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 cont aining astrophysically relevant species such as CO, CO2, H2O, CH4, CH3OH, N H3, O-2, and N-2 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 t he CO/CO2 ratio decreases with the irradiation dose (amount of energy depos ited on the sample). In all of the studied mixtures, as expected, it decrea ses with increasing temperature because of CO sublimation. However the temp erature where CO sublimes strongly depends on the initial mixture, remainin g 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 s uch as planetary icy surfaces and ice mantles on grains in the interstellar medium.