  Vacuum-UV (VUV) photodesorption from water-rich ice mantles coating
interstellar grains is known to play an important role in the gas-to-ice ratio
in star- and planet-forming regions. Quantitative photodesorption yields from
water ice are crucial for astrochemical models. We aim to provide the first
quantitative photon-energy dependent photodesorption yields from water ice in
the VUV. This information is important to understand the photodesorption
mechanisms and to account for the variation of the yields under interstellar
irradiation conditions. Experiments have been performed on the DESIRS beamline
at the SOLEIL synchrotron, delivering tunable VUV light, using the SPICES
(Surface Processes and ICES) set-up. Compact amorphous solid water ice (H$_2$O
and D$_2$O) has been irradiated from 7 to 13.5 eV. Quantitative yields have
been obtained by detection in the gas phase with mass-spectrometry for sample
temperatures 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 at
higher 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 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 $\pm$10) K, suggesting an ice restructuration at this
temperature.
