Electron mobilities in low density helium and nitrogen gases: Momentum transfer cross sections at very low energies

Electron mobilities have been measured in low density helium and nitrogen gases as reference standards in the ongoing study of effects of molecular shape and density on electron transport in fluids. Wider temperature ranges were used than heretofore. Electron mobilities in helium gas at temperatures T=11–466 K and densities n=4.9–7.3 (1025 molec/m3) are consistent with previously reported scattering cross sections. The value of σm at 10−23 J (10−4 eV) is 4.9×10−20 m2, corresponding to a scattering length of 6.2×10−11 m. Density normalized mobilities nμ in nitrogen gas at T=66–460 K and n=0.9–6.7 (1025 molec/m3) decrease with increasing density. Values of nμ extrapolated to low field strengths and zero density gave values which were used to estimate momentum transfer cross sections. Further study of electron scattering by nitrogen at energies <3×10−21 J (20 meV) is desired.