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National audience; In the coldest regions (≈10 K) of the interstellar medium (ISM), most molecular species apart from H2 accrete on dust grains to form ice mantles, acting as particularly rich molecular reservoirs. Nevertheless, large amounts of gas phase species are also detected in these regions and their existence can be explained by desorption processes at the ice surface. Among these processes, the desorption induced by UV radiations, known as UV photodesorption, has been the subject of several experimental studies on different compounds, such as CO1,2, the second most abundant species in the interstellar medium. It has been shown that the UV photodesorption in pure CO and mixed CO:N2 ices follows an indirect "Desorption Induced by Electronic Transition" (DIET) mechanism.3 In this astrochemical context, the theoretical work presented here aims to understand, at the molecular level, the vibrational energy redistribution mechanism in pure CO and binary CO:N2 ices. The chosen approach is the molecular dynamics one, either ab initio (AIMD) with the VASP software or based on classical force fields.4 In particular, the focus is on the end of the DIET mechanism where the electronic energy of the excited molecule, redistributed on some high vibrational states of its electronic ground state, is transferred to neighbouring molecules inducing or not a desorption. To do this, an aggregate approach is used to model the amorphous structure of the simulated ice. These aggregates, initially created, optimised, and then thermalised at temperatures similar to those of the ISM, are then used in molecular dynamics simulations in which a randomly selected CO or N2 molecule is excited at given vibrational levels (from v=5 to 40). The energy redistribution in translational, rotational, and vibrational modes taking place in the aggregate, after a single molecule vibrational excitation, is analysed and compared for the two molecular dynamics approaches used in this study.References[1] M. Bertin, E.C. Fayolle, C. Romanzin, K.O. Öberg, X. Michaut, A. Moudens, L. Philippe, P. Jeseck, H. Linnartz, and J.-H. Fillion, Phys. Chem. Chem. Phys. 14, 9929 (2012).[2] E.C. Fayolle, M. Bertin, C. Romanzin, X. Michaut, K.O. Öberg, H. Linnartz, and J.-H. Fillion, Astrophys. J. 739, L36 (2011).[3] M. Bertin, E.C. Fayolle, C. Romanzin, H.A.M. Poderoso, X. Michaut, K.O. Öberg, H. Linnartz, and J.-H. Fillion, Astrophys. J. 779, 120 (2013).[4] M.C. van Hemert, J. Takahashi, and E.F. van Dishoeck, J. Phys. Chem. A 119, 6354 (2015).

International audience; In the coldest parts (~10K) of the interstellar medium (ISM), most of the molecular species except H2 accrete onto dust grains leading to the formation of chemically rich physisorbed molecular ice mantles, referred to as molecular ices. However, surprisingly large abundances of gas phase molecules are also observed in these cold regions. Due to the cold temperature, the existence of these gaseous species is naturally suggested to be the result of non-thermal desorption processes at the ice surfaces. One of these processes is the desorption induced by UV photons, namely UV photodesorption, which has been studied for various molecular ices including CO1,2 for which it has been shown that the UV photodesorption follows a “Desorption Induced by Electronic Transition (DIET)” mechanism.3 In this astrochemical context and in the framework of the ANR PIXyES (Photodesorption Induced by UV-X-rays and Electrons on ice Surfaces), this study aims at understanding, at the molecular level, the energy redistribution after VUV excitation in pure CO ice by means of molecular dynamics using a classical force field.4 The focus is on the end of the DIET mechanism where the electronic energy of the excited molecule redistributes on the vibrational states of its electronic ground state which leads to photodesorption of a neighbouring molecule from the ice surface. To do so, a cluster approach is used to model the ice amorphous structure. The obtention, optimization and characterization of the theoretical samples is detailed in this work. Then, the energy profile to observe the desorption of a CO molecule from the cluster surface is presented using different approximations. These preliminary results show that the energy required to desorb a CO molecule from the cluster surface is much lower (» 60 meV) than the vibrational energy acquired by the excited CO molecule after the VUV irradiation (» 8 eV). This observation suggests that the CO photodesorption should be highly probable as has been reported in the experimental works.1 M. Bertin, E.C. Fayolle, C. Romanzin, K.I. Öberg, X. Michaut, A. Moudens, L. Philippe, P. Jeseck, H. Linnartz, and J.-H. Fillion, Phys. Chem. Chem. Phys. 14, 9929 (2012).2 E.C. Fayolle, M. Bertin, C. Romanzin, X. Michaut, K.I. Öberg, H. Linnartz, and J.-H. Fillion, Astrophys. J. 739, L36 (2011).3 M. Bertin, E.C. Fayolle, C. Romanzin, H.A.M. Poderoso, X. Michaut, L. Philippe, P. Jeseck, K.I. Öberg, H. Linnartz, and J.-H. Fillion, Astrophys. J. 779, 120 (2013).4 M.C. van Hemert, J. Takahashi, and E.F. van Dishoeck, J. Phys. Chem. A 119, 6354 (2015).

National audience; Dans les régions les plus froides (≈ 10 K) du milieu interstellaire, la plupart des espèces moléculaires à part H2 s’accumulent sur les grains de poussière pour former des manteaux de glaces, jouant le rôle de réservoirs moléculaires particulièrement riches. Néanmoins, de larges quantités d’espèces en phase gazeuse sont également détectées dans ces régions et leur existence peut être expliquée par des processus de désorption à la surface des glaces. Parmi ces processus, on retrouve la désorption induite par rayonnement ultraviolet (UV), appelée photodésorption UV, qui a fait l’objet de plusieurs études expérimentales notamment dans le cas de CO1,2, deuxième espèce la plus abondante du milieu interstellaire. Il a été montré que cette photodésorption UV dans des glaces pures et mixtes de CO suivait un mécanisme indirect de « Desorption Induced by Electronic Transition » (DIET).3Le travail présenté ici, dans ce contexte astrochimique et dans le cadre de l’ANR PIXyES « Photodesorption Induced by UV-X-rays and Electrons on ice Surfaces », concerne l’étude théorique, à l’échelle moléculaire, de la redistribution de l’énergie après excitation UV dans des glaces pures de CO. L’approche retenue est la dynamique moléculaire, soit ab initio (AIMD) avec le logiciel VASP, soit basée sur un champ de force classique4. En particulier, l’accent est mis sur la fin du mécanisme DIET où l’énergie électronique de la molécule excitée, redistribuée sur certains états vibrationnels élevés de son état fondamental électronique, se transfère aux molécules voisines induisant ou non une désorption. Pour se faire, une approche par agrégats est utilisée pour modéliser la structure amorphe de la glace de CO. Ces agrégats optimisés puis thermalisés à des températures similaires à celles du milieu interstellaire sont ensuite utilisés dans des simulations de dynamique moléculaire dans lesquelles une molécule de CO sélectionnée aléatoirement, est excitée dans un niveau vibrationnel élevé (v ≈40 « E ≈8 eV) correspondant à l’énergie acquise par la molécule après excitation UV. La redistribution de cette énergie vibrationnelle dans les agrégats est analysée pour les deux méthodes de dynamique moléculaire utilisées dans cette étude.1 M. Bertin, E.C. Fayolle, C. Romanzin, K.I. Öberg, X. Michaut, A. Moudens, L. Philippe, P. Jeseck, H. Linnartz, and J.-H. Fillion, Phys. Chem. Chem. Phys. 14, 9929 (2012). 2 E.C. Fayolle, M. Bertin, C. Romanzin, X. Michaut, K.I. Öberg, H. Linnartz, and J.-H. Fillion, Astrophys. J. 739 , L36 (2011). 3 M. Bertin, E.C. Fayolle, C. Romanzin, H.A.M. Poderoso, X. Michaut, L. Philippe, P. Jeseck, K.I. Öberg, H. Linnartz, and J.-H. Fillion, Astrophys. J. 779 , 120 (2013). 4 M.C. van Hemert, J. Takahashi, and E.F. van Dishoeck, J. Phys. Chem. A 119 , 6354 (2015).