Formation of Para -H 2 O by Vacuum-UV Photolysis of O 2 in Solid Hydrogen: Implication for Astrochemistry.

The observation that the ortho to para ratio (OPR) of interstellar H 2 O is smaller than 3 is an important yet unresolved subject in astronomy. We irradiated O 2 embedded in solid H 2 at 3 K with vacuum-ultraviolet (VUV) light and observed IR lines associated with para -H 2 O (denoted as p H 2 O) and nonrotating H 2 O-( o H 2 ) n (where o H 2 denotes ortho -H 2 ) but no lines associated with ortho -H 2 O (denoted as o H 2 O). After maintaining the matrix in darkness for ∼30 h, the amount of p H 2 O decreased, accompanied by an increase in H 2 O-( o H 2 ) n via diffusion of o H 2 . After that, the continuous nuclear-spin conversion from o H 2 to para -H 2 (denoted as p H 2 ) in solid H 2 over time resulted in the conversion of nonrotating H 2 O-( o H 2 ) n to rotating p H 2 O in solid p H 2 . The observation of the formation and conversion of p H 2 O in our experiment suggests a plausible route in which VUV irradiation of O 2 and H 2 adsorbed on grain surfaces might be responsible for the smaller OPR of interstellar H 2 O.