Dynamic structure factor of quantum liquid films

We employ a generalized Feynman theory for the calculation of the excited states of films of liquid He4 adsorbed to a substrate. The spectrum consists of a continuum and, depending on the areal density of the film, one or several discrete modes. The continuum shows a number of well-defined modes corresponding to sound waves propagating in different layers of the films. We discuss the structure of collective excitations and their transition from the thin, surface-mode-dominated to the thick, zero-sound-dominated films. A nearly momentum-independent mode is identified as a transverse oscillation of the discrete layers of the film relative to each other.