AN EXPERIMENTAL INVESTIGATION OF THE DECOMPOSITION OF CARBON MONOXIDE AND FORMATION ROUTES TO CARBON DIOXIDE IN INTERSTELLAR ICES

The formation of carbon dioxide from the processing of carbon monoxide (CO) and molecular oxygen (O-18(2)) via radiolysis is studied within the context of its formation in interstellar ices in quiescent clouds. With the help of isotopic labeling, we were able to "trace" the atoms and provide mechanistical information on how carbon monoxide is decomposed, and carbon dioxide is formed in interstellar ices. Here, we quantify the production of O-18(3), (OO2)-O-18, (OOO)-O-18-O-18, (CO)-O-18, CO2, (OCO)-O-18, and (CO2)-O-18. In contrast to experiments using ultraviolet irradiation, we find that upon exposure to energetic electrons, isolated carbon monoxide molecules are able to undergo unimolecular decomposition to give suprathermal carbon (C) and oxygen (O) atoms. Molecular oxygen decomposes to two oxygen atoms. The free oxygen atoms can react with carbon monoxide via addition to form the carbon dioxide isotopomers as observed experimentally. This mechanism to form carbon dioxide is distinctly different to the one observed in pure carbon monoxide ices where electronically excited carbon monoxide reacts with a neighboring carbon monoxide molecule to form solely carbon dioxide and a carbon atom.