The observation that the to ratio (OPR) of interstellar HO is smaller than 3 is an important yet unresolved subject in astronomy. We irradiated O embedded in solid H at 3 K with vacuum-ultraviolet (VUV) light and observed IR lines associated with -HO (denoted as HO) and nonrotating HO-(H) (where H denotes -H) but no lines associated with -HO (denoted as HO). After maintaining the matrix in darkness for ∼30 h, the amount of HO decreased, accompanied by an increase in HO-(H) via diffusion of H. After that, the continuous nuclear-spin conversion from H to -H (denoted as H) in solid H over time resulted in the conversion of nonrotating HO-(H) to rotating HO in solid H. The observation of the formation and conversion of HO in our experiment suggests a plausible route in which VUV irradiation of O and H adsorbed on grain surfaces might be responsible for the smaller OPR of interstellar HO.
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