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

H. Carrascosa; G. A. Cruz-Díaz; G. M. Muñoz Caro; E. Dartois; Y. -J. Chen

doi:10.1093/mnras/staa334

paper

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

arXiv:2002.00173 · doi:10.1093/mnras/staa334

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.

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

Abstract

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