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

Observations by ISO and Spitzer toward young stellar objects showed that CO₂ segregates in the icy mantles covering dust grains. Thermal processing of the ice mixture was proposed as being responsible for the segregation. Although several laboratories 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 and Reflection Absorption InfraRed Spectroscopy to study how CO₂ molecules interact on a non-porous amorphous solid water (np-ASW) surface. We found that CO₂ diffuses significantly on an 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 10¹² s^-1. This energy should also apply to the diffusion of CO₂ on the wall of pores. The binding energy of CO₂ from CO₂ clusters and CO₂ from H₂O ice has been found to be 2415 ±20 K 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 the 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 the np-ASW surface. This study should be of help to uncover the thermal history of CO₂ on the icy mantles of dust grains.