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Amorphous solid water (ASW) films grown by vapor deposition below 110 K develop negative surface voltages Vs with respect to the substrate. This polarization is due to a partial alignment of the water molecules during condensation. Kelvin probe measurements show that the magnitude of the surface potential, |Vs|, increases linearly with film thickness at a rate that decreases with increasing deposition temperature. |Vs| decreases with increasing deposition temperature and increasing incidence angle of the vapor source. After film growth, |Vs| decreases irreversibly by 80% when the ice film is heated to ∼30 K above the deposition temperature. The measurements of |Vs| as a function of film porosity indicate that polarization in ASW is governed by incompletely coordinated water molecules, dangling with unbalanced dipoles at the internal surface of the pores and weakly aligned by the anisotropic film-vacuum interface. This idea is supported by the strikingly similar behavior of |Vs| and the infrared absorption due to the most pliable, two-coordinated surface molecules with annealing temperature.

We have conducted experiments on the sputtering of water ice by 100 keV Ar + between 20 and 150 K. Our findings indicate that the temperature dependence of the total sputtering yield is heavily influenced by the thermal and irradiation history of the ice, showing a complex dependence on irradiation fluence that is correlated to the ejection of O 2 molecules. The results suggest that O 2 produced by the ions inside the ice diffuses to the surface where it is trapped and then ejected via sputtering or thermal desorption. A high concentration of O 2 can trap in a subsurface layer during bombardment at 130 K, which we relate to the formation of hydrogen and its escape from that region. A simple model allows us to determine the depth profile of the absolute concentration of O 2 trapped in the ice.

The energy to desorb atomic oxygen from an interstellar dust grain surface, E des , is an important controlling parameter in gas-grain models; its value impacts the temperature range over which oxygen resides on a dust grain. However, no prior measurement has been done of the desorption energy. We report the first direct measurement of E des for atomic oxygen from dust grain analogs. The values of E des are 1660±60 K and 1850±90 K for porous amorphous water ice and for a bare amorphous silicate film, respectively, or about twice the value previously adopted in simulations of the chemical evolution of a cloud. We use the new values to study oxygen chemistry as a function of depth in a molecular cloud. For n = 10 4 cm −3 and G 0 =10 2 (G 0 =1 is the average local interstellar radiation field), the main result of the adoption of the higher oxygen binding energy is that H 2 O can form on grains at lower visual extinction A V , closer to the cloud surface. A higher binding energy of O results in more formation of OH and H 2 O on grains, which are subsequently desorbed by far-ultraviolet radiation, with consequences for gas-phase chemistry. For higher values of n and G 0 , the higher binding energy can lead to a large increase in the column of H 2 O but a decrease in the column of O 2 .

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