Influence of helium-ion bombardment on the surface properties of pure and ammonia-adsorbed water thin films

The influence of the ion bombardment on the surface properties of water-ice films has been investigated. The films are irradiated with 1.5 keV He+ ions and analyzed sequentially on the basis of time-of-flight secondary-ion mass spectrometry (TOF-SIMS). In order to minimize any temperature-induced effects, the measurements were made at 15 K. The damage of the films, as estimated from the H/D exchange between NH3 and the D2O ice and the intermixing of NH3 with the H218O ice, is recognized at the fluence above 2 × 1014 ions/cm2. The sputtering yield of the D2O ice is determined as 0.9 ± 0.2 molecules per incoming He+ ion. The temperature-programmed TOF-SIMS analysis of the water-ice films has been completed within the fluence of 5.8 × 1012 ions/cm2, so that no appreciable damage of the film should be induced during the measurement.