Electron transmission spectroscopy: Resonances in triatomic molecules and hydrocarbons

Compound states of triatomic molecules and simple hydrocarbons are studied using electron transmission spectroscopy. Structures in the derivative of the current transmitted through a gas-filled collision chamber are interpreted as resonances in the electron-molecule cross sections. At low electron energies (0–6 eV) we observe, in N2O, H2S, and C2H4, broad and featureless structures which we identify as shape resonances. In the same energy range, the molecules CO2, NO2, C6H6, and SO2 exhibit narrow structures which form vibrational progressions. In CO2, NO2, and C6H6, these vibrational progressions are identified as shape resonances; in SO2, the interpretation is not clear cut. No low-energy resonances are observed in H2O and in CH4. At higher energies (9–17 eV) we observe sharp structures for H2O, H2S, N2O, CO2, and C2H4 (but not for C6H6 and CH4). These structures form bands, each band consisting of a vibrational progression. The states which are responsible for the bands consist of two Rydberg electrons moving in the field of a particular positive-ion core. These bands are similar to those found previously in diatomic molecules.