Low-Temperature Chemistry Between Water and Hydroxyl Radicals: H/D Isotopic Effects
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Mon. Not. R. Astron. Soc. 000, 1{?? (2014) Printed 3 October 2018 (MN LaTEX style file v2.2) Low-temperature chemistry between water and hydroxyl radicals: H/D isotopic effects T. Lamberts1;2?, G. Fedoseev1, F. Puletti3, S. Ioppolo2, H. M. Cuppen2, and H. Linnartz1 1 Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, University of Leiden, P.O. Box 9513, NL 2300 RA Leiden, The Netherlands. 2 Theoretical Chemistry, Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands 3 Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK Released 2014 Xxxxx XX ABSTRACT Sets of systematic laboratory experiments are presented - combining UHV cryo- genic and plasma-line deposition techniques - that allow to compare H/D isotopic effects in the reaction of H2O (D2O) ice with the hydroxyl radical OD (OH). The latter is known to play a key role as intermediate species in the solid state formation of water on icy grains in space. The main finding of our work is that the reaction H2O+OD −−! OH+HDO occurs and that this may affect the HDO/H2O abundances in space. The opposite reaction D2O + OH −−! OD + HDO is much less effective, and also given the lower D2O abundances in space not expected to be of astronomical relevance. The experimental results are extended to the other four possible reactions between hydroxyl and water isotopes and are subsequently used as input for Kinetic Monte Carlo simulations. This way we interpret our findings in an astronomical con- text, qualitatively testing the influence of the reaction rates. Key words: astrochemistry { methods: laboratory: molecular { methods: laboratory: solid state { solid state: volatile { ISM: molecules 1 INTRODUCTION tios as found in our oceans (Rodgers & Charnley 2002; Hartogh et al. 2011). Therefore, both the origin of the Surface reactions on grains have been proposed as an effec- HDO/H O ratio in the ices and their subsequent chem- tive way to form water at the low temperatures typical for 2 ical and thermal processing are currently widely studied the interstellar medium (van de Hulst 1949; Tielens & Hagen (Caselli & Ceccarelli 2012). It is in fact during the water for- arXiv:1511.02010v1 [astro-ph.SR] 6 Nov 2015 1982). Over the last ten years many of the possible reactions mation on dust surfaces when the deuterium fractionation have been tested in various laboratories and reaction routes, commences. The preferential incorporation of D over H in rates, and branching ratios have been determined (Hiraoka molecules can lead to D/H ratios in molecules that are much et al. 1998; Miyauchi et al. 2008; Ioppolo et al. 2008, 2010; larger than the primordial ratio of ∼ 1:5 × 10−5 (Piskunov Cuppen et al. 2010; Romanzin et al. 2011). The general con- et al. 1997; Oliveira et al. 2003). To understand the origin of clusions obtained in these studies are in line with each other this fractionation, it is neccessary to consider both the gas- (Dulieu 2011) and the astronomical relevance has been sum- phase and solid-state processes that are at play. The driving marized by van Dishoeck et al. (2013). forces behing these processes are the lower zero-point energy A detailed study of the solid state formation of wa- of an X-D bond with respect to an X-H bond and the dif- ter also holds the potential to characterize deuteration ef- ference in tunneling behavior, both as a result of the larger fects. With the goal to understand how water was deliv- mass of deuterium (Roberts et al. 2003; Tielens 1983; Caselli ered to Earth, there has been much interest in linking the & Ceccarelli 2012; Lipshtat et al. 2004). One of the possi- HDO/H O ratio in cometary, interstellar, and laboratory 2 ble enhancement routes for hydrocarbon bonds is the simple ices as well as astronomical (gas-phase) observations to ra- replacement of a hydrogen by a deuterium via a deuterium mediated abstraction mechanism: −CH + D −−!−C + HD ? corresponding author, current address: Computational Chem- followed by −C + D −−!−CD (Nagaoka et al. 2005, 2007). istry Group, Inst. of Theoretical Chemistry, University of An example is substitution of H for D in solid methanol, Stuttgart, Pfaffenwaldring 55, D-70569, Stuttgart, Germany which has been found to be efficient, whereas the reverse c 2014 RAS 2 T. Lamberts et al. reaction (substituting D for H) is not. Note that Nagaoka Table 1. Summary of the performed experiments with additional et al. (2005) have also attempted to substitute hydrogen in calibration and control experiments and corresponding parame- water by the same process, but did not find any deuteration ters, Tsurf = 15 K and texp = 90 min in all cases. The deposition upon D exposure. The non-occurence of reaction H2O + D rate of the species is denoted as fdep and the angle represents the has been confirmed in our laboratory (unpublished data). angle of the deposition line with respect to the surface. Another abstraction process, so far studied in less de- ◦ ◦ tail, involves OH and OD radicals. Hydroxyl radicals play an 90 fdep 45 Discharge fdep important role as reactive intermediates in water formation (cm2 s−1) (cm2 s−1) (see e.g. Cuppen et al. 2010). Furthermore, recent micro- 18 12 13 a 1a H2 O 4 × 10 D2O 6 1 × 10 scopic models have shown that the radical concentration in 18 12 13 1b H2 O 4 × 10 D2O { 1 × 10 the ice can be very high if photon penetration is included 13 a 1c { { D2O 6 1 × 10 (Chang & Herbst 2014). Garrod (2013) indicated the im- 1d H18O 4 × 1012 {{{ portance of abstraction reactions by the hydroxyl radical 2 12 18 13 a in the framework of complex hydrocarbon molecules. Such 2a D2O 4 × 10 H2 O 6 1 × 10 12 18 13 an OH induced abstraction mechanism is particularly im- 2b D2O 4 × 10 H2 O { 1 × 10 18 13 a portant in water-rich ices, because both OH and OD radi- 2c { { H2 O 6 1 × 10 2d D O 4 × 1012 {{{ cals are produced on the surface by radical chemistry and 2 by photodissociation of water isotopologues (Ioppolo et al. aThe upper limit is derived from the D O and H18O deposition ¨ 2 2 2008; Andersson & van Dishoeck 2008; Oberg et al. 2009). rate when the microwave source is turned off. Considering again the formation of HDO in the ice, the water surface reaction network needs to be duplicated, in- (Lamberts et al. 2015). Therefore, here, only reactions R1 volving both reactions with hydrogen and deuterium. As a and R4 are tested experimentally, at low temperature and result of the large number of reactions that are in compe- using reflection absorption infrared spectroscopy (RAIRS) tition with each other and with diffusion, it is experimen- as an in-situ diagnostic tool. tally challenging to study hydrogenation and deuteration, Note that from an astrochemical point-of-view, reac- simultaneously. Therefore, in the past, either the deutera- tions R2, R4, and R6 are unlikely to be relevant as a result tion pathways have been studied separately, e.g.,O + D 2 of the low concentrations of the reactants (D2O and OD) (Chaabouni et al. 2012), or specific reaction routes are present in the ice. Nevertheless, we put efforts in character- tackled theoretically and experimentally, e.g., OH (OD) + izing R4, as an understanding of the underlying mechanism H2 (HD or D2) and H2O2 (D2O2) + H (D) (Kristensen et al. helps in painting the full picture. Reactions R1 and R3, on 2011; Oba et al. 2012, 2014). Here, we add to these studies the contrary, could occur in regions with a high photon flux and investigate the cross links between the hydrogenation as this causes water and isotopologues to dissociate, thus and deuteration networks by the following four hydrogen generating additional hydroxyl radicals. abstraction reaction via hydroxyl radicals: In the following, we outline the experimental setup and k sets of experiments performed (Section 2), the analysis of H O + OD −!1 OH + HDO (R1) 2 the resulting RAIR spectra (Section 3), the astrochemical k2 HDO + OD −! OH + D2O (R2) implications by means of a Kinetic Monte Carlo model (Sec- tion 4), and we conclude with summarizing remarks (Sec- k3 HDO + OH −! OD + H2O (R3) tion 5). k4 D2O + OH −! OD + HDO : (R4) Hydrogen abstraction of OH from H O (or the fully 2 2 EXPERIMENTAL METHODS deuterated analog) can also take place Two sets of representative experiments and their corre- k5 H2O + OH −! OH + H2O (R5) sponding control experiments are summarized in Table 1. k6 All measurements are performed at a surface temperature D O + OD −! OD + D O (R6) 2 2 of 15 K for a duration of 90 minutes. The two experiments and although this does not have a net effect on the abun- that are used for this study are part of a larger set, varying dances in the ice, it can be seen as an analog of bulk dif- mixing ratios and temperatures, and found to be optimum fusion, of which models indicate it is of great importance for the goals set in this work. The findings of the other ex- in ice chemistry (Vasyunin & Herbst 2013; Lamberts et al. periments are largely in line with the ones discussed here, 2014; Chang & Herbst 2014). The isolated OH-H2O com- but do not provide additional information. 2 plex has been detected in a variety of matrices (Ar, Ne, O2) Experiments are performed using the SURFRESIDE as has been summarized by Do (2014). This shows that a setup, which was constructed to systematically investigate pre-reactive complex indeed can be formed.