Dust grains in the interstellar medium and the outer Solar System commonly have a coating of water ice, which affects their optical properties and surface chemistry. The thickness of these icy mantles may be determined in part by the extent of photodesorption (photosputtering) by background ultraviolet radiation. But this process is poorly understood, with theoretical estimates of the photodesorption rate spanning several orders of magnitude. Here we report measurements of the absolute ultraviolet photodesorption yield of low-temperature water ice. Our results indicate that the rate of photodesorption is appreciable. In particular, it can account for the absence of icy mantles on grains in diffuse interstellar clouds, it exceeds solar-wind ion erosion and sublimation in the outer Solar System, and it is important in determining the lifetimes of icy mantles in dense molecular clouds.

The density of 0.5-3 m thick vapor-deposited films of water ice were measured by combined optical interferometry and microbalance techniques during deposition on an optically flat gold substrate from a capillary array gas source. The films were of high optical quality with an index of refraction of 1.29Ϯ0.01 at 435.8 nm, a density of 0.82Ϯ0.01 g/cm 3 , and a porosity of 0.13Ϯ0.01. In contrast to previous studies, none of the measured properties exhibited any significant variation with growth rate or temperature over the range studied ͑0.6-2 nm/min, 20-140 K͒.

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State selective differential cross sections for rotationally inelastic scattering of NO (Ji=0.5, 1.5, F1→Jf=2.5–12.5, F1 and Jf=1.5–9.5, F2) from He and D2 measured by crossed molecular beam product imaging are reported. The differential cross sections were extracted from the data images using a new basis image iterative fitting technique. The images typically exhibit a single broad rotational rainbow maximum that shifts from the forward to the backward scattering direction with increasing ΔJ. The angle of the rainbow maximum was lower at a given ΔJ for D2 than for He as a collision partner. At a collision energy of ∼500 cm−1, primarily the repulsive part of the potential surface is probed, which can be modeled with a two-dimensional hard ellipse potential. This model for rotationally inelastic scattering is shown to qualitatively match the experimental differential cross sections. A more advanced correlated electron pair approximation potential energy surface for NO+He does not give substantially better agreement with the experiment. The differences between scattering of He and D2 are partially attributed to their differing structure and partially to a small difference in collision energy used in the two experiments.

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