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

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, H-2, and O-2, 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.