Total cross-section for low-energy and very low-energy electron collisions with O₂

An absolute total cross-section for electron scattering from O₂ is obtained in the energy range from 20 eV down to 16 meV with a very narrow electron energy width of 7 meV, using the threshold-photoelectron source employing synchrotron radiation. The total cross-section obtained in the present study generally agrees well with those obtained in previous experimental works in the energy region where the previous experimental total cross-sections agree with each other. At lower energies, several features are found, including the very large contribution of the ${}^2{\Pi }_g$ resonance to the total cross-section. The resonance energies and the resonance widths of the individual vibrational states of the ${}^2{\Pi }_g$ resonance are determined from the analysis of the measured absolute total cross-section. The resonance width of the ${}^2{\Pi }_g$ resonance of O₂ is found to be much narrower than the value estimated from previous high-resolution experiments and theoretical calculations. (paper)