Diffusion and Clustering of Carbon Dioxide on Non-porous Amorphous Solid Water

Observations by ISO and Spitzer towards young stellar objects (YSOs) showed that CO2 segregates in the icy mantles covering dust grains. Thermal processing of ice mixture was proposed as responsible for the segregation. Although several laboratory studied thermally induced segregation, a satisfying quantification is still missing. We propose that the diffusion of CO2 along pores inside water ice is the key to quantify segregation. We combined Temperature Programmed Desorption (TPD) and Reflection Absorption InfraRed Spectroscopy (RAIRS) to study how CO2 molecules interact on a non-porous amorphous solid water (np-ASW) surface. We found that CO2 diffuses significantly on a np-ASW surface above 65 K and clusters are formed at well below one monolayer. A simple rate equation simulation finds that the diffusion energy barrier of CO2 on np-ASW is 2150±50 K, assuming a diffusion pre-exponential factor of 10 s. This energy should also apply to the diffusion of CO2 on wall of pores. The binding energy of CO2 from CO2 clusters and CO2 from H2O ice have been found to be 2415± 20 and 2250± 20 K, respectively, assuming the same prefactor for desorption. CO2-CO2 interaction is stronger than CO2-H2O interaction, in agreement with the experimental finding that CO2 does not wet np-ASW surface. For comparison, we carried out similar experiments with CO on np-ASW, and found that the CO-CO interaction is always weaker than CO-H2O. As a result, CO wets np-ASW surface. This study should be of help to uncover the thermal history of CO2 on the icy mantles of dust grains.