Mechanisms of methyl formate production during electron-induced processing of methanol-carbon monoxide ices

The formation of methyl formate (CH3OCHO) upon electron irradiation of mixed ices of carbon monoxide (CO) and methanol (CH3OH) has been monitored by post-irradiation thermal desorption spectrometry (TDS). The energy dependence of the product yields obtained with electron energies between 3 and 18 eV was studied. These energies are characteristic of secondary electrons that are released in vast numbers under the effect of ionizing radiation. Our results reveal that the reactions leading to methyl formate are initiated by a number of different electron-molecule interactions that produce CH3O˙ radicals. Dissociative electron attachment (DEA) to CH3OH around 5.5 eV and neutral dissociation (ND) above 7 eV release CH3O˙ radicals that can add to CO to initiate a reaction sequence leading to formation of methyl formate. Around 10 eV, DEA to CO yields an oxygen radical anion that reacts with CH3OH to also produce CH3O˙ radicals. Alternatively, CH3OH can also release H˙ radicals upon both DEA and ND. These can also add to CO to form HCO˙ radicals as an intermediate to formaldehyde (H2CO), which was also investigated to unravel the reaction mechanisms leading to formation of methyl formate. The recombination of HCO˙ and CH3O˙ as minority radical species is considered as an alternative but less probable pathway to the formation of methyl formate. To the best of our knowledge, this is the first study showing considerable contributions of DEA to the formation of methyl formate in CH3OH containing ices. Thus, our study has important implications for current astrochemical models.