VUV irradiation of carbon dioxide (CO 2 ) and ammonia (NH 3 ) complexes in argon matrix J.B. Bossa, F. Duvernay * , P. Theulé, F. Borget, T. Chiavassa Physique des Interactions Ioniques et Moléculaires, UMR 6633, Université de Provence et CNRS, Centre de St Jérôme, case 252, 13397 Marseille Cedex 20, France article info Article history: Received 23 June 2008 Accepted 23 October 2008 Available online 30 October 2008 Keywords: Matrix isolation FTIR spectroscopy Photochemistry Carbamic acid Molecular complexes abstract In this study, we characterized by FTIR spectroscopy the 1:1 and 2:1 molecular complexes between NH 3 and CO 2 in argon matrix at low temperature. In addition we tentatively identified the 1:2 molecular com- plex. The structures of the complexes were established from B3LYP calculations. Moreover, we showed that the VUV irradiation of the 1:1 complex forms the NH 2 OH:CO complex. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction The interstellar medium (ISM) is a rich reservoir of molecules. Two of them, CO 2 and NH 3 coexist in a variety of astrophysical environments and are two of the most abundant molecules to have been identified into the icy mantle of the interstellar grains [1]. In several places of the ISM, those grains are exposed to VUV radia- tion coming from the stars that can induce the formation of new products [2,3]. Low temperature irradiation of a NH 3 :CO 2 binary ice in laboratory has been investigated in the past and these stud- ies showed that carbamic acid can be produced in such conditions [4,5]. Moreover, it has been shown that the photolysis of a mixture of methylamine and CO 2 leads to the formation of glycine, the sim- plest amino acid [6]. Therefore, the understanding of the photo- chemical processes of amine–CO 2 binary mixtures are of particular importance for astrochemistry. The rare gas cryogenic matrices isolation technique is a conve- nient tool to study molecular complexes by infrared spectroscopy and provides a simplified model of photoreactivity in an interstel- lar grain. Experimental and theoretical vibrational modes fre- quency provide valuable information on the complex formation and its possible geometrical structure. Moreover, the isolated molecular complex formed in a cryogenic matrix can induce a selective mechanism for a particular reaction induced by UV pho- tolysis and thus give an insight into the mechanism [7]. The goal of these studies is twofold. First, we characterize the 1:1, 2:1 and 1:2 molecular complexes between CO 2 and NH 3 in a cryogenic matrix by FTIR spectroscopy. We identify the complex structures and give their vibrational assignments with the help of quantum calculation. Secondly, we focus on the VUV photolysis of the 1:1 NH 3 :CO 2 molecular complex. We show that the main pho- tochemical product is a 1:1 molecular complex between NH 2 OH and CO. We proposed a mechanism to explain the formation of this complex. 2. Experimental Argon (Air liquide, 99.99% purity), carbon dioxide (Linde, 99.99% purity) and ammonia (Air liquide, 99.9% purity) gases are mixed in different ratios in a pyrex bulb using standard manometric tech- niques. The gaseous sample is deposited at a 8 mbar/min rate on a gold platted surface kept at 10 K at a constant pressure of 10 À7 mbar. The annealing of the sample is achieved by a 4 K/min heating rate warming up of the surface at 10 K, 20 K, 30 K and 36 K and then re-cooled at 10 K. The VUV irradiation (k > 120 nm) is realized using a microwave discharge hydrogen flow lamp (Opthos instru- ments). The infrared spectra of the samples are recorded at 10 K, in a reflection mode between 4000 and 600 cm À1 using a Nicolet Magna 750 FTIR spectrometer with a MCT detector. Each spectrum is averaged over one hundred scans and has a 0.12 cm À1 resolution. 3. Computational All calculations are performed using the GAUSSIAN 98 package [8]. The structures and energies of the monomers and complexes 0301-0104/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2008.10.026 * Corresponding author. Tel.: +33 49 128 8580; fax: +33 49 163 6510. E-mail address: fabrice.duvernay@univ-provence.fr (F. Duvernay). Chemical Physics 354 (2008) 211–217 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys