e--Ar scattering length from drift velocities measured in argon-hydrogen mixtures

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 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 a0 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 x 1020 cm-3. (Author)