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

Abstract

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) n H+ series of ions, the spectra from ice using all three primary ions are principally composed of two series of cluster ions (H2O) n H+ 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.

Ionization of Water Clusters Mediated by Exciton Energy Transfer from Argon Clusters

“…A process termed active parent ion cooling was coined, where argon atoms effectively carried away the excess energy upon evaporation. Electron impact ionization of water clusters embedded in helium nanodroplets has also shown evidence of water cluster ions (typically at the 10% level when compared to protonated ones), , as does ion bombardment of ice in a recent secondary ion emission experiment . These studies suggest that nondissociative ionization of small water clusters may be feasible only with an ultrasoft ionization method, and here, we show how an argon cluster can take the role of both electronic energy transport and a heat bath to achieve ultrasoft ionization.…”

The influence of polyatomic primary ion chemistry on matrix effects in secondary ion mass spectrometry analysis

“…This suggests that the presence of the water ice matrix mediates to a large extent a similar protonation enhancement to that of the water cluster beam. Examination of the mass spectra from the frozen‐hydrated samples confirms that a series of water cluster secondary ions is detected, as reported previously, and that the yields of these species, which may themselves provide additional routes to analyte protonation, are greatest for C 60 bombardment (Figure S4, supporting information).…”

“…This is consistent with our previously published conclusion that the water beam promotes protonation (and not it would appear deprotonation at E / n = 5 eV/atom). In considering the formation probability and relative stability of positive or negative reactive species from water cluster beams, it may also be relevant that on sputtering ice with C 60 the yield of positive ion clusters exceeds that of negative ion clusters by an order of magnitude . Similarly, there may be lower concentrations of ions of the form (H 2 O) n OH − , (H 2 O) n O − or (H 2 O) n OH 3 − resulting from shattered water clusters than (H 2 O) n H + , etc.…”

Water Radical Cations in the Gas Phase: Methods and Mechanisms of Formation, Structure and Chemical Properties

“…Characterization of generated water radical cations is usually done using mass spectrometry, which can be combined with optical spectroscopy [ 15 , 16 ]. Two series of cluster ions ((H 2 O) n H + and (H 2 O) n +• ) in water ice have been detected simultaneously in experiments involving secondary ion mass spectrometry (SIMS), with Au + , Au 3 + , and C 60 + as primary ions [ 17 ]. Typically, protonated (H 2 O) n H + cations have been observed as the predominant products of water ionization [ 18 , 19 , 20 ].…”