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The HDO/H2O ratio in interstellar gas is often used to draw conclusions onthe origin of water in star-forming regions and on Earth. In cold cores and inthe outer regions of protoplanetary disks, gas-phase water comes fromphotodesorption of water ice. We present fitting formulae for implementation inastrochemical models using photodesorption efficiencies for all water iceisotopologues obtained using classical molecular dynamics simulations. Weinvestigate if the gas-phase HDO/H2O ratio reflects that present in the ice orwhether fractionation can occur during photodesorption. Probabilities for thetop four monolayers are presented for photodesorption of X (X=H,D) atoms, OXradicals, and X2O and HDO molecules following photodissociation of H2O, D2O,and HDO in H2O amorphous ice at temperatures from 10-100 K. Isotope effects arefound for all products: (1) H atom photodesorption probabilities from H2O iceare larger than those for D atom photodesorption from D2O ice by a factor of1.1; the ratio of H and D photodesorbed upon HDO photodissociation is a factorof 2. This process will enrich the ice in deuterium atoms over time; (2) theOD/OH photodesorption ratio upon D2O and H2O photodissociation is on average afactor of 2, but the ratio upon HDO photodissociation is almost constant atunity for all temperatures; (3) D atoms are more effective in kicking outneighbouring water molecules than H atoms. However, the ratio of thephotodesorbed HDO and H2O molecules is equal to the HDO/H2O ratio in the ice,therefore, there is no isotope fractionation upon HDO and H2O photodesorption.Nevertheless, the enrichment of the ice in D atoms due to photodesorption canover time lead to an enhanced HDO/H2O ratio in the ice, and, whenphotodesorbed, also in the gas. The extent to which the ortho/para ratio of H2Ocan be modified by the photodesorption process is also discussed. (Abridged)

Photodesorption of H2O, HDO, and D2O ice and its impact on fractionation