In liquid helium, an electron is surrounded by a cavity called an electron bubble of 20 Ångstroms in diameter. A positive helium ion is solvated by an electrostriction induced solid helium-ice shell called a snowball of 7 Ångstroms in diameter. By studying their transport properties, these objects are well suited for the testing of the microscopic properties of superfluidity. At low temperatures and with small electric fields, the drift velocity of the charges depends on their interaction with the elementary excitations of the superfluid: phonons, rotons, and 3He atomic impurities. At higher fields, ions produce quantized vortex rings and vortex lines and studying these sheds light on quantum hydrodynamics. In the fermionic liquid, the 3He isotope ion transport properties display important pieces of information on the coupling of a charge to a Fermi liquid and on the richer topological structure of the superfluid phases appearing at ultralow temperatures. In the normal liquid phases of both isotopes, ions and electrons are used to probe classical hydrodynamics at the λ-transition and at the liquid-vapor transition at which long-range critical fluctuations of the appropriate order parameter occur. Several experiments have investigated the structure of electron bubbles. Electron drift velocity measurements in dense helium gas have elucidated the dynamics of electron bubble formation. This book provides a review of the more than forty-year-long experimental and theoretical research on the transport properties of electrons and ions in liquid and gaseous helium.
