Experimental studies of surface reactions among OH radicals that yield H2O and CO2 at 40–60 K

Abstract

We investigated the OH-related formation routes of two astrophysically important molecules, H₂O and CO₂, under relatively warm astrophysical conditions. OH radicals, together with other neutral species such as H, O, H₂, and O₂, were produced in H₂O microwave-discharge plasma and cooled to 100 K before being deposited on an Al substrate at 40–60 K. H₂O formed at 40 and 50 K, but not at 60 K. Taking the experimental conditions into account, a possible route of H₂O formation is via reactions involving OH + OH, which yield H₂O₂ as the main reaction product. The present study is the first to show experimentally that surface reactions of two OH radicals can yield H₂O at low temperatures. The products' branching ratio was 0.2 and 0.8 for H₂O and H₂O₂, respectively. When CO was co-deposited with neutral species that formed in the H₂O plasma, CO₂ was formed at 40–60 K. H₂CO₃ formed at 40 and 50 K. The present results may suggest that chemical reactions related to OH radicals are effective at yielding various molecules in relatively warm astrophysical environments, such as protostars.