Excitations in Quantum Liquids

Progress made in measuring and interpreting the elementary excitations of superfluid and normal liquid \4he in the past 25 years is reviewed. The goal is to bring up to date the data, calculations and our understanding of the excitations since the books and reviews of the early 1990s. Only bulk liquid \4he is considered. Reference to liquid \3he, mixtures, reduced dimensions (films and confined helium) is made where useful to enhance interpretation. The focus is on the excitations as measured by inelastic neutron scattering methods. The review covers the dynamic response of liquid \4he from the collective excitations at low energy and long wavelength (i.e. phonon-roton modes) to the single particle excitations at high energy from which the atomic momentum distribution and Bose-Einstein condensate fraction are determined. A goal is to show the interplay of these excitations with other spectacular properties such as superfluidity and the test of fundamental calculations of quantum liquids that is possible. The role of Bose-Einstein condensation in determining the nature of the \pr~ mode and particularly it's temperature dependence is emphasized. The similarity of normal liquid \4he with other quantum and classical liquids is discussed. .