Photon-induced desorption of larger species in UV-irradiated methane (CH4) ice

Abstract

At the low temperatures found in the interior of dense clouds and circumstellar regions, along with H${}_2$O and smaller amounts of species such as CO, CO${}_2$, or CH${}_3$OH, the infrared features of CH${}_4$ have been observed on icy dust grains. Ultraviolet (UV) photons induce different processes in ice mantles, affecting the molecular abundances detected in the gas-phase. This work aims to understand the processes that occur in a pure CH${}_4$ ice mantle submitted to UV irradiation. We studied photon-induced processes for the different photoproducts arising in the ice upon UV irradiation. Experiments were carried out in ISAC, an ultra-high vacuum chamber equipped with a cryostat and an F-type UV-lamp reproducing the secondary UV-field induced by cosmic rays in dense clouds. Infrared spectroscopy and quadrupole mass spectrometry were used to monitor the solid and gas-phase, respectively, during the formation, irradiation, and warm-up of the ice. Direct photodesorption of pure CH${}_4$ was not observed. UV photons form CH${}_x\cdot$ and H$\cdot$ radicals, leading to photoproducts such as H${}_2$, C${}_2$H${}_2$, C${}_2$H${}_6$, and C${}_3$H${}_8$. Evidence for the photodesorption of C${}_2$H${}_2$ and photochemidesorption of C${}_2$H${}_6$ and C${}_3$H${}_8$ was found, the latter species is so far the largest molecule found to photochemidesorb. ${}^{13}$CH${}_4$ experiments were also carried out to confirm the reliability of these results.

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Cited by in corpus (7)

• Laboratory and Computational Studies of Interstellar Ices
• An overview of desorption parameters of Volatile and Complex Organic Molecules: A systematic dig on experimental literature
• Synthesis of the first nitrogen-heterocycles in interstellar ice analogs containing methylamine (CH${}_3$NH${}_2$) exposed to UV radiation: Formation of trimethylentriamine (TMT, c-(-CH${}_2$-NH)${}_3$) and hexamethylentetramine (HMT, (CH${}_2$)${}_6$N${}_4$)
• Photochemistry of interstellar ice forming Complex Organic Molecules
• Catalytic role of HI in the interstellar synthesis of complex organic molecule
• Photocleavage of aliphatic C--C bonds in the interstellar medium
• In situ characterization of volatile and refractory hydrocarbons produced by UV photolysis of interstellar C${}_2$H${}_2$ ice