Abstract
The motion of negative ions at supercritical drift velocities in pressurized liquid helium is governed at low temperatures by the spontaneous emission of rotons. Assuming a constant matrix element the transition probability for two-roton emission processes is calculated using perturbation theory. The effect of recoil on the ionic motion is studied qualitatively by means of a simple kinetic approach and quantitative calculations are performed using the Boltzmann equation. The calculated dependence of drift velocity on electric field $E$ shows that an ${E}^{\frac{1}{3}}$ variation occurs over a substantial range of fields as found experimentally. Comparison with experimental data for fields $2<E<{10}^{3}$ kV ${\mathrm{m}}^{\ensuremath{-}1}$ is made and deviations from the ${E}^{\frac{1}{3}}$ law at high fields are accurately accounted for. The role of vortex nucleation is discussed and further experiments are suggested.
Published Version
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