CO/CO2 Molecular Number Ratio Produced by ion Irradiation of Ices

We have quantitatively studied, by infrared absorption spectroscopy, the CO/CO₂ molecular number ratio after ion irradiation of ices and mixtures containing astrophysically relevant species such as CO, CO₂, H₂O, CH₄, CH₃OH, NH₃, O₂, and N₂ at 12–15 K. The ratios have also been measured after warm up to temperatures between 12 and 200 K. As a general result we find that the CO/CO₂ ratio decreases with the irradiation dose (amount of energy deposited on the sample). In all of the studied mixtures, as expected, it decreases with increasing temperature because of CO sublimation. However the temperature where CO sublimes strongly depends on the initial mixture, remaining at a temperature over 100 K in some cases. Our results might be relevant to interpret the observed CO/CO₂ ratio in several astrophysical scenarios such as planetary icy surfaces and ice mantles on grains in the interstellar medium. This revised version was published online in July 2006 with corrections to the Cover Date.