Dissociative electron attachment in nanoscale ice films: Temperature and morphology effects

The electron-stimulated desorption (ESD) of D- ions from condensed D2O films is investigated. Three low-energy peaks are observed which are identified as arising from excitation of B-2(1), (2)A(1), and B-2(2) dissociative electron attachment (DEA) resonances. A fourth, higher energy feature is also seen in the D- yield which is likely due to the formation of a transient anion state that dissociates and/or decays into a dissociative excited state. The energies and ion yields of the resonances vary with the temperature and morphology of the D2O film. Below 60 K, the work function of the ice films changes with temperature and the DEA resonances shift in energy. The D- ESD yield generally increases with temperature, but it deviates from this trend at temperatures corresponding to structural phase transitions in ice. The (B-2(1)) D- temperature dependence is remarkably similar to that observed for the ESD of low-energy D+ ions from D2O ice, even though the two originate from different electronic excitations. These results are attributed to thermally induced changes in the hydrogen bonding network, which changes the lifetimes of the predissociative states that lead to ESD and which also allows for the reorientation of surface molecules. (C) 1997 American Institute of Physics. [S0021-9606(97)02344-1].