Mobility of charges in liquid, solid, and dense gaseous helium

The aim of this review is to provide a systematic survey of the extensive experimental and theoretical data obtained recently as a result of studies of the motion of charged particles in liquid, solid, and gaseous helium. The great variety of the available material has forced a subdivision of the general question of the motion of charged particles in helium into a number of “autonomous” areas. The motion of charges in a homogeneous medium at sufficiently low velocities is discussed. Chapter 1 describes the structure of positive (cations) and negative (anions) helium ions in liquid helium. It is noted that there is good agreement between theoretical descriptions and observed characteristics of ions in helium. The nucleation of electron bubbles in dense gaseous helium is investigated. Chapter 2 surveys theoretical and experimental results on the mobility of helium ions under kinetic conditions. The structure of helium ions ensures that the ion scattering cross sections of different thermal and impurity excitations exhibit very varied properties. Each of the basic scattering mechanisms, i.e., the phonon mechanism, scattering of impurities in weak He$^3$–He$^4$ solutions, the Fermi-fluid mechanism, and the roton mechanism, which are listed here in order of increasing complexity of interpretation, must therefore be specially considered. There have been notable successes in the theoretical interpretation of the kinetic mobility of helium ions, but there are also the characteristic difficulties which impede the development of a complete theory of the kinetic mobility of helium ions. Chapter 3 reviews the results obtained as a result of studies of the mobility of helium ions when the hydrodynamic approximation can be used in dense gaseous and in solid helium.