A fresh look at the 2A1 CO2- potential 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.