QUANTUM STICKING, SCATTERING, AND TRANSMISSION OF HE-4 ATOMS FROM SUPERFLUID HE-4 SURFACES

We develop a microscopic theory of the scattering, transmission, and s ticking of He-4 atoms impinging on a superfluid He-4 slab at near norm al incidence, and inelastic neutron scattering from the slab. The theo ry includes coupling between different modes and allows for inelastic processes. We find a number of essential aspects that must be observed in a physically meaningful and reliable theory of atom transmission a nd scattering; all are connected with multiparticle scattering, partic ularly the possibility of energy less. These processes are (a) the cou pling to low-lying (surface) excitations (ripplons/third sound) which is manifested in a finite imaginary part of the self-energy, and (b) t he reduction of the strength of the excitation in the maxon/roton regi on.