A Comparative Study of Secondary Ion Emission from Water Ice under Ion Bombardment by Au+, Au3+, and C60+

Secondary ion emission from water ice has been studied using Au+, Au3+, and C60+ primary ions. In contrast to the gas phase in which the spectra are dominated by the (H2O)nH+ series of ions, the spectra from ice using all three primary ions are principally composed of two series of cluster ions (H2O)nH+ and (H2O)n+. Dependent on the conditions, the unprotonated series can dominate the spectra. Since in the gas phase (H2O)n+ is unstable with respect to the formation of the protonated ion series, the presence of the solid must provide a means to stabilize their formation. The cluster ion yields under Au+ bombardment are very low and can be understood in terms of sputtering on the borderline between linear cascade and thermal spike behavior. There is a 104 increase in yield across the whole spectrum compared to Au+ when Au3+ and C60+ species are used as primary ions. The character of the spectra differed between these two primary ions, but insights into the mechanism of secondary ion emission for both is discussed within an energy deposition framework provided by the fluid flow-based mesoscale energy deposition footprint (MEDF) model that predicts a cone-shaped zone of activation and emission. C60+ differs from Au3+ in that it delivers its energy closer to the surface, and it is argued this has consequences for the cluster ion distribution and yield. Increasing the ion dose by sputtering suppresses the yield of (H2O)n+ and increases the yield of the protonated ions in the small cluster region, whereas the yield in the large cluster regime is suppressed significantly. The three primary ions show rather different behavior, and this is discussed in the light of the sputtering models. Finally, negative ion spectra including cluster ions have been observed for the first time. C60+ delivers the highest yields, but these are less than 10 times the positive ion yields, probably because the O and OH fragment ions on which the clusters are based are easily neutralized by protons.