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

1981; American Institute of Physics; Volume: 24; Issue: 2 Linguagem: Inglês

10.1103/physreva.24.714

0556-2791

Sam Huang, Gordon R. Freeman,

Spectroscopy and Laser Applications

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