ELECTRONIC EXCITATION OF CONDENSED CO - SPUTTERING AND CHEMICAL-CHANGE

We have bombarded condensed CO "ice" films with keV and MeV ions at low temperatures to investigate the physical sputtering mechanisms and chemical modifications of the ice. The sputtering yield measured for keV and MeV H+ and He+ ions indicates that the yield has a quadratic dependence on the electronic stopping power of the ions. Energy spectra of CO molecules sputtered by bombardment with 53 keV He+ and 34 keV Ar+ ions are very similar and show a collision-cascade-like behavior. We explore the origin of this quadratic dependence in terms of several models proposed for sputtering by electronic excitation and conclude that the quadratic dependence is intrinsic to the electronic relaxation process for CO. RBS measurements of the residue produced from sputtering various thicknesses of the initial CO film (∼ (3-25) × 1017 CO/cm2) indicate the production of the residue is approximately proportional to the initial thickness, accounts for ∼ 2% of the initial mass, and has a stoichiometric composition of approximately C3O. The production and composition of the residue is discussed in terms of existing mechanisms for the radiolysis of gas phase CO. © 1990.