Thermal desorption of water ice in the interstellar medium

Water (H2O) ice is an important solid constituent of many astrophysical env ironments. To comprehend the role of such ices in the chemistry and evoluti on of dense molecular clouds and comets, it is necessary to understand the freeze-out, potential surface reactivity and desorption mechanisms of such molecular systems. Consequently, there is a real need from within the astro nomical modelling community for accurate empirical molecular data pertainin g to these processes. Here we give the first results of a laboratory progra mme to provide such data. Measurements of the thermal desorption of H2O ice , under interstellar conditions, are presented. For ice deposited under con ditions that realistically min-tic those in a dense molecular cloud, the th ermal desorption of thin films (much less than 50 molecular layers) is foun d to occur with zeroth-order kinetics characterized by a surface binding en ergy, E-des, of 5773 +/- 60 K, and a pre-exponential factor, A, of 10(30 +/ -2) molecules cm(-2) s(-1). These results imply that, in the dense interste llar medium, thermal desorption of H2O ice will occur at significantly high er temperatures than has previously been assumed.