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Vacuum-UV (VUV) photodesorption from water-rich ice mantles coatinginterstellar grains is known to play an important role in the gas-to-ice ratioin star- and planet-forming regions. Quantitative photodesorption yields fromwater ice are crucial for astrochemical models. We aim to provide the firstquantitative photon-energy dependent photodesorption yields from water ice inthe VUV. This information is important to understand the photodesorptionmechanisms and to account for the variation of the yields under interstellarirradiation conditions. Experiments have been performed on the DESIRS beamlineat the SOLEIL synchrotron, delivering tunable VUV light, using the SPICES(Surface Processes and ICES) set-up. Compact amorphous solid water ice (H$_2$Oand D$_2$O) has been irradiated from 7 to 13.5 eV. Quantitative yields havebeen obtained by detection in the gas phase with mass-spectrometry for sampletemperatures ranging from 15 K to 100 K. Photodesorption spectra of H$_2$O(D$_2$O), OH (OD), H$_2$ (D$_2$) and O$_2$ peak around 9-10 eV and decrease athigher energies. Average photodesorption yields of intact water at 15 K are 5$\times$ 10$^{-4}$ molecule/photon for H$_2$O and 5 $\times$ 10$^{-5}$molecule/photon for D$_2$O over the 7-13.5 eV range. The strong isotopic effectcan be explained by a differential chemical recombination between OH (OD) and H(D) photofragments originating from lower kinetic energy available for the OHphotofragments upon direct water photodissociation and/or possibly by anelectronic relaxation process. It is expected to contribute to waterfractionation during the building-up of the ice grain mantles in molecularclouds and to favor OH-poor chemical environment in comet-formation regions ofprotoplanetary disks. The yields of all the detected species except OH (OD) areenhanced above (70 $\pm$10) K, suggesting an ice restructuration at thistemperature.

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