High temperature electron localization in dense He gas

Measurements of the mobility of excess electrons in a high density helium gas are reported. Extended ranges of temperature [(26 /spl les/ T /spl les/ 77) K] and density [(0.05 /spl les/ N /spl les/ 12.0) atoms.nm /sup -3/] are investigated to ascertain the effect of temperature 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 disorder of the medium and, hence, on the gas density. Moreover, it appears that the transition from delocalized to localized states shifts to larger densities as the temperature is increased. This behavior can be understood in terms of a simple model of electron self-trapping in a spherically symmetric square well.