Measurement by Cyclotron Resonance of Molecular Cross Sections for Elastic Collisions with 295°K Electrons

Cyclotron resonance absorption is used to obtain molecular scattering cross sections for electrons at 295°K. The electrons were present in active nitrogen obtained by passing N₂ through a microwave discharge, and various gases were introduced into the active nitrogen downstream from the discharge. Very weak rf fields, with electric components parallel to the dc magnetic field, were used in order not to ``heat up'' the electrons. The observed width of the cyclotron resonance line is proportional to the molecular‐electronic cross section since electron—electron collisions are not significant in these experiments. But suitable averages over electron velocities must be carried out to account for the observed line shapes and to relate the line widths to the cross sections. The spectral lines are sometimes quite asymmetric and this asymmetry can then be used as a rather sensitive measure of electron concentration. Dipolar gases were observed to have large cross sections which exhibited approximately a $\frac{3}{2}$ power dependence on the dipole moment; theoretical calculations based on a partial wave treatment of dipole scatterers agreed well with experiments. The cross section for N₂ is 4.96 Ų; for CO₂ 302 Ų; and for the dipolar gases N₂O, NH₃, SO₂, HCF₃, H₂O, HCN, and CH₃NO₂, the cross sections are 56, 1560, 1840, 1760, 2360, 5280, and 6020 Ų, respectively. The dipolar cross sections are inversely proportional to the energy of the electrons, and both theoretical and experimental values of the cross sections refer to energies of 3k(295°)/2. The large cross section for CO₂ probably arises from its large quadrupole moment.