Effect of the UV dose on the formation of complex organic molecules in astrophysical ices: irradiation of methanol ices at 20 K and 80 K

Methanol is a ubiquitous complex organic molecule (COM) in the interstellar medium, thought to be a precursor of larger COMs when it is submitted to different energetic processes, that can trigger chemical reactions in solid and gas phases. Using laboratory experiments, we report the characterization of the evolution of photoproducts generated by the UV irradiation of methanol ice at different UV doses and temperatures (20 and 80 K). We used gas chromatography coupled to mass spectrometry (GC-MS) to analyse the volatile organic compounds (VOCs) recovered during the warming of the photoprocessed methanol ice. We identified 21 molecules (with up to five carbon atoms, including alcohols, aldehydes, ketones, ester, and ethers) and followed their abundance as a function of the UV fluence and ice temperatures. With increasing UV fluence, an increase in the production of heavier COMs is observed, while species with 1 or 2 carbon atoms are depleted or do not increase. Species within a same chemical family show the same pattern of evolution, with heavier molecules present in smaller quantities. Ketones and esters are the chemical families that lead to more complex molecules and start forming at the earliest stages of irradiation. Their formation pathways are driven by radical recombinations with CO as the main building blocks. Aldehydes are formed before their alcohol counterparts, implying they do not form through alcohol dehydrogenation, but via radical recombination around HCO. Ethers seem to be the precursors of a large set of COMs, and alcohols present a steady profile throughout irradiation.