Effective mass of self-trapped electrons in gases

The effective mass $m^{*}$ of self-trapped electrons in gases is calculated, and used to estimate the translational contribution $-\left(\frac{3}{2}\right)\mathrm{kT}\ln \left(\frac{m^{*}}{m}\right)$ to the free-energy difference between self-trapped and free states for electrons in helium gas. When added to the free-energy difference between stationary self-trapped and free states this translational contribution has the effect of converting metastable stationary self-trapped states into stable moving self-trapped states.