Electron transport in gaseous and liquid argon: Effects of density and temperature

The scattering cross section of gaseous argon as a function of electron energy (2-300 meV) has been redetermined from the temperature dependence of the thermal electron mobility μₜₕ. The cross section at ε<10 meV is larger than previously reported. The threshold drift velocity above which electron heating occurs is vdᵗʰʳ≈100 m/s in the low-density gas at 121 K; the ratio of vdᵗʰʳ to the speed of sound is vdᵗʰʳc≈0.5, characteristic of energy loss by elastic collisions. In the dense gas at nnc0.2, where nc is the critical density: (1) the value of nμₜₕ increases; (2) the maximum in the plot of μn against field strength E/n shifts to lower E/n; (3) the temperature coefficient of μₜₕ at constant density increases. (1) is due to the mutual screening of the attractive, long-range scattering interactions; (2) is due to (1) and the constant "saturation" drift velocity; (3) is due to quasilocalization of the electrons. Quasilocalization or enhanced scattering in the coexistence vapor and liquid is significant at 0.2nnc1.6, and maximizes near nnc=0.6. Quasilocalization occurs to a smaller extent in argon than in xenon at the same nnc and TTc. The low-energy wing of the Ramsauer-Townsend effect is obliterated by screening at n≥1.0×10²² molecule/cm³ in both argon and xenon, which corresponds to nnc=1.2 in the former and 2.0 in the latter. The maximum in μₜₕ occurs at 1.2 ×{} 10²² molecule/cm³ in both liquids, corresponding to nnc=1.5 and 2.4, respectively. The magnitude of the maximum in μₜₕ is reasonably interpreted by the Lekner zero-scattering-length model, but it is not yet possible to explain quantitatively the density at which the maximum occurs. The drift velocities at high fields, E/n>5 mTd (5×10^-20 V cm²/molecule) little affected by density up to nnc=1.6 in argon. At higher density the drift velocities increase. Relatively large densities are required to affect the behavior at these fields because the scattering cross section in the vicinity of the Ramsauer-Townsend minimum is low ({~}1 ×{} 10^-17 cm²).