VERY-LOW ENERGY ELECTRON COLLISIONS WITH MOLECULAR CHLORINE

Electron transmission experiments are reported involving the measureme nt of absolute total cross-sections for scattering of electrons by Cl- 2 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-max imum. These data represent the first absolute values of scattering cro ss-sections recorded for Cl-2. Results are also reported for scatterin g in the presence of an axial magnetic field of 20 G over an energy ra nge which extends from 16 meV to 250 meV. Strong evidence is found for electron attachment at very low impact energies, in agreement with th e 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 axi al magnetic field. A qualitative mechanism is suggested for the format ion of the observed structure involving virtual excitation of the firs t vibrational quantum of Cl-2 The doublet nature of the resonance is a ttributed to the intervention of the excited Pi-states of Cl-2(-). The plausibility of the proposed mechanism could be checked by accurate c alculations of excited state potentials for Cl-2(-) and by measurement of the yield of Cl- ions as a function both of the electron energy an d the temperature of the target gas.