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 CO_1,2 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).