2008ApJ...686.1493Z

We investigated the formation of two C₃H₂ O isomers, i.e., cyclopropenone (c-C₃H₂O) and propynal (HCCCHO), in binary ice mixtures of carbon monoxide (CO) and acetylene (C₂H₂) at 10 K in an ultrahigh vacuum machine on high-energy electron irradiation. The chemical evolution of the ice samples was followed online and in situ via a Fourier transform infrared spectrometer and a quadrupole mass spectrometer. The temporal profiles of the cyclopropenone and propynal isomers suggest (pseudo-) first-order kinetics. The cyclic structure (c-C₃H₂O) is formed via an addition of triplet carbon monoxide to ground-state acetylene (or vice versa); propynal (HCCCHO) can be synthesized from a carbon monoxide-acetylene complex via a [HCO…CCH] radical pair inside the matrix cage. These laboratory studies showed for the first time that both C₃H₂ O isomers can be formed in low-temperature ices via nonequilibrium chemistry initiated by energetic electrons as formed in the track of Galactic cosmic ray particles penetrating interstellar icy grains in cold molecular clouds. Our results can explain the hitherto unresolved gas phase abundances of cyclopropenone in star-forming regions via sublimation of c-C₃H₂ O as formed on icy grains in the cold molecular cloud stage. Implications for the heterogeneous oxygen chemistry of Titan and icy terrestrial planets and satellites suggest that the production of oxygen-bearing molecules such as C₃H₂ O may dominate on aerosol particles compared to pure gas phase chemistry.