Strong mode dependence of the 3.8-eV resonance in CO₂ vibrational excitation by electron impact

We have made a systematic observation for integrated cross sections that vibrational excitation modes are found to show quite different energy dependence in the 3.8-eV shape resonance peak in a CO₂ molecule. This feature arises from the different interaction scheme for excitation to each mode, i.e., the weak polarization interaction for excitation to the symmetric stretching mode, weak dipole interaction to the bending mode, and strong dipole interaction to the asymmetric stretching mode. The asymmetric stretching shows a sharp increase in differential cross sections (DCSs) toward small-angle scattering due to the strong dipole interaction, which washes out resonance features seen at intermediate angles. DCSs for the symmetric and bending modes clearly show conspicuous features at a wide range of the scattering angle around 3.8-eV shape resonance.