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Gaseous H2O has been detected in several cold astrophysical environments,where the observed abundances cannot be explained by thermal desorption of H2Oice or by H2O gas phase formation. These observations hence suggest anefficient non-thermal ice desorption mechanism. Here, we present experimentallydetermined UV photodesorption yields of H2O and D2O ice and deduce theirphotodesorption mechanism. The ice photodesorption is studied under ultra highvacuum conditions and at astrochemically relevant temperatures (18-100 K) usinga hydrogen discharge lamp (7-10.5 eV), which simulates the interstellar UVfield. The ice desorption during irradiation is monitored using reflectionabsorption infrared spectroscopy of the ice and simultaneous mass spectrometryof the desorbed species. The photodesorption yield per incident photon isidentical for H2O and D2O and depends on both ice thickness and temperature.For ices thicker than 8 monolayers the photodesorption yield Y is linearlydependent on temperature due to increased diffusion of ice species such thatY(T) = 1E-3(1.3+0.032*T) UV photon-1, with a 60% uncertainty for the absoluteyield. The increased diffusion also results in an increasing H2O:OH desorptionproduct ratio with temperature. The yield does not depend on the substrate, theUV photon flux or the UV fluence. The yield is also independent on the initialice structure since UV photons efficiently amorphize H2O ice. The results areconsistent with theoretical predictions of H2O photodesorption and partly inagreement with a previous experimental study. Applying the experimentallydetermined yield to a Herbig Ae/Be star+disk model shows that UVphotodesorption of ices increases the H2O content by orders of magnitude in thedisk surface region compared to models where non-thermal desorption is ignored.

Photodesorption of Ices II: H2O and D2O