Sputtering of ice by low-energy ions

We measured the total sputtering yield Y of amorphous water ice at 80 K for 0.35-4 keV He and Ar ions. We found that Y depends linearly on the elastic stopping cross section at low energies as predicted by the standard linear cascade theory of sputtering. As the energy increases, a quadratic dependence with the electronic stopping cross section arises due to cooperative effects between excitations produced by the projectile. We also studied how sputtering depends on the projectile incidence angle and found that Y follows a cos−f(θ) dependence with f = 1.45 ± 0.05 for 2 keV He+ and f = 1.78 ± 0.08 for 2 keV Ar+. Measurements of Y for 2 keV ions vs. ice temperature between 30 and 140 K show the same temperature dependence as that reported previously for high-energy ions. We introduce a general formula to calculate Y below 100 keV once the ice temperature, projectile type, energy and angle of incidence are known. This formula can be used for modeling the production of extended neutral atmospheres around astrophysical icy bodies subject to ion bombardment.