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Physical Review A

Excess electron mobility in high-density argon gas

A. F. Borghesani, M. Santini, Peter Lamp

Published Dec 1, 1992• 83 citations
Publisher

Abstract

We report accurate measurements of excess electron mobility in high-density Ar gas at a temperature T\ensuremath{\simeq}163 K in the density range (4N70)\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ as a function of the electric-field strength. The positive-density effect shown by the zero-field density-normalized mobility ${\mathrm{\ensuremath{\mu}}}_{0}$N can be explained within the same theoretical framework recently used for neon gas, where ${\mathrm{\ensuremath{\mu}}}_{0}$N shows a negative-density effect. Thus the analysis of our present measurements indicates that there is no need for two separate theories for both negative- and positive-scattering-length gases.

DOI
10.1103/physreva.46.7902

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We report accurate measurements of excess electron mobility in high-density Ar gas at a temperature T\ensuremath{\simeq}163 K in the density range (4N70)\ifmmode\times\else\texttimes\fi{}1020{10}^{20}1020 cm\ensuremath−3{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}cm\ensuremath−3 as a function of the electric-field strength. The positive-density effect shown by the zero-field density-normalized mobility \ensuremathμ0{\mathrm{\ensuremath{\mu}}}_{0}\ensuremathμ0​N can be explained within the same theoretical framework recently used for neon gas, where \ensuremathμ0{\mathrm{\ensuremath{\mu}}}_{0}\ensuremathμ0​N shows a negative-density effect. Thus the analysis of our present measurements indicates that there is no need for two separate theories for both negative- and positive-scattering-length gases.
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We present experimental data for drift velocities of very low energy electrons in a 5.45% hydrogen-argon mixture and the analysis of these data to determine the e - -Ar scattering length. The addition of a small fraction of hydrogen to argon enables experiments to be made with very low-energy electron swarms without the use of very high gas densities, thus reducing the uncertainty in the e - -Ar cross section derived from the experimental data. A scattering length of -1.459 a 0 is found, lying between the previous swarm determination and that obtained from time-of-flight (TOF) beam measurements. Pressure effects on the transport coefficients measured in earlier experiments are discussed, and evidence presented for a large departure from the Einstein relation at gas densities of 3*10 20 cm -3 .
The paper addresses counterintuitive behavior of electrons injected into dense cryogenic media with negative scattering length a0. Instead of the expected polaronic effect (formation of density enhancement clusters) which should substantially reduce the electron mobility, an opposite picture is observed: with increasing ∣a0∣ (the trend taking place for inert gases with the growth of atomic number) and the medium density, the electrons remain practically free. An explanation of this behavior is provided based on consistent accounting for the nonlinearity of electron interaction with the gaseous medium in the gas atom number density.
In a recent paper, Reisfeld and Asaf [Phys. Rev. A 49, 348 (1994)] suggested a simple model explaining an observed correlation between the electron scattering lengths and electric polarizabilities of rare-gas atoms. They proposed an electron-target interaction potential consisting of a short-range hard-core repulsive part plus a long-range dipole polarization potential, and used the Born approximation to evaluate the scattering length. We show that for this model the scattering length may be evaluated analytically and that theoretical predictions based on an exact expression disagree with experimental data.
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A theoretical relation between the electron scattering length and the van der Waals approximation to the equation of state is described. The theory provides a simple calculation for the scattering length of neutral atomic scatterers such as rare gases, using two experimental parameters: the hard-core radius (calculated from the van der Waals equation of state, using critical data) and the atomic polarizability. The results for the calculated electron scattering length agree with measured values derived either from the energy shifts of highly excited molecular Rydberg states, perturbed by He, Ne, Ar, Kr, and Xe, or from swarm electron cross-section data for the same gases.