Quantification of O₂ formation during UV photolysis of water ice: H₂O and H₂O:CO₂ ices

Abstract

Context. The Rosetta and Giotto missions investigated the composition of the cometary comae of 67P/Churyumov-Gerasimenko and 1P/Halley, respectively. In both cases, a surprisingly large amount of molecular oxygen (O 2 ) was detected and was well correlated with the observed abundances of H 2 O. Laboratory experiments simulating chemical processing for various astronomical environments already showed that formation of solid state O 2 is linked to water. However, a quantitative study of O 2 formation upon UV photolysis of pure H 2 O and H 2 O dominated interstellar ice analogues is still missing. Aims. The goal of this work is to investigate whether the UV irradiation of H 2 O-rich ice produced at the earliest stages of star formation is efficient enough to explain the observed abundance of cometary O 2 . Methods. The photochemistry of pure H 16 2 O (H 18 2 O) as well as mixed H 2 O:CO 2 (ratio of 100:11, 100:22, 100:44) and H 2 O:CO 2 :O 2 (100:22:2) ices was quantified during UV photolysis. Laser desorption post-ionisation time of flight mass spectrometry (LDPI TOF MS) was used to probe molecular abundances in the ice as a function of UV fluence. Results. Upon UV photolysis of pure amorphous H 2 O ice, deposited at 20 K, formation of O 2 and H 2 O 2 is observed at abundances of, respectively, (0.9 ± 0.2)% (O 2 /H 2 O) and (1.3 ± 0.3)% (H 2 O 2 /H 2 O). To the best of our knowledge, this is the first quantitative characterisation of the kinetics of this process. During the UV photolysis of mixed H 2 O:CO 2 ices, the formation of the relative amount of O 2 compared to H 2 O increases to a level of (1.6 ± 0.4)% (for H 2 O:CO 2 ratio of 100:22), while the (H 2 O 2 /H 2 O) yield remains similar to experiments with pure water. In an ice enriched with O 2 (2%), the O 2 level increases up to 7% with regard to H 2 O, at low UV fluence, which is higher than expected on the basis of the enrichment alone. The resulting O 2 /H 2 O values derived for the H 2 O and H 2 O:CO 2 ices may account for a (substantial) part of the high oxygen amounts found in the comae of 67P and 1P.

Vacuum UV photodesorption of organics in the interstellar medium: an experimental study of formic acid HCOOH and methyl formate HCOOCH3-containing ices

“…These desorptions may originate from the desorption from the solid water itself, since the measured yields are comparable to the O 2 and OH photodesorption yields from pure water ice. [50][51][52] However, O 2 desorption is not modied in the case of HCOOCH 3 mixed with water, and no OH signal could be measured for these systems. However, contrary to the case of methyl formate, OH is expected to be formed via the dissociation of HCOOH, even if its desorption from pure formic acid could not be detected.…”

Disentangling UV photodesorption and photoconversion rates of H2O ice at 20 K

“…We expected the depletion of the parent species to be due to the formation of photoproducts, H 2 , OH, O 2 , HO 2 , and H 2 O 2 . Only recently, the first quantitative study of the formation of typically elusive O 2 and H 2 O 2 was reported -it also applied LDPI TOF MS (Bulak et al 2022). In experiments favoring bulk processes, the authors of the study exposed an H 2 O ice (100 ML thickness) deposited at 20 K to a UV photon fluence of 1.8 × 10 18 photons cm −2 .…”

“…At higher photon fluence, the depletion of H 2 O continues at a linear rate, while the abundances of detected photoproducts are expected to either remain on the same level or decrease (Bulak et al 2022). This means that the formation of O 2 and H 2 O 2 cannot account for the continuing loss of H 2 O.…”

“…In our experiments, the photoconversion depletes the H 2 O column density twice as fast as the photodesorption. The formation of photoproducts O 2 and H 2 O 2 can account for the initial photoconversion of water until a UV fluence of 9.0 × 10 17 photon cm −2 , each at a formation level of 1% of H 2 O (Bulak et al 2022). The continuing formation of OH radicals can be of significance for the ice chemistry in mixed ices, opening pathways toward oxygen-carrying molecules such as CO 2 , CH 3 OH, HCOOH, or HCOOCH 3 (Öberg et al 2009aGarrod et al 2008).…”

The formation of CO2 through consumption of gas-phase CO on vacuum-UV irradiated water ice

“…An indepth investigation of O 2 formation is beyond the scope of this work, but it is briefly discussed to explain the decrease in CO 2 production. For more information on O 2 production, we refer to a recent study that quantitatively investigated the production of O 2 and H 2 O 2 by VUV irradiation of H 2 O (Bulak et al 2022).…”