Dynamics of helium in restricted geometries

We review microscopic calculations for excitations, the dynamics of Andreev states, as well as neutron and atom scattering processes off helium mixtur es, films, slabs, and clusters. Modern theoretical methods are built upon q uantitative, microscopic descriptions of the ground state of both the host liquid as well as potential impurity atoms. A thorough analysis of the theo retical approach and the physical circumstances point towards the essential role played by multi-particle excitation and inelastic processes. In parti cular, inelastic processes determine almost exclusively atom scattering pro perties. (C) 2000 Elsevier Science B.V. All rights reserved.