Photon and electron induced desorption from molecular ices

The deposition of energy in the form of electronic excitations in molecules condensed on cold surfaces (10-100 K) can lead to the desorption of some of these molecules. This basic surface science process has consequences in a variety of fields, two of which are of concern here: astrochemistry and vacuum dynamics in accelerators. Photon and Electron-Induced desorption are studied in this manuscript for thin films of condensed molecules (ices), e.g. CO, H$_2$O, NO or CH$_4$. The first objective is to obtain a quantification of the desorption of the various desorbing species, and to look for the parameters that affect the efficiency of the process. The second objective is to understand the mechanisms of evolution and relaxation of the initial electronic excitations that lead to desorption. Photon-induced desorption is studied at LERMA using synchrotron radiation in the VUV range (5-14 eV) and soft X-ray range (520-600 eV). This allows to obtain spectrally-resolved information, which is crucial both for model implementation and fundamental understanding of the mechanisms. Electron-induced desorption is studied at CERN in the 150-2000 eV range. The results expand the available data on UV photodesorption and allow to determine the relevance of electron or X-ray desorption for astrochemistry. Progress has also been made on the understanding of mechanisms, particularly on the role of energy or particle transport from the bulk to the surface of the ice, on indirect desorption, or on the desorption of ions in the soft X-ray range. A new experimental set-up has also been developed at LERMA for laser desorption and laser spectroscopy experiments, allowing quantum-state and kinetic energy resolved measurements of desorbed molecules.