On The Photodesorption of CO${}_2$ Ice Analogues: The Formation of Atomic C in The Ice and the Effect of The VUV Emission Spectrume

Abstract

CO${}_2$ ice has a phase transition at 35 K when its structure changes from amorphous to crystalline. Using Reflection absorption Infrared Spectroscopy (RAIRS), Öberg et al. observed that the photodesorption yield of CO${}_2$ ice deposited at 60 K and irradiated at 18 K is 40% lower than that of CO${}_2$ ice deposited and irradiated at 18 K. In this work, CO${}_2$ ices were deposited at 16-60 K and UV-irradiated at 16 K to rule out the temperature effect and figure out the relationship between photodesorption yield and ice structure. IR spectroscopy is a common method used for measurement of the photodesorption yield in ices. We found that undetectable C atoms produced in irradiated CO${}_2$ ice can account for 33% of the amount of depleted CO${}_2$ molecules in the ice. A quantitative calibration of QMS was therefore performed to convert the measured ion current into photodesorption yield. During various irradiation periods, the dominant photodesorbing species were CO, O${}_2$, and CO${}_2$, and their photodesorption yields in CO${}_2$ ices deposited at different temperature configurations were almost the same, indicating that ice morphology has no effect on the photodesorption yield of CO${}_2$ ice. In addition, we found that the lower desorption yield reported by Martín-Doménech et al. is due to a linear relationship between the photodesorption yield and the combination of energy distribution of Microwave-Discharge Hydrogen-flow Lamp (MDHL) and UV absorption cross section of ices.

11 pages, 11 figures

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• Photochemistry of interstellar ice forming Complex Organic Molecules
• Flux and fluence effects on the Vacuum-UV photodesorption and photoprocessing of CO${}_2$ ices