Atoms and ions in superfluid helium

Optical spectra, level population and nonradiative deexcitation processes of atomic impurities in liquid helium have been investigated. The recombination of electrons and positive metal ions are described by the tunneling model. This model explains the population gap of 1.8 eV below the ionisation limit which was observed for several neutral defect atoms in liquid helium. In the framework of a pseudopotential theory excited singlet levels of Ba, Ca and Na atoms are recalculated and compared to experimental data. Non-spherical atomic defects for excited p-states are treated also. Quadrupole vibrations of these distorted defect structures are assumed to be responsible for inducing multiphonon transitions between excited atomic states.