Ripplons in 4He films observed by neutron scattering

We report the first direct observation of ripplon excitations at large wave vectors. The experiments were performed on liquid-4He films of atomic thickness adsorbed on a graphite substrate. Neutron-scattering techniques were used to determine the inelastic structure factor of surface (ripplon) and bulklike (phonon-roton) elementary excitations. The lowest lying mode at low wave vectors is experimentally shown to be a surface mode; its dispersion relation agrees well with the quantum-hydrodynamic theory of Edwards and Saam. The predictions of recent microscopic models are compared to the data at high energies and momenta