Low-energy electron-stimulated production of molecular hydrogen from amorphous water ice

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Abstract
We have observed, via quadrupole mass spectrometry (QMS), stimulated production of D2 (H2) during low‐energy (5–50 eV) electron–beam irradiation of D2O (H2O) amorphous ice. The upper limit for the D2 (H2) production threshold is 6.3±0.5 eV; well below the first excited state of condensed water at 7.3 eV. The D2 (H2) yield increases gradually until another threshold is reached at ∼17 eV and continues to increase monotonically (within experimental error) up to 50 eV. We assign the 6.3 eV threshold to D− (H−)+D2O (H2O)→D2 (H2)+OD− (OH−) condensed phase (primarily surface) reactions that are initiated by dissociative attachment. We associate the yield below ∼11 eV with the dissociation of Frenkel‐type excitons and attribute the yield above ∼11 eV mainly to the recombination of D2O+, or D3O+, with quasifree or trapped electrons. Exciton dissociation and ion–electron recombination processes can produce reactive energetic D (H) atom fragments or D2 (H2) directly via molecular elimination. The importance of D+ (H+) interactions increases at ∼17 eV (dipolar threshold) and at energies ≥21 eV where multihole and multielectron final states are energetically accessible.