Vacuum-UV photodesorption from compact Amorphous Solid Water : photon energy, isotopic and temperature effects

Context. Vacuum-UV (VUV) photodesorption from water-rich ice mantles coating interstellar grains is known to play an important role on the gas-to-ice ratio in stars and planets formation regions. Quantitative photodesorption yields from water ice are crucial for astrochemical models. Aims. The aims is to provide the first quantitative photon-energy dependent photodesorption yields from water ices in the VUV. This information is important for the understanding of the photodesorption mechanisms and to account for the variation of the yields under interstellar irradiation conditions. Methods. Experiments have been performed on the DESIRS beamline at the SOLEIL synchrotron facility (St Aubin, France), delivering tunable VUV radiation, using the ultra-high SPICES (Surface Processes and ICES) vacuum chamber. Thick compact amorphous solid water ice (H2O and D2O) grown onto a cold Au substrate have been irradiated from 7 to 13.5 eV. Quantitative yields have been obtained by detection into the gas phase with mass-spectrometry for sample temperatures ranging from 15 K to 100 K. Results. Photodesorption spectra of H2O (D2O), OH (OD), H2 (D2) and O2 peak around 9-10 eV and decrease at higher energies. Average photodesorption yields of intact water at 15 K are 5 × 10−4 molecule/photon for H2O and 5 × 10−5 molecule/photon for D2O over the 7-13.5 eV range. The strong isotopic effect can be explained by a differential chemical recombination between OH (OD) and H (D) photofragments originating from lower kinetic energy available for the OH photofragments upon direct water photodissociation and/or possibly by an electronic relaxation process. It is expected to contribute to water fractionation during the building-up of the ice grain mantles in molecular clouds and to favor OH-poor chemical environment in comet-formation regions of protoplanetary disks. The yields of all the detected species except OH (OD) are enhanced above (70 ±10) K, suggesting an ice restructuration at this threshold temperature.