An interpretation of dispersionless modes in neutron diffraction experiments on helium films

We analyze the dynamic structure function S(q, omega) for neutron scatterin g off helium films in the direction perpendicular to the substrate. "Disper sionless-modes" that have been found in neutron diffraction experiments on helium films adsorbed on graphite powder (H. J. Lauter, H. Godfrin, and H. Wiechert, Excitations in He-4 films, in "Proceedings of the Second Internat ional Conference on Phonon Physics," World Scientific, 1985) are interprete d as standing waves or resonances of the helium liquid perpendicular to the substrate. Our interpretation is supported by microscopic calculations of the He-4 dynamic structure function.