Untangling the formation of the cyclic carbon trioxide isomer in low temperature carbon dioxide ices

The formation of the cyclic carbon trioxide isomer, CO3(X-1 A(1)), in carbon-dioxide-rich extraterrestrial ices and in the atmospheres of Earth and Mars were investigated experimentally and theoretically. Carbon dioxide ices were deposited at 10 K onto a silver (111) single crystal and irradiated with 5 keV electrons. Upon completion of the electron bombardment, the samples were kept at 10 K and were then annealed to 293 K to release the reactants and newly formed molecules into the gas phase. The experiment was monitored via a Fourier transform infrared spectrometer in absorption-reflection-absorption ( solid state) and through a quadruple mass spectrometer ( gas phase) on-line and in situ. Our investigations indicate that the interaction of an electron with a carbon dioxide molecule is dictated by a carbon - oxygen bond cleavage to form electronically excited (D-1) and/or ground state (P-3) oxygen atoms plus a carbon monoxide molecule. About 2% of the oxygen atoms react with carbon dioxide molecules to form the C-2v symmetric, cyclic CO3 structure via addition to the carbon - oxygen double bond of the carbon dioxide species; neither the Cs nor the D-3h symmetric isomers of carbon trioxide were detected. Since the addition of O(D-1) involves a barrier of a 4 - 8 kJ mol(-1) and the reaction of O(P-3) with carbon dioxide to form the carbon trioxide molecule via triplet-singlet intersystem crossing is endoergic by 2 kJ mol(-1), the oxygen reactant(s) must have excess kinetic energy (suprathermal oxygen atoms which are not in thermal equilibrium with the surrounding 10 K matrix). A second reaction pathway of the oxygen atoms involves the formation of ozone via molecular oxygen. After the irradiation, the carbon dioxide matrix still stores ground state oxygen atoms; these species diffuse even at 10 K and form additional ozone molecules. Summarized, our investigations show that the cyclic carbon trioxide isomer, CO3(X (1)A(1)), can be formed in low temperature carbon dioxide matrix via addition of suprathermal oxygen atoms to carbon dioxide. In the solid state, CO3(X (1)A(1)) is being stabilized by phonon interactions. In the gas phase, however, the initially formed C-2v structure is rovibrationally excited and can ring-open to the D-3h isomer which in turn rearranges back to the C-2v structure and then loses an oxygen atom to 'recycle' carbon dioxide. This process might be of fundamental importance to account for an O-18 enrichment in carbon dioxide in the atmospheres of Earth and Mars.