Virtual-state effects on elastic scattering and vibrational excitation of CO₂ by electron impact

Near-threshold cross sections for elastic scattering and for excitation of the 100 symmetric stretch vibrational mode of CO₂ have been calculated using the energy-modified adiabatic phase matrix method. These calculations are based on fixed-nuclei R matrices at several internuclear geometries that incorporate short-range exchange and correlation effects. The representation of the polarization response of the target inside the R-matrix box is improved using an alternative numerical technique, the ΔR procedure. These calculations over a range of internuclear distances reveal unanticipated behavior of the near-threshold scattering matrix. A fixed-nuclei virtual state occurs only in a limited range of internuclear separation, changing to a weakly bound state outside this range. Thus an energy pole of the scattering matrix remains near threshold and has a large effect on both elastic and vibrational cross sections, but cannot be simply described as a virtual state. Our quantitative results, in good agreement with experiment, indicate that the present modified adiabatic methodology is adequate to treat this complex behavior.