低温星間塵表面における水素分子のオルソ–パラ転換

抄録

Molecular hydrogen (H₂) has nuclear spin isomers: ortho-H₂ and para-H₂. Under the isolated condition, the nuclear spin conversion (NSC) between these isomers is forbidden. The ortho-to-para ratio, OPR, of H₂ is important in chemistry and physics of interstellar media. The OPR in space has been considered to evolve only via the gas phase reactions with proton and hydrogen atoms. Because interstellar H₂ is produced predominantly on the surface of cosmic dust and gaseous H₂ molecules inevitably collide with dust, the NSC process on the surface of cosmic dust should be quantitatively qualified to know the evolution of OPR. Recently, we revealed that the H₂ NSC occurs on cosmic dust analogs within laboratory timescale. In this article, we introduce the methods for observing NSC processes, the NSC time constants determined for the surfaces of amorphous solid water, amorphous-Mg₂SiO₄, and diamond-like carbon at low temperatures, and implication of the results to astrophysics.