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 ${\mu }_{\mathrm{th}}$. The cross section at $\epsilon <10\mathrm{}$ meV is larger than previously reported. The threshold drift velocity above which electron heating occurs is $v_d^{\mathrm{thr}}\approx 100$ m/s in the low-density gas at 121 K; the ratio of $v_d^{\mathrm{thr}}$ to the speed of sound is $\frac{v_d^{\mathrm{thr}}}{c}\approx 0.5$, characteristic of energy loss by elastic collisions. In the dense gas at $\frac{n}{n_c}\gtrsim 0.2$, where $n_c$ is the critical density: (1) the value of $n{\mu }_{\mathrm{th}}$ increases; (2) the maximum in the plot of $\mu n$ against field strength $\frac{E}{n}$ shifts to lower $\frac{E}{n}$; (3) the temperature coefficient of ${\mu }_{\mathrm{th}}$ 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 $0.2\lesssim \frac{n}{n_c}\lesssim 1.6$, and maximizes near $\frac{n}{n_c}=0.6\mathrm{}$. Quasilocalization occurs to a smaller extent in argon than in xenon at the same $\frac{n}{n_c}$ and $\frac{T}{T_c}$. The low-energy wing of the Ramsauer-Townsend effect is obliterated by screening at $n\ge 1.0\times {10}^{22}$ molecule/${\mathrm{cm}}^3$ in both argon and xenon, which corresponds to $\frac{n}{n_c}=1.2\mathrm{}$ in the former and 2.0 in the latter. The maximum in ${\mu }_{\mathrm{th}}$ occurs at 1.2 × ${10}^{22}$ molecule/${\mathrm{cm}}^3$ in both liquids, corresponding to $\frac{n}{n_c}=1.5\mathrm{} \mathrm{and} 2.4$, respectively. The magnitude of the maximum in ${\mu }_{\mathrm{th}}$ 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, $\frac{E}{n}>5\mathrm{}$ mTd ($5\times {10}^{-20\mathrm{}}$ V ${\mathrm{cm}}^2$/molecule) little affected by density up to $\frac{n}{n_c}=1.6\mathrm{}$ 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 × ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$).