  CO$_2$ ice has a phase transition at 35 K when its structure changes from
amorphous to crystalline. Using Reflection absorption Infrared Spectroscopy
(RAIRS), \"Oberg 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 Mart\'in-Dom\'enech 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.
