Potential energy surface of the CO2- anion

Electron attachment to CO2 is studied by exploring the parts of the potential energy surface where an electron can be bound in the fixed-nuclei picture. High level ab initio methods are used, and in particular the question of an adequate description of the threshold region, where the attachment energy goes to zero and the electronically bound anionic states become unstable, is discussed. Since the relevant coordinates are the symmetric stretching and the bending mode, we consider a two dimensional cut (C-2v symmetry constraint) through the three dimensional nuclear coordinate space, and specifically C-O bond lengths between 1.1 and 1.6 Angstrom and bending angles between 180 and 125degrees are examined. Three bound CO2- states are identified in this region. The minimum associated with the long-lived CO2- state is localised on the lowest surface of the anion, and only a small barrier separates it from the region where the anion becomes unstable. Using earlier results from R-matrix calculations and our findings we can then answer the question which of the short-lived scattering states connects to the long-lived CO2- species. All three states of the CO2- anion are vibronically coupled, and the implications for the nuclear dynamics of these states are discussed, in particular in view of the recently observed vibrational excitation spectra.