Virtual-state effects on elastic scattering and vibrational excitation of CO2 by electron impact - art. no. 040701

Near-threshold cross sections for elastic scattering and for excitation of the 100 symmetric stretch vibrational mode of CO2 have been calculated usin g the energy-modified adiabatic phase matrix method. These calculations are based on fixed-nuclei R matrices at several internuclear geometries that i ncorporate short-range exchange and correlation effects, The representation of the polarization response of the target inside the R-matrix box is impr oved using an alternative numerical technique, the AR procedure. These calc ulations over a range of internuclear distances reveal unanticipated behavi or of the near-threshold scattering matrix. A fixed-nuclei virtual state oc curs only in a limited range of internuclear separation, changing to a weak ly bound state outside this range. Thus an energy pole of the scattering ma trix remains near threshold and has a large effect on both elastic and vibr ational 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 comple x behavior.