ICOM formation from radical chemistry: a mechanistic study from cryogenic matrix coupled with IR and EPR spectroscopies

Abstract

ABSTRACT Interstellar complex organic molecules (iCOMs) have been identified in different interstellar environments including star forming regions as well as cold dense molecular clouds. Laboratory studies show that iCOMs can be formed either in gas phase or in the solid state, on icy grains, from ‘non-energetic’ (atom-addition/abstraction) or energetic (UV-photon, particle bombardments) processes. In this contribution, using a new experimental approach mixing matrix isolation technique, mass spectrometry, and infrared and EPR spectroscopies, we want to investigate the COM formation at 35 K from a complex mixture of ground state radicals trying to draw a general reaction scheme. We photolyse (121 nm) CH3OH diluted in Ar at low temperature (below 15 K) to generate $\mathrm{H^.CO}$, $\mathrm{HO^.CO}$, $\mathrm{^.CH_2OH}$, $\mathrm{CH_3O^.}$, $\mathrm{^.OH}$, and $\mathrm{^.CH_3}$ radicals and ‘free’ H-atoms within the matrix. Radicals have been identified using infrared and EPR spectroscopies. With the disappearance of the Ar matrix (at 35 K), these unstable species are then free to react, forming new species in a solid film. Some recombination products have been detected using infrared spectroscopy and mass spectrometry in the solid film after Ar removal, namely methyl formate (CH3OCHO), glycolaldehyde (HOCH2CHO), ethylene glycol (HOCH2CH2OH), glyoxal (CHOCHO), ethanol (CH3CH2OH), formic acid (HCOOH), dimethyl ether (CH3OCH3), methoxymethanol (CH3OCH2OH), and CH4O2 isomers (methanediol and/or methyl hydroperoxide). The detected molecules are fully consistent with the radicals detected and strongly support the solid state scenario of iCOM formation in interstellar ices based on radical–radical recombination. We then discuss astrophysical implications of the radical pathways on the observed gas phase iCOMs.