2018ApJ...852...75C

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.