Formation of Para-H2O by Vacuum-UV Photolysis of O2 in Solid Hydrogen: Implication for Astrochemistry
Shu-Yu Lin, Wen-Jian Huang, Sheng-Lung Chou, Hui-Fen Chen, and Yu-Jong Wu*
2023/05/25
The observation that the ortho to para ratio (OPR) of interstellar H2O is smaller than 3 is an important yet unresolved subject in astronomy. We irradiated O2 embedded in solid H2 at 3 K with vacuum-ultraviolet (VUV) light and observed IR lines associated with para-H2O (denoted as pH2O) and nonrotating H2O–(oH2)n (where oH2 denotes ortho-H2) but no lines associated with ortho-H2O (denoted as oH2O). After maintaining the matrix in darkness for ∼30 h, the amount of pH2O decreased, accompanied by an increase in H2O–(oH2)n via diffusion of oH2. After that, the continuous nuclear-spin conversion from oH2 to para-H2 (denoted as pH2) in solid H2 over time resulted in the conversion of nonrotating H2O–(oH2)n to rotating pH2O in solid pH2. The observation of the formation and conversion of pH2O in our experiment suggests a plausible route in which VUV irradiation of O2 and H2 adsorbed on grain surfaces might be responsible for the smaller OPR of interstellar H2O.
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