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Electron-Promoted Desorption from Water Ice Surfaces: Neutral Gas-Phase Products Ali G. M. Abdulgalil, Alexander Rosu-Finsen, Demian Marchione, John D. Thrower, Mark P. Collings, and Martin R. S. McCoustra ACS Earth Space Chem., Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 28, 2017
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ACS Earth and Space Chemistry
Electron-Promoted Desorption from Water Ice Surfaces: Neutral Gas-Phase Products Ali G. M. Abdulgalil, Alexander Rosu-Finsen,* Demian Marchione,† John D. Thrower,ǂ Mark P. Collings and Martin R. S. McCoustra* Institute of Chemical Sciences, Heriot-Watt University, Riccarton, EH14 4AS, Edinburgh, United Kingdom. KEYWORDS: ISM, molecular cloud, non-thermal desorption, electron irradiation, H2O
ABSTRACT
Electron-promoted desorption (EPD) from compact amorphous solid water (c-ASW) has been studied. Low-energy electron bombardment with 200 to 300 eV electrons leads to H2O depletion as monitored by reflection-absorption infrared spectroscopy (RAIRS) of the remaining c-ASW film. Cross-sections for H2O depletion were calculated to be in the range 1.6±1.0 × 10-16 to 5.2±3.0 × 10-16 cm2. However, mass spectrometric measurements identify a major component of the desorbing material as H2, which appears with similar kinetics to those for H2O loss. Molecular H2O is observed as a minor desorption product in the gas phase.
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ACS Earth and Space Chemistry
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1. Introduction Water (H2O), in its solid and gaseous forms, is a key molecule in the Solar System and beyond. It has been detected in both forms in many galactic environments including planets, comets, and the interstellar medium (ISM) itself.1 Observations show that H2O is a major component of interstellar ices along most lines-of-sight, with a typical abundance of 1×10−4 with respect to atomic hydrogen.2 The formation of H2O ice in astrophysical environments occurs at low pressure and low temperatures (10-90 K). The dominant H2O formation pathway in the ISM is through reactive accretion involving efficient atomic oxygen (O),3 molecular oxygen (O2),4,5 and ozone (O3)6 hydrogenation directly on grain surfaces. Gas-phase H2O formation followed by freeze-out onto grain surfaces does not account for observed solid H2O abundances.7 In most astrophysical environments, H2O is found to exist as amorphous solid water (ASW) which is metastable with respect to the crystalline state due to a high activation barrier for the structural transformation.8,9 During the formation of dense cores, freeze-out of gas phase species onto the surfaces of dust grains takes place, adding to the icy mantles created by reactive accretion. At the low temperatures (
