arXiv:1405.7839v1 [astro-ph.SR] 30 May 2014 Mon. Not. R. Astron. Soc. 000, 1–7 (2013) Printed 2 June 2014 (MN L A T E X style file v2.2) Vacuum-UV absorption spectroscopy of interstellar ice analogs. Isotopic effects. G. A. Cruz-Diaz 1 ⋆ , G. M. Mu ˜ noz Caro 1 , and Y.-J. Chen 2,3 1 Carretera de Ajalvir, km 4, Torrejon de Ardoz, 28850 Madrid, Spain 2 Space Sciences Center and Department of Physics and Astronomy, University of Southern California, Los Angeles, CA 90089-1341, USA 3 Department of Physics, National Central University, Jhongli City, Taoyuan Country 32054, Taiwan Accepted 2013 ——–. Received 2013 ——–; in original form 2013 ——– ABSTRACT This paper reports the first measurements of solid-phase vacuum-ultraviolet (VUV) absorption cross sections of heavy isotopologues present in icy dust grain mantles of dense interstellar clouds and cold circumstellar environments. Pure ices composed of D 2 O, CD 3 OD, 13 CO 2 , and 15 N 15 N were deposited at 8 K, a value similar to the coldest dust temperatures in space. The column density of the ice samples was measured in situ by infrared spectroscopy in transmittance. VUV spectra of the ice samples were collected in the 120-160 nm (10.33-7.74 eV) range using a commercial microwave discharged hydrogen flow lamp as the VUV source. Prior to this work, we have recently submitted a similar study of the light isotopologues (Cruz- Diaz et al. 2013a; Cruz-Diaz et al. 2013b). The VUV spectra are compared to those of the light isotopologues in the solid phase, and to the gas phase spectra of the same molecules. Our study is expected to improve very significantly the models that estimate the VUV absorption of ice mantles in space, which have often used the available gas phase data as an approximation of the absorption cross sections of the molecular ice components. We will show that this work has also important implications for the estimation of the photodesorption rates per absorbed photon in the ice. Key words: interstellar ice analogs – VUV-absorption cross section. 1 INTRODUCTION After molecular hydrogen (H 2 ), the molecules H 2 O, CO, CO 2 , and CH 3 OH, are among the most abundant in the interstellar medium, as it has been inferred from observations of the gas and solid phase (Mumma & Charnley 2011, and references therein). The main el- ements, and their corresponding isotopes, which compose most volatile molecules in the interstellar medium, are H:D, 12 C: 13 C, 14 N: 15 N, and 16 O: 17 O: 18 O. Deuterium enrichment can be the result of low temperature gas-grain reactions because of the differences in zero-point ener- gies between deuterated and non-deuterated species (Wilson et al. 1973). Observation toward prestellar cores indicates, in gas phase, that abundances of singly deuterated molecules are typically higher than the cosmic atomic D/H ratio of 1.5 × 10 -5 (Linsky 2003), also, doubly and triply deuterated molecules have been observed with D/H ratios reaching ∼ 30 % for D 2 CO and ∼ 3 % for CD 3 OH (see, Ceccarelli et al. 1998; Loinard et al. 2002; Parise et al. 2004; Rata- jczak et al. 2009). Deuterated methanol molecules were detected in the gas phase toward low-mass class 0 protostars with abundances up to about 60 % relative to CH 3 OH (Parise et al. 2006). D 2 O has ⋆ E-mail: cruzdga@cab.inta-csic.es been detected toward the solar-type protostar IRAS 16293-2422 (Butner et al. 2007; Vastel et al. 2010). Roberts (2003) showed that the multiply deuterated isotopologues of H 3 + can efficiently trans- fer deuterons to other neutral molecules in very cold ( 20 K) gas depleted of its CO (because the CO molecules are frozen onto re- fractory dust grain mantles). Isotopic substitution often alters the chemical and physical properties of atoms and molecules, resulting in differences in ab- sorption spectra and reaction rates. Therefore, measurements of the isotopic compositions of various species can be used to interpret the physico-chemical histories and the chemical reaction pathways in these environments. In particular, isotope effects in the non- dissociative photoionization region of molecular nitrogen play an important role in isotopic fractionation in planetary atmospheres and other environments (e.g., interstellar molecular clouds, the so- lar nebula, and in the atmospheres of Earth, Mars, and Titan) in which N 2 and VUV radiation are present (see, Croteau et al. 2011, and references therein). Carbon dioxide is an important constituent of quiescent and star forming molecular clouds (Gerakines et al. 1999, and ref- erences therein). It is primarily present in the solid state (van Dishoeck et al. 1996). The 13 CO 2 isotope has been detected with a two orders of magnitude lower abundance with respect to CO 2