NASA Astrobiology Institute (NAI)

Higher carbon oxides of the form COn (n=3-8) have long been known as important molecules in atmospheric (Earth, Mars) and solid state chemical reactions. For instance, the CO3 molecule is considered as an important reaction intermediate in the atmospheres of Earth and Mars for quenching electronically excited oxygen atoms and in contributing to the anomalous 18O isotope enrichment. The geometry of the CO3 intermediate plays an important role in explaining these effects; however, only the cyclic (C2v) isomer has been experimentally confirmed prior to the project.

We have identified for the first time five higher carbon oxides via low temperature spectroscopy at 10 K by irradiating carbon dioxide ices with energetic electrons and probing the newly formed molecules via infrared spectroscopy (solid state) and a quadrupole mass spectrometer (gas phase): CO₃ [D₃h], CO₄ [C_2v; D₂d], CO₅ [C₂], and CO₆ [C_s]. These assignments have been confirmed in ¹²C¹⁸O₂, ¹³C¹⁶O₂, and ¹³C¹⁸O₂ systems. The time-dependent concentration profiles of the carbon oxides suggest that the CO3 molecules are initially formed by an addition of a suprathermal oxygen atom, generated via unimolecular decomposition of the carbon dioxide molecule, to the carbon-oxygen double bond and to the carbon atom to form CO₃ [C_2v] and CO₃ [D₃h], respectively. As the irradiation time increased, the ring structure is expanded successively to form CO₄ [C_2v], CO₅ [C₂], and CO₆ [Cs]. The investigation of the formation route to CO₄ [D₂d] is still in progress; preliminary fits of the temporal profile suggest that suprathermal oxygen atoms add to the carbon-oxygen double bond of CO₃ [C_2v].