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

Abstract

The formation of the cyclic carbon trioxide isomer, CO₃(X ¹A₁), 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 (¹D) and/or ground state (³P) 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 C_s nor the D_3h symmetric isomers of carbon trioxide were detected. Since the addition of O(¹D) involves a barrier of a 4–8 kJ mol⁻¹ and the reaction of O(³P) 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 ¹A₁), can be formed in low temperature carbon dioxide matrix via addition of suprathermal oxygen atoms to carbon dioxide. In the solid state, CO₃(X ¹A₁) 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 ¹⁸O enrichment in carbon dioxide in the atmospheres of Earth and Mars.