Sputtering of carbon monoxide ice by hydrogen ions

Carbon monoxide is an important constituent of comet comae, of icy surfaces of planetary bodies, and of interstellar grains. We present new laboratory studies of sputtering of frozen CO by hydrogen ions for energies below 10 keV. The sputtering yield turned out to depend critically on the energy and the electronic stopping power, (dE/dx)e, for the ions. The yield for a 9 keV H+ incident on solid CO was ∼34 CO/H+. For proton bombardment the yield is proportional to (dE/dx)e1.3, similar to the behavior of the sputtering yield for water ice. It means that the particle ejection occasionally requires species from two ionization/excitation events. For molecular ions the yield for CO ice increases with the square of the stopping power (dE/dx)e. The distribution of the sputtered CO molecules exhibits a maximum at 13 meV and falls off strongly with ejection energy E1. A complicating feature is the formation of a residue, possibly CO2 ice, during bombardment of solid CO. The sputtering yield depends slightly on the initial residue, similar to the case of sputtering by keV electrons. The sputtering yield and the chemical efficiency are high compared to similar properties of solid N2, which means that mixtures of these solids are preferentially depleted of CO.