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