Vacuum-UV Photodesorption from Compact Amorphous Solid\nWater: Photon Energy Dependence, Isotopic and Temperature Effects

Abstract

Photodesorption from water-rich interstellar\nice mantles is known\nto play an important role on the gas-to-ice ratio in stars and planet\nformation regions. Quantitative determination of the photodesorption\nyields in the laboratory is crucial to astrochemical models. This\nstudy presents for the first time, the photon-energy dependence of\nthe photodesorption yields from water ice samples in the vacuum-UV\n(VUV) range. Experiments have been performed with the Surface Processes\nand ICES (SPICES) setup coupled to the DESIRS beamline at the SOLEIL\nsynchrotron facility (St. Aubin, France). Thick (20–100 ML)\ncompact amorphous solid water ices (H₂O and D₂O) grown onto a cold Au substrate have been irradiated at sample\ntemperatures ranging from 15 to 110 K. Photodesorption yields of water\nand photoproducts have been obtained by mass-spectrometry from 7 to\n13.5 eV. In interstellar conditions, average H₂O photodesorption\nyields are (5 ± 2) × 10^–4 molecule/photon\nat low temperature (15 K) whereas, lower yields, by a factor of ∼6–9\non average, were found for D₂O. This strong isotopic effect\ncan be explained by a differential chemical recombination between\nOH (OD) and H (D) photofragments at the surface of the samples. In\naddition, an enhancement of the yields above (70 ± 10) K suggests\na thermally induced ice restructuration at this threshold temperature.