Threshold density for electron self-localization in gaseous Helium

Experiments in dense helium gas have shown that the measured electron mobility $\mu$ is a weighted sum of the contributions of fast quasi-free electrons and of slow electrons self-trapped in cavities carved in the gas. Self-trapped electrons appear if the excess free energy with respect to their quasi-free companions is favorable. There is a temperature dependent, threshold density $N_{th}$, above which stable, self-trapped states exist. $N_{th}$ can be determined by examining the behavior of $\mu$ as a function of density and electrical field. We show that the optimum fluctuation model of an electron localized within a partially empty bubble is in agreement with the experimental data in the temperature range $7\mathrm{K} \le \quad T \le 64\mathrm{K}$.