THEORETICAL-ANALYSIS OF NEUTRON-SCATTERING SPECTRA OF HE-4 FILMS ON GRAPHITE

We compare microscopic calculations of the dynamic structure function of helium films adsorbed to a graphite substrate with neutron scatteri ng data. Starting from a generalized Feynman theory of collective exci tations, we include successively, three-phonon interactions, self-ener gy corrections, and experimental broadening and thereby improve the ag reement between theoretical predictions and experiments. The inclusion of three-phonon vertices allows high-lying excited states to decay in to lesser energetic ones and thus leads to a natural linewidth. A is s hown that the theoretical linewidth of the excitations is typically sm aller than the experimental resolution, and that much structure of the spectrum in the neutron scattering data is obscured under a, broad pl ateau, between the ripplon and the phonon excitations. This analysis l eaves little doubt of the existence of layer-phonons and that the obse rved shoulder in the dynamic structure function, near the roton minimu m, is actually a 2D roton propagating in. the inner-most liquid layer.