Heavy ion irradiation of condensed CO2: sputtering and molecule formation

Context : Ices present in different astrophysical environments are exposed to ion irradiation from cosmic rays (H to heavier than Fe) in the keV to GeV energy range. Aims : The objective of this work is to study the effects produced in astrophysical ices by heavy ions at relatively high energies (MeV) in the electronic energy loss regime and compare them with those produced by protons. Methods : C18O2 was condensed on a CsI substrate at 13 K and it was irradiated by 46 MeV 58Ni11+ up to a final fluence of 1.5 × 1013 cm−2 at a flux of 2 × 109 cm−2 s−1. The ice was analyzed in situ by infrared spectroscopy (FTIR) in the 5000−600 cm−1 range. Results : The CO2 destruction was observed, as well as the formation of other species such as CO, CO3, O3, and C3. The destruction cross section of CO2 is found to be 1.7×10−13 cm2, while those for the formation of CO, CO3, and O3 molecules are 1.6 × 10−13 cm2, 4.5 × 10−14 cm2, and 1.5 × 10−14 cm2, respectively. The sputtering yield of the CO2 ice is 4.0 × 104 molecules/impact, four orders of magnitude higher than for H projectiles at the same velocity. This allows us to estimate the contribution of the sputtering by heavy ions as compared to protons in the solar winds and in cosmic rays. Conclusions : The present results show that heavy ions play an important role in the sputtering of astrophysical ices. Furthermore, this work confirms the quadratic stopping power dependence of sputtering yields.