Water formation by surface O3 hydrogenation

Three solid state formation routes have been proposed in the past to explain the observed abundance of water in space: the hydrogenation reaction channels of atomic oxygen (O + H), molecular oxygen (O-2 + H), and ozone (O-3 + H). New data are presented here for the third scheme with a focus on the reactions O-3 + H, OH + H and OH + H-2, which were difficult to quantify in previous studies. A comprehensive set of H/D-atom addition experiments is presented for astronomically relevant temperatures. Starting from the hydrogenation/deuteration of solid O-3 ice, we find experimental evidence for H2O/D2O (and H2O2/D2O2) ice formation using reflection absorption infrared spectroscopy. The temperature and H/D-atom flux dependence are studied and this provides information on the mobility of ozone within the ice and possible isotope effects in the reaction scheme. The experiments show that the O-3 + H channel takes place through stages that interact with the O and O-2 hydrogenation reaction schemes. It is also found that the reaction OH + H-2 (OH + H), as an intermediate step, plays a prominent (less efficient) role. The main conclusion is that solid O-3 hydrogenation offers a potential reaction channel for the formation of water in space. Moreover, the nondetection of solid ozone in dense molecular clouds is consistent with the astrophysical picture in which O-3 + H is an efficient process under interstellar conditions. (C) 2011 American Institute of Physics. [doi:10.1063/1.3532087]