Reactivity of water–electron complexes on crystalline ice surfaces

Abstract

The interactions between long-living electrons trapped in defects of crystalline D₂O and electronegative molecules have been investigated using two-photon photoemission spectroscopy. When covered by a Xe adlayer, the spectroscopic signature of the trapped electrons vanishes, which provides evidence that the trapping sites are located on the surface of the crystalline ice. The reactive character of these surface-trapped electrons with molecules has been studied. In the case of CFCl₃ adsorbed on top of the ice, we show that the trapped electrons induce the dissociation of the molecules, via a dissociative electron attachment process, resulting in ˙CFCl₂ and Cl⁻ formation. The latter species are responsible for the observed increase of the work function and presumably for the deactivation of the surface trapping sites with respect to subsequent light-induced population by excited electrons. This process is thought to be of high efficiency since it is observed for a very low CFCl₃ coverage of only ∼0.004 monolayer (ML). In the case of exposure of the crystalline ice to a partial pressure of gaseous O₂, the deactivation of the trapping site has also been observed. The mechanism is thought to involve the formation of the O₂^*− transient anion by electron attachment, followed by its reactive interaction with the water molecules of the defect. In both cases, the mechanisms are triggered by negative ion resonances which are known from experiments using a primary electron beam to be effective for isolated molecules for ballistic electrons of ∼0 eV. We thereby demonstrate a similarity between the processes induced by these primary, very low kinetic-energy electrons and by the long-living surface electrons on the crystalline ice surface. These results suggest that the photoexcited trapped electrons can play an important role in the heterogeneous chemical processes on condensed water surfaces and could be relevant in the polar stratosphere chemistry.