Cross sections for electron attachment resonances peaking at subthermal energies

A simple procedure is described whereby both the magnitude and the energy dependence of the cross section for subthermal peaking electron attachment resonances are determined from electron swarm data alone. For an initial energy region it is shown that the attachment cross section can be approximated by σ_a(ε) = A_γ/ε^γ, and a determination of the power γ of the electron energy ε as well as of the corresponding constant A_γ yields both the magnitude and the energy dependence of the cross section at thermal and epithermal energies. The values of A_γ [in (ergs)^γ square centimeters], γ, and σ_a(0.05 eV) (in square centimeters) are 2.5×10^-29,1.12,1.17×10^-14; 7×10 ^-34, 1.412, 2.17×10^-15;9.1×10 ^-32,1.148, 1×10^-16; and 1.66× 10 ^-29, 1.031, 5.2×10^-16 for sulfur hexafluoride, 1,4-naphthoquinone, anthracene, and 1,2-benzanthracene, respectively. The thermal (T=298°K) value of the attachment rate for SF₆ in the carrier gases N₂ and C₂H₄ is found to be 8.7×10⁹ sec^-1·torr1 and 9×10 ⁹ sec^-1·torr^-1, respectively. These values are in excellent agreement with the most reliable data available which are summarized. Cross sections are reported also for SF_5- at 298°K. This work demonstrates further the potential of the electron swarm method in electron attachment studies.