RESONANT ELECTRON-IMPACT EXCITATION OF VIBRATIONAL-MODES IN POLYATOMIC-MOLECULES

Measured differential cross sections (DCSs) for electron-impact excita tion of bending vibrational modes involving an odd number of vibration al quanta in CO2 by 4 eV incident energy electrons display a strong tr end to zero for forward and backward scattering which is characteristi c of 'symmetry-forbidden' transitions. This DCS behaviour is postulate d 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 m ay play a role in dissociative attachment in this 3.5-5.0 eV energy re gion. The resonant vibrational excitation mechanism proposed here is a lso believed to be operative in other polyatomic molecules and could b e investigated by performing selected electron energy-loss measurement s within the lowest energy resonance regions of the molecules CO2, N2O , OCS, CS2 and SO2.