CO₂ synthesis in solid CO by Lyman-α photons and 200 keV protons

We have studied the synthesis of carbon dioxide from solid carbon monoxide at 16 K induced by photolysis with Lyman-α photons and by irradiation with 200 keV protons to quantitatively compare the effects of photolysis and ion irradiation on CO ice and to determine the importance of these processes in interstellar ice grains. The CO and CO2 concentrations during irradiation of an initially pure CO film evolve with fluence to a saturation value, a behaviour that is explained by a two-state model. Our results indicate that the initial CO2 production rates for both radiation processes are similar when normalized to the absorbed energy and that the solid CO2 abundance observed in the interstellar ices cannot be explained only by radiolysis and photolysis of pure solid CO.