Scattering Cross Sections of Argon and Atomic Oxygen to Thermal Electrons

Abstract

The electron-scattering cross section for momentum transfer of several neutral species has been experimentally determined. An $S$-band microwave interferometer was employed to investigate the plasma produced by the incident shock wave in a shock tube. This plasma was in the state of equilibrium, so the relative species concentrations and the electron velocity distribution were known. For electron-collision frequencies small compared with the signal frequency, the microwave data could be intepreted to yield the scattering cross sections. The cross section of the argon atom was measured from 1800 to 5500°K. The results agree with previous drift measurements when those cross sections are averaged over the Maxwellian distribution of electron velocities. Above 3000°K the oxygen molecule becomes dissociated and the oxygen-atom cross section can be measured. From 3000 to 4000°K, this atomic cross section is (1.2±0.2) × ${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$. For molecular nitrogen and oxygen the small-collision-frequency requirement could not be fully satisfied. The cross sections for these gases were approximately ${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$ from 2000 to 3500°K, with no observable dependency on gas density.