ice: Surface structure and electronic excitations

We present a study of the electron-stimulated desorption of deuterium cations ~D ) from thin ~1–40 ML! D 2O ice films vapor deposited on a Pt~111! substrate. Measurements of the total yield and velocity distributions as a function of temperature from 90 to 200 K show that the D yield changes with film thickness, surface temperature, and ice phase. We observe two energy thresholds for cation emission, near 25 and 40 eV, which are weakly dependent upon the ice temperature and phase. The cation time-of-flight ~TOF! distribution is at least bimodal, indicating multiple desorption channels. A decomposition of the TOF distributions into ‘‘fast’’ and ‘‘slow’’ channels shows structure as a function of excitation energy, film thickness, and temperature. The D 1 yield generally increases with temperature, rising near 120 K on amorphous ice, and near 135 K on crystalline ice. The amorphous-crystalline phase transition at ;160 K causes a drop in total desorption yield. The temperature dependence of D 2 desorption via the B1 dissociative electron attachment resonance is very similar to the slow D 1 yield, and likely involves similar restructuring and lifetime effects. The data collectively suggest that a thermally activated reduction of surface hydrogen bonding increases the lifetime of the excited states responsible for ion desorption, and that these lifetime effects are strongest for excited states involving a1 bands @S0163-1829~97!05931-6#