A fresh look at the²A₁CO₂potential energy surface

In a recent letter detailing the observation of vibrational structure in vibrationally inelastic electron–CO2 scattering cross sections (Allan M 2002 J. Phys. B: At. Mol. Opt. Phys. 35 L387), it was concluded that the potential energy surface of CO2− has to look considerably different from the picture anticipated so far. The experimental data suggest that either the adiabatic electron affinity of CO2 is markedly lower than the best theoretical values, or the potential energy surface of CO2− in the vicinity of the surface of the neutral shows a qualitatively different behaviour from what had been predicted. Here we present a theoretical study of the CO2− ground state potential energy surface in the region where the anion is electronically stable, using extended basis sets and highly correlated levels of theory. Our findings support the latter conjecture: starting from the minimum energy structure of bent CO2−, the surface of the anion is found to exhibit a barrier and then drops to values below that of the minimal energy structure, before crossing the surface of neutral CO2. This qualitative behaviour is only revealed when at least three sets of diffuse functions are added to the one-particle basis set, indicating a change in character of the anionic state. The theoretical results suggest a new way of thinking about electron detachment from metastable CO2−, and have profound implications for the calculation of potential energy surfaces of polar anions.