Resonant electron-impact excitation of vibrational modes in polyatomic molecules

Measured differential cross sections (DCSs) for electron-impact excitation of bending vibrational modes involving an odd number of vibrational quanta in CO2 by 4 eV incident energy electrons display a strong trend to zero for forward and backward scattering which is characteristic of 'symmetry-forbidden' transitions. This DCS behaviour is postulated here to be produced by a Feshbach resonant mechanism involving a low-lying bent excited state of CO2. The model described here identifies three additional low-lying bent excited states of CO2 which could also be parent states for core-excited Feshbach resonances, one of which may play a role in dissociative attachment in this 3.5-5.0 eV energy region. The resonant vibrational excitation mechanism proposed here is also believed to be operative in other polyatomic molecules and could be investigated by performing selected electron energy-loss measurements within the lowest energy resonance regions of the molecules CO2, N2O, OCS, CS2 and SO2.