We report accurate measurements of excess electron mobility in high-density Ar gas at a temperature T 163 K in the density range (4 & N & 70) x 10 cm as a function of the electric-field strength. The positive-density efFect shown by the zero-field density-normalized mobility p, oN can be explained within the same theoretical framework recently used for neon gas, where p,oN 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.PACS number(s): 51.50+v, 52.25 Fi

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The electron drift mobility has been measured in a wide range of temperatures and densities in neon gas and saturated vapor. The "zero-Geld" density-normalized mobility poS exhibits a strong density dependence, which cannot be accounted for by the existing multiple-scattering theories. The data, however, can be well 6tted by assuming a density dependence in the e-Ne scattering cross section.

We report accurate measurements of the mobility of excess electrons in high-density helium gas in extended ranges of temperature [(26 < or = T < or = 77) K] and density [(0.05 < or = N < or = 10.0) atoms nm(-3)]. The aim is the investigation of the combined effect of temperature and density on the formation and dynamics of localized electron states. The main result of the experiment is that the formation of localized states essentially depends on the relative balance of fluid dilation energy, repulsive electron-atom interaction energy, and thermal energy. As a consequence, the onset of localization depends on the medium disorder through gas temperature and density. The transition from delocalized to localized states shifts to larger densities as temperature is increased. This behavior can be understood in terms of a simple model of electron self-trapping in a spherically symmetric square well.

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