Ion-induced molecular ejection from D 2O ice

Abstract

Studies of molecules ejected from water ice by fast ions provide insight into the electronic relaxation processes and subsequent chemistry occurring in ice at very low temperatures. The ion-induced ejection of D 2O, D 2, and O 2 molecules from thin films of D 2O ice has been measured as a function of the fluence of incident MeV ions at temperatures between 10 and 140 K. For a given beam current, the O 2 yield exhibits initial transients which are slow compared with the prompt ejection of D 2O. We interpret these results as due to the build-up of O 2 in the films following fragmentation of D 2O molecules by incident ions. The fragmens re-form into new molecular species which diffuse to and escape from the surface, aided by subsequent bombardment. The D 2 transient has a prompt component, which we postulate is due to rapid formation during electronic recombination near the surface. A slow component of the D 2 transient is also observed, which may arise through a two-step process similar to that of O 2. Time-of-flight energy spectra of the ejected D 2O molecules have also been measured. Incident ion masses and energies range from those for which nuclear elastic energy deposition dominates (50 keV argon) to those for which electronic energy deposition dominates (1.5 MeV helium). The spectra cannot be described by models typically employed for the sputtering of metals. For instance, the spectra do not have maxima characteristic of the sublimation energy of the solid. In addition, the sputtering yield in the high energy part of the ejection spectrum of D 2O is too large to arise from nuclear elastic energy deposition. It must result instead from relatively energetic non-radiative relaxation of electronic excitation. For incident MeV ions that deposit their energy predominantly in electronic excitation, the low energy part of the D 2O ejection spectrum is greatly enhanced, indicative of a weakly antibinding region formed along an incident particle track. Enhanced ion yields are also found in the collision cascade region which are attributed to electronic processes.