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water, ortho-to-para ratio, OPR, vacuum ultraviolet, VUV
The Collaborative Research of NSRRC Scientist Yu-Jong Wu and User Hui-Fen Chen Highlighted as a Cover of Journal of Physical Chemistry Letters
The collaborative research of Dr. Yu-Jong Wu and Prof. Hui-Fen Chen revealed that vacuum–UV irradiation of O2 and H2 adsorbed on a grain surface might be a plausible route for contributing to a smaller ortho-to-para ratio of interstellar H2O. The research was highlighted in a cover of Journal of Physical Chemistry Letters.
Water is an important species widely found in various astro-environments. It exists in two isomers with distinct total nuclear spin, namely para-H2O and ortho-H2O. Following spin statistics, the ortho-to-para ratio (OPR) of H2O is 3 at temperatures greater than 50 K. In the past 30 years, non-statistical OPR values of water have been obtained in various astro-enviroments, which were considered to convey the message of the thermal process under 30 K during its formation. However, recent experimental result revealed that the OPR of H2O desorbed from interstellar ices was 3 regardless of the formation temperature. Thus, the OPR of interstellar H2O smaller than 3 is an important yet unresolved subject in astrochemistry.

The research team led by the NSRRC Scientist, Dr. Yu-Jong Wu and NSRRC user, Prof. Hui-Fen Chen in Medicinal and Applied Chemistry of Kaohsiung Medical University worked in collaboration on this issue. They proposed a plausible route for explaining the observation of low OPR in space. Vacuum-UV light generated from TLS 21A2 was used to irradiate O2 in solid H2 which made the preferential formation of interstellar para-H2O via the reaction of O + H2. According to their experimental results, the H2O formed in this scheme can produce an OPR as small as ~1.6 upon photolysis. This paper has been featured on the inside cover of Journal of Physical Chemistry Letters.