PHOTODESORPTION OF ICES. II. H2O AND D2O

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 nonthermal ice desorption mechanism. Here, we present experimentally determined UV photodesorption yields of H2O and D2O ices and deduce their photodesorption mechanism. The ice photodesorption is studied under ultrahigh 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, Y-pd(T, x), is identical for H2O and D2O and its dependence on ice thickness and temperature is described empirically by Y-pd(T, x) = Y-pd(T, x > 8)(1 - e(-x/l(T))), where x is the ice thickness in monolayers (MLs) and l(T) is a temperature-dependent ice diffusion parameter that varies between similar to 1.3 ML at 30 K and 3.0 ML at 100 K. For thick ices, the yield is linearly dependent on temperature due to increased diffusion of ice species such that Y-pd(T, x > 8) = 10(-3) (1.3 + 0.032 x 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 from 0.7:1.0 at 20 K to 2.0:1.2 at 100 K. The yield does not depend on the substrate, the UV photon flux, or the UV fluence. The yield is also independent of the initial ice structure since UV photons efficiently amorphize H2O ice. The results are consistent with theoretical predictions of H2O photodesorption at low temperatures and partly in agreement with a previous experimental study. Applying the experimentally determined yield to a Herbig Ae/Be star+disk model provides an estimate of the amount of gas-phase H2O that may be observed by, e. g., Herschel in an example astrophysical environment. The model shows that UV photodesorption of ices increases the H2O content by orders of magnitude in the disk surface region compared to models where nonthermal desorption is ignored.