Carbon dioxide (CO2) is one of the most abundant molecules present in interstellar ice after H2O and CO. In the high mass protostars, CO2 represents an important tracer of the chemical and physical history. Further, the photoprocess of CO2 ice has been being the main subject of experimental studies quite recently.
In this study, we present CO2 ice deposited and VUV irradiated at different temperatures (14 K, 50 K, 70 K), that aims to understand the thermal effect on the mechanism of photodesorption of CO2 ice. We used a microwave-discharge hydrogen-flow lamp to mimic the interstellar UV field (114−170 nm). A quartz crystal microbalance (QCM) and a fouier transform infrared spectroscopy (FTIR) were selected to be the principal instruments in two different experimental systems, and both systems equipped with a quadrupole mass spectrometer (QMS) to measure the desorbed species during the whole experimental process.
Mass spectra show that desorbed molecules mostly originated from the event, CO2 ice decomposed to CO. Secondary desorbed molecules come from the photolysis of CO2 ice as VUV irradiation proceeding to form O2. Finally, a few CO2 molecules directly desorbed from CO2 ice. According to our results, photodesorption yield increased with the increased accumulation of photolysis products, reached a limited value due to the photolysis product only can desorb the surface layers at the temperature below sublimation of CO (14 K). After the ice temperature was raised up to 50K and 70K, Photo-product CO will either desorbed from the surface and also from the deep layers of ice. The amorphous ice structure will turn into crystalline above 50K and beyond, so CO will struggle to migrate to the surface gradually. Until the crystalline structure of CO2 was fully destroyed, CO will sublimate freely within the ice interior.
Moreover, when CO2 ice was deposited and VUV irradiated at the temperature which is above the sublimation temperature of CO, we determined the maximum mobile distance of CO molecule in CO2 ice at 50 K and 70 K.
Keyword：carbon dioxide，interstellar ice，photolysis，photodesorption，VUV，QMS||en_US|