Very low energy electron collisions with molecular chlorine

Electron transmission experiments are reported involving the measurement of absolute total cross-sections for scattering of electrons by Cl₂ for energies between 20 meV and 9.5 eV, with an energy resolution in the incident electron beam of between 3 and 4 meV full-width half-maximum. These data represent the first absolute values of scattering cross-sections recorded for Cl₂. Results are also reported for scattering in the presence of an axial magnetic field of 20 G over an energy range which extends from 16 meV to 250 meV. Strong evidence is found for electron attachment at very low impact energies, in agreement with the results of other groups. Data suggest that attachment occurs through the p-partial wave. A resonance in the form of a doublet in the total scattering cross-section has been found between 70 and 200 meV. This structure is absent in the spectrum obtained in the presence of an axial magnetic field. A qualitative mechanism is suggested for the formation of the observed structure involving virtual excitation of the first vibrational quantum of Cl₂. The doublet nature of the resonance is attributed to the intervention of the excited Π-states of Cl^- ₂. The plausibility of the proposed mechanism could be checked by accurate calculations of excited state potentials for Cl^- ₂ and by measurement of the yield of Cl^- ions as a function both of the electron energy and the temperature of the target gas.