Elastic electron scattering and vibrational excitation of CO₂in the threshold energy region

In a crossed-beam experiment e - +CO 2 elastic scattering and the excitation of the three fundamental vibrational modes have been investigated in the energy region below the 2 Pi u shape resonance. Angular and energy dependences of the differential cross sections have been determined in absolute units below 1.05 eV collision energy. For elastic scattering the large increase of the cross section towards zero energy is consistent with the existence of a virtual state. The angular dependences of the elastic differential cross sections below 1 eV indicate that direct scattering is important even at very low collision energies. The excitation of the two infrared-active modes (010) and (001) below 1 eV is mainly determined by the direct excitation via the dipole potential, whereas the excitation of the symmetric-stretch mode (100) is dominated by s-wave scattering. A comparison with recent theoretical investigations shows that angular and energy dependences for this mode near threshold are strongly influenced by the virtual state of the e - +CO 2 potential.