VUV Photolysis of CH4–H2O mixture in methane-rich ices: Formation of large complex organic molecules in astronomical environments

Abstract

The present work aims to highlight the influence of water molecules in the photodecomposition of methane ice and reveal the photoproducts formed in solid phase upon VUV irradiation of CH 4-H 2 O mixture in methane-rich ices. The analysis of our IR spectra shows that even with very low concentrations of water in methane ices, several oxygenated hydrocarbons are formed as photoproducts derived from the photodecomposition of water and methane at cryogenic temperatures. We show that both alka[e]nes and oxygen bearing organics are efficiently formed at temperatures as low as 3 K. However, while the IR signatures of the alka[e]nes such as C 2 H 6 , C 2 H 4 and C 2 H 2 dominate the IR spectra of the irradiated CH 4-H 2 O ices at temperatures lower than 50 K, the heating of the sample to 110 K reveals the formation of large carbon chain complex organic molecules such as ethanol, pronanol, porpanal and glycolaldehyde.

Proton and Electron Irradiations of CH4:H2O Mixed Ices

“…In their study on the irradiation of CH4:H2O ices by 40 MeV nickel ions at 15 K, Mejía et al [23] were able to additionally demonstrate the formation of ethanol, C2H5OH; ethanal, CH3CHO; and methanoic acid, HCOOH along with simpler molecules such as carbon monoxide, CO; CO2, and H2CO. Similar results were reported by Krim and Jonusas [24], who also reported the formation of a number of ether molecules as well as glycolaldehyde, HOCH2CHO after the irradiation of a CH4:H2O ice by vacuum-ultraviolet photons at 3 K.…”

“…The necessary precursor radicals in the reactions leading to the formation of CH3OH highlighted above have all been observed in experimental studies [24], and thus all likely contribute to its formation in our work to at least some extent.…”

“…The processing of CH4:H2O mixed ices results in the appearance of a number of new absorption bands due to the formation of various daughter molecules as a result of the radiation chemistry occurring within the ices. The most likely first steps leading to the formation of daughter molecules is the radiation-induced dehydrogenation of CH4 and H2O to respectively yield CH3 and OH radicals [21][22][23][24].…”

“…Of interest to this present study is the radiation chemistry occurring in CH4:H2O mixed ices due to the ubiquity of H2O ices within astronomical environments [20], as well as the apparent association of CH4 formation with H2O-rich environments [2,3]. This radiation chemistry has been considered by previous experimental studies [ [21][22][23][24] which have shown that alcohols, higher hydrocarbons, and methanal, H2CO are among the readily produced closed-shell daughter molecules. For instance, the study of Wada et al [22] considered the low-energy electron irradiation of CH4:H2O ices at 10 K and found that methanol, CH3OH is the major daughter molecule of this process and is formed by the combination of hydroxyl, OH and methyl, CH3 radicals or alternatively by the insertion reaction between methylene radicals, CH2 and H2O.…”

Matrix-Assisted Processes in CH4-Doped Ar Ices Irradiated with an Electron Beam

“…[9]. Various types of ionizing radiation were used to study the radiation behavior of methane and methane-rich ices-ions [10][11][12][13][14][15][16][17][18][19], electrons [20][21][22][23][24][25][26][27][28][29] and photons [26,[30][31][32][33][34][35].…”