Water formation by surface O3 hydrogenation

Abstract

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 H 2 O/D 2 O (and H 2 O 2 /D 2 O 2 ) 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.

Solid state chemistry of nitrogen oxides – Part II: surface consumption of NO2

“…Since the formation of ozone through the oxygenation of molecular oxygen is efficient, reaction NO 2 + O also has to be fast (barrierless or with a small barrier) in the solid phase. 48,49 3.3 NO 2 + N 70 and 80 K when N 2 O is exposed to N atoms, confirming that new species are formed in the ice upon N-atom exposure. Here, NO 2 + N is also studied in an apolar astrochemically relevant CO ice at 15 K. Fig.…”

Surface formation routes of interstellar molecules: hydrogenation reactions in simple ices

“…A spectral resolution between 1 and 4 cm -1 is used and several scans are co-added. In Fuchs et al (2009), Ioppolo et al (2008, 2011a and Romanzin et al (2011), the ice is first deposited and then hydrogenated/deuterated. In this case, RAIR difference spectra with respect to the initial deposited ice are acquired during H/D exposure.…”

“…This approach makes it possible to derive fundamental and molecule specific parameters, like reaction rates and diffusion barriers, which can then be included in astrochemical models to simulate the ice evolution under much longer timescales (10 5 years) than accessible in the laboratory (\1 day). The work presented in the next section follows a bottom-up approach and summarizes a representative sample of relevant experiments (e.g., Watanabe and Kouchi 2002;Watanabe et al 2004Watanabe et al , 2006Fuchs et al 2009;Miyauchi et al 2008;Ioppolo et al 2008Ioppolo et al , 2010Ioppolo et al , 2011aMatar et al 2008;Oba et al 2009Oba et al , 2010Cuppen et al 2010;Mokrane et al 2009;Romanzin et al 2011;Ö berg et al 2009). These experiments prove that species like H 2 CO, CH 3 OH and H 2 O can be formed at low temperatures by simple hydrogenation (i.e., without the need for thermal, UV or cosmic ray processing) and provide the basic molecular data to simulate their formation on astronomical timescales (e.g., Cuppen et al 2009), even though the ice as a whole is not representative for a realistic astronomical ice.…”

“…The latter work shows that the initially proposed reaction network (Tielens and Hagen 1982) of only three mainly isolated channels is too simple and that several of the channels are actually linked through additional reactions. The third water formation channel (the hydrogenation of solid O 3 ) was tested by Mokrane et al (2009) and more recently by Romanzin et al (2011). Since this channel is connected to the O 2 channel after the first reaction step, special care was taken in Romanzin et al (2011) to deposit a pure O 3 ice by keeping the substrate temperature between the O 2 and O 3 desorption temperature during deposition.…”

“…The third water formation channel (the hydrogenation of solid O 3 ) was tested by Mokrane et al (2009) and more recently by Romanzin et al (2011). Since this channel is connected to the O 2 channel after the first reaction step, special care was taken in Romanzin et al (2011) to deposit a pure O 3 ice by keeping the substrate temperature between the O 2 and O 3 desorption temperature during deposition. If such a temperature is also kept during H-atom addition, the O 2 molecules formed upon O 3 hydrogenation will desorb from the surface of the ice.…”