Probing the quantal identity of low-lying electronic states of CO²⁺ by quantum-chemical calculations and ion-translational-energy spectrometry

Abstract

Potential-energy curves of various electronic states of CO²⁺ and CO⁺ are computed using all-electron ab initio molecular-orbital methods. Configuration-interaction effects are treated by perturbative techniques (using Moller-Plesset perturbation theory to fourth order) and by variational methods (using the coupled-cluster approach). In the case of CO²⁺, calculations indicate that the lowest-energy ³∏ and ¹Σ⁺ states are nearly degenerate in the Franck-Condon region but that only the latter is likely to be metastable; the former is expected to predissociate rapidly due to a curve crossing with a purely repulsive ³Σ- state. Experimental measurements have been carried out on the kinetic energy released when metastable CO²⁺ ions dissociate by a tunneling mechanism, using an ion-translational-energy spectrometer. The kinetic-energy spectra are measured of fragment ions produced when CO²⁺ dissociates via an intermediate highly excited (dissociative) CO^+∗ state populated in an electron-capture reaction in collision with He. The experimental results remain difficult to interpret within the framework of the computed potential-energy curves.