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

Jiao He; Shahnewaj M. Emtiaz; Gianfranco Vidali

doi:10.3847/1538-4357/aa5f52

paper

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

arXiv:1611.02182 · doi:10.3847/1538-4357/aa5f52

Abstract

Observations by ISO and Spitzer towards young stellar objects (YSOs) showed that CO $_{2}$ 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 CO $_{2}$ 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 CO $_{2}$ molecules interact on a non-porous amorphous solid water (np-ASW) surface. We found that CO $_{2}$ 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 CO $_{2}$ on np-ASW is 2150 $\pm$ 50 K, assuming a diffusion pre-exponential factor of 10 $^{12}$ s $^{- 1}$ . This energy should also apply to the diffusion of CO $_{2}$ on wall of pores. The binding energy of CO $_{2}$ from CO $_{2}$ clusters and CO $_{2}$ from H $_{2}$ O ice have been found to be $2415 \pm 20$ and $2250 \pm 20$ ~K, respectively, assuming the same prefactor for desorption. CO $_{2}$ -CO $_{2}$ interaction is stronger than CO $_{2}$ -H $_{2}$ O interaction, in agreement with the experimental finding that CO $_{2}$ 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-H $_{2}$ O. As a result, CO wets np-ASW surface. This study should be of help to uncover the thermal history of CO $_{2}$ on the icy mantles of dust grains.

8 pages, 11 figures, submitted to ApJ

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

Abstract

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