We have conducted experiments on the sputtering of water ice by 100 keV ${\mathrm{Ar}}^{+}$ between 20 and 150 K. Our findings indicate that the temperature dependence of the total sputtering yield is heavily influenced by the thermal and irradiation history of the ice, showing a complex dependence on irradiation fluence that is correlated to the ejection of ${\mathrm{O}}_2$ molecules. The results suggest that ${\mathrm{O}}_2$ produced by the ions inside the ice diffuses to the surface where it is trapped and then ejected via sputtering or thermal desorption. A high concentration of ${\mathrm{O}}_2$ can trap in a subsurface layer during bombardment at 130 K, which we relate to the formation of hydrogen and its escape from that region. A simple model allows us to determine the depth profile of the absolute concentration of ${\mathrm{O}}_2$ trapped in the ice.

• Received 24 August 2005

DOI:https://doi.org/10.1103/PhysRevB.72.245422