LOW-ENERGY ELECTRON-STIMULATED PRODUCTION OF MOLECULAR-HYDROGEN FROM AMORPHOUS WATER ICE

We have observed, via quadrupole mass spectrometry (QMS), stimulated production of D-2 (H-2) during low-energy (5-50 eV) electron-beam irradiation of D2O (H2O) amorphous ice. The upper limit for the D-2 (H-2) production threshold is 6.3+/-0.5 eV; well below the first excited state of condensed water at 7.3 eV. The D-2 (H-2) yield increases gradually until another threshold is reached at similar to 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)-->D-2 (H-2)+OD- (OH-) condensed phase (primarily surface) reactions that an initiated by dissociative attachment. We associate the yield below similar to 11 eV with the dissociation of Frenkel-type excitons and attribute the yield above similar to 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 D-2 (H-2) directly via molecular elimination. The importance of D+(H+) interactions increases at similar to 17 eV (dipolar threshold) and at energies greater than or equal to 21 eV where multihole and multielectron final states are energetically accessible. .