Angular dependence of the sputtering yield of water ice by 100 keV proton bombardment

We measured the total sputtering yield of amorphous water ice for 100 keV H+ as a function of the projectile incidence angle, and the angular distribution of the ejected H2O and O-2 molecules, using a quartz-crystal microbalance and mass spectrometry, respectively, at temperatures of 20 K and 100 K. The total sputtering yield follows a cos(-f)theta dependence, with f congruent to 1.3, regardless of the irradiation temperature. This is explained by the action of fast binary delta-electrons that relocate the electronic energy deposited by the ion near the surface into the bulk of the material. We found that the O-2 emission follows a cosine dependence, as expected from isotropic collision cascades or if transport of the oxygen to the surface is by thermal diffusion. In contrast, H2O emission is more outward peaked than cosine, which could be attributed to the blocking of large angle emission by the transient crater formed during sputtering of multiple water molecules by a given projectile. (c) 2005 Elsevier B.V. All rights reserved.