Formation of complex organic molecules in methanol and methanol–carbon monoxide ices exposed to ionizing radiation – a combined FTIR and reflectron time-of-flight mass spectrometry study

Abstract

The radiation induced chemical processing of methanol and methanol–carbon monoxide ices at 5.5 K exposed to ionizing radiation in the form of energetic electrons and subsequent temperature programmed desorption is reported in this study. The endogenous formation of complex organic molecules was monitored online and in situ via infrared spectroscopy in the solid state and post irradiation with temperature programmed desorption (TPD) using highly sensitive reflectron time-of-flight (ReTOF) mass spectrometry coupled with single photoionization at 10.49 eV. Infrared spectroscopic analysis of the processed ice systems resulted in the identification of simple molecules including the hydroxymethyl radical (CH₂OH), formyl radical (HCO), methane (CH₄), formaldehyde (H₂CO), carbon dioxide (CO₂), ethylene glycol (HOCH₂CH₂OH), glycolaldehyde (HOCH₂CHO), methyl formate (HCOOCH₃), and ketene (H₂CCO). In addition, ReTOF mass spectrometry of subliming molecules following temperature programmed desorption definitely identified several closed shell C/H/O bearing organics including ketene (H₂CCO), acetaldehyde (CH₃COH), ethanol (C₂H₅OH), dimethyl ether (CH₃OCH₃), glyoxal (HCOCOH), glycolaldehyde (HOCH₂CHO), ethene-1,2-diol (HOCHCHOH), ethylene glycol (HOCH₂CH₂OH), methoxy methanol (CH₃OCH₂OH) and glycerol (CH₂OHCHOHCH₂OH) in the processed ice systems. Additionally, an abundant amount of molecules yet to be specifically identified were observed sublimating from the irradiated ices including isomers with the formula C₃H_(x=4,6,8)O, C₄H_(x=8,10)O, C₃H_(x=4,6,8)O₂, C₄H_(x=6,8)O₂, C₃H_(x=4,6)O₃, C₄H₈O₃, C₄H_(x=4,6,8)O₄, C₅H_(x=6,8)O₄ and C₅H_(x=6,8)O₅. The last group of molecules containing four to five oxygen atoms observed sublimating from the processed ice samples include an astrobiologically important class of sugars relevant to RNA, phospholipids and energy storage. Experiments are currently being designed to elucidate their chemical structure. In addition, several reaction pathways were identified in the irradiated ices of mixed isotopes based upon the results of both in situ FTIR analysis and TPD ReTOF gas phase analysis. In general, the results of this study provide crucial information on the formation of a variety of classes of organics including alcohols, ketones, aldehydes, esters, ethers, and sugars within the bulk ices upon exposure to ionizing radiation that are relevant to the molecular clouds within the interstellar medium.