Formation of Para-H₂O by Vacuum-UV Photolysis of O₂ in Solid Hydrogen: Implication for Astrochemistry

Abstract

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.

A Plausible Model for the Galactic Extended Red Emission: Graphene Exposed to Far-ultraviolet Light

“…Photoabsorption and PL experiments were performed at beamline TLS-03 of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan; the experimental setup is described elsewhere (Chou et al 2020(Chou et al , 2022. Exposure of graphene samples to intense far-UV light in vacuum was performed at undulator beamline TLS-21A2, NSRRC (Wu et al 2012;Lin et al 2022). Conditions of exposure to far-UV light under large and small densities of photon flux were achieved via an adjustment of the position of a sample relative to the focal point of the beam.…”

Vacuum UV Photolysis of Benzene in Solid Nitrogen

“…The experimental setup was similar to that reported previously. 41,42 VUV light was generated from the undulator beamline TLS-21A2, NSRRC; the photon flux was estimated to be ∼1.0 × 10 15 photons s −1 , with a bandwidth of approximately 1.5%. A nickel-plated copper target was cooled to serve as the cold substrate for the matrix samples, which also served as a mirror to reflect the probed IR beam to the detector.…”