On the Photodesorption of CO ₂ Ice Analogs: The Formation of Atomic C in the Ice and the Effect of the VUV Emission Spectrum

CO ₂ ice has a phase transition at 30 K when its structure changes from amorphous to crystalline. Using reflection absorption infrared spectroscopy, berg 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 the energy distribution of the Microwave-Discharge Hydrogen-flow Lamp and UV absorption cross section of ices.