Electron-stimulated reactions in thin D2O films on Pt(111) mediated by electron trapping.

We have measured the electron-stimulated desorption (ESD) of D(2), O(2), and D(2)O, the electron-stimulated dissociation of D(2)O at the D(2)O/Pt interface, and the total electron-stimulated sputtering in thin D(2)O films adsorbed on Pt(111) as a function of the D(2)O coverage (i.e., film thickness). Qualitatively different behavior is observed above and below a threshold coverage of approximately 2 monolayers (ML). For coverages less than approximately 2 ML electron irradiation results in D(2)O ESD and some D(2) ESD, but no detectible reactions at the water/Pt interface and no O(2) ESD. For larger coverages, electron-stimulated reactions at the water/Pt interface occur, O(2) is produced and the total electron-stimulated sputtering of the film increases. An important step in the electron-stimulated reactions is the reaction between water ions (generated by the incident electrons) and electrons trapped in the water films to form dissociative neutral molecules. However, the electron trapping depends sensitively on the water coverage: For coverages less than approximately 2 ML, the electron trapping probability is low and the electrons trap preferentially at the water/vacuum interface. For larger coverages, the electron trapping increases and the electrons are trapped in the bulk of the film. We propose that the coverage dependence of the trapped electrons is responsible for the observed coverage dependence of the electron-stimulated reactions.