Vacuum-UV Photodesorption from Compact Amorphous Solid Water: Photon Energy Dependence, Isotopic and Temperature Effects

International audience ; Photodesorption from water-rich interstellar ice mantles is known to play an important role on the gas-to-ice ratio in stars and planet formation regions. Quantitative determination of the photodesorption yields in the laboratory is crucial to astrochemical models. This study presents for the first time, the photon-energy dependence of the photodesorption yields from water ice samples in the vacuum-UV (VUV) range. Experiments have been performed with the Surface Processes and ICES (SPICES) setup coupled to the DESIRS beamline at the SOLEIL synchrotron facility (St. Aubin, France). Thick (20−100 ML) compact amorphous solid water ices (H 2 O and D 2 O) grown onto a cold Au substrate have been irradiated at sample temperatures ranging from 15 to 110 K. Photodesorption yields of water and photoproducts have been obtained by massspectrometry from 7 to 13.5 eV. In interstellar conditions, average H 2 O photodesorption yields are (5 ± 2) × 10 −4 molecule/photon at low temperature (15 K) whereas, lower yields, by a factor of ∼6−9 on average, were found for D 2 O. This strong isotopic effect can be explained by a differential chemical recombination between OH (OD) and H (D) photofragments at the surface of the samples. In addition, an enhancement of the yields above (70 ± 10) K suggests a thermally induced ice restructuration at this threshold temperature.