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

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.