Zero-Field Mobility of an Excess Electron in Fluid Argon

Drift velocities of electrons in fluid argon have been measured at temperatures from 90 to 160 °K and at pressures from 10 to 100 atm for applied electric fields in the range -25 to -200 V/cm. The electron drift velocity is found to be linear with respect to electric field strength only to -100 V/cm at temperatures from 90 to 125 °K and to become increasingly nonlinear at temperatures greater than 125 °K. Mobilities can be obtained from these data by extrapolation to zero field; maxima are found in the zero-field mobilities as a function of density, in the region of 0.81 g/${\mathrm{cm}}^3$. Using the model proposed by Lekner for electron scattering by a system of fluctuating potentials and assuming that the scattering length for electrons in fluid argon approaches zero at some density, it is possible to obtain a semiempirical relation for the zero-field mobility as a function of density. Excellent agreement between calculated and observed mobilities is found in the high-density range studied 1.0-1.4 g/${\mathrm{cm}}^3$. At densities less than 1.0 g/${\mathrm{cm}}^3$, several qualitative aspects of the experimental data are accounted for by the theory, but quantitative agreement is lacking. It is possible that at these lower densities, gas-like scattering is of dominant importance.