Absolute Measurements of Total Cross Sections for the Scattering of Low-Energy Electrons by Atomic and Molecular Oxygen

The atom-beam recoil technique has been used to measure absolute total cross sections for the scattering of electrons by atomic and molecular oxygen at 22 energies between 0.5 and 11.3 eV, with additional data obtained up to 100 eV. In this method, a modulated electron beam cross-fires an atom beam, whose attenuation at the modulation frequency is observed. A mechanical chopper is utilized so that both the total atom beam and its scattered component can be measured by the same ac detection and amplification systems. It is therefore possible to determine absolute cross sections without requiring knowledge of the beam detection efficiency or the over-all gain of the amplification system. The angular resolution in the electron polar scattering angle ranges from about 15.6° at 1 eV to 9.6° at 12 eV for O, and from about 18.5° at 1 eV to 11.4° at 12 eV for ${\mathrm{O}}_2$. Our absolute values for ${\mathrm{O}}_2$ between 0.5 and 11.3 eV are between 10% and 20% higher than those of Brüche, while the shape of the curve agrees well. The atomic cross sections vary from about 5.3 ${\mathrm{\AA }}^2$ at 0.5 eV to 8.3 ${\mathrm{\AA }}^2$ at 11.3 eV, with an estimated error of 20%, except for the 0.5-eV point, which has an error of 30%. These results are in excellent agreement with semiempirical calculations of Cooper and Martin and of Robinson and Geltman, as well as with the polarized-orbitals calculation of Temkin, and the the static central field exchange approximation of Myerscough.