Low-Temperature Mobility of the Electron Bubble in DiluteHe3-He4Mixtures and PureHe3

Recent low-temperature measurements of the mobility of negative ions in dilute ${\mathrm{He}}^3$-${\mathrm{He}}^4$ mixtures are explained. A new method based on the Boltzmann equation is used to derive the general formula for the drag force of a heavy impurity in an arbitrary gas. It is shown that the structure of the negative ion (electron bubble) depends on the ${\mathrm{He}}^3$ concentration $C_3$ and temperature. Also, for $C_3$ above about 1% the distortion of the ${\mathrm{He}}^3$ distribution by the moving ion becomes important. The application of our theory to pure, highly degenerate ${\mathrm{He}}^3$ shows that the mobility remains finite at $T=0\mathrm{}$ and rises with temperature. Both results agree with experiment.