho-to-para ratios of hydrogen molecules desorbed from ice at around 10 K: What happens on cosmic ice dust?

253rd National Meeting (2017)

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