Accurate Electron Drift Mobility Measurements in Moderately Dense Helium Gas at Several Temperatures

We report new accurate measurements of the drift mobility mu of quasifree electrons in moderately dense helium gas in the temperature range 26 K <= T <= 300 K for densities lower than those at which states of electrons localized in bubbles appear. By heuristically including multiple-scattering effects into classical kinetic formulas, as previously done for neon and argon, an excellent description of the field E, density N, and temperature T dependence of mu is obtained. Moreover, the experimental evidence suggests that the strong decrease of the zero-field density-normalized mobility N-mu 0 with increasing N from the low up to intermediate density regime is mainly due to weak localization of electrons caused by the intrinsic disorder of the system, whereas the further decrease of N-mu 0 for even larger N is due to electron self-trapping in cavities. We suggest that a distinction between weakly localized and electron bubble states can be done by inspecting the behavior of N-mu 0 as a function of N at intermediate densities.