CO Diffusion and Desorption Kinetics in CO2 Ices

The diffusion of species in icy dust grain mantles is a fundamental process that shapes the chemistry of interstellar regions; yet, measurements of diffusion in interstellar ice analogs are scarce. Here we present measurements of CO diffusion into CO2 ice at low temperatures (T = 11-23 K) using CO2 longitudinal optical phonon modes to monitor the level of mixing of initially layered ices. We model the diffusion kinetics using Fick's second law and find that the temperature-dependent diffusion coefficients are well fit by an Arrhenius equation, giving a diffusion barrier of 300 +/- 40 K. The low barrier along with the diffusion kinetics through isotopically labeled layers suggest that CO diffuses through CO2 along pore surfaces rather than through bulk diffusion. In complementary experiments, we measure the desorption energy of CO from CO2 ices deposited at 11-50 K by temperature programmed desorption and find that the desorption barrier ranges from 1240 +/- 90 K to 1410 +/- 70 K depending on the CO2 deposition temperature and resultant ice porosity. The measured CO-CO2 desorption barriers demonstrate that CO binds equally well to CO2 and H2O ices when both are compact. The CO-CO2 diffusion-desorption barrier ratio ranges from 0.21 to 0.24 dependent on the binding environment during diffusion. The diffusion-desorption ratio is consistent with the above hypothesis that the observed diffusion is a surface process and adds to previous experimental evidence on diffusion in water ice that suggests surface diffusion is important to the mobility of molecules within interstellar ices.