Scattering of ³He and ⁴He atoms from ⁴He clusters

We develop a microscopic theory of elastic and inelastic scattering and sticking of ³He and ⁴He atoms impinging on superfluid ⁴He clusters. A number of essential aspects must be observed in a physically meaningful and reliable theory of such scattering processes. These are all connected with multiparticle processes, particularly the possibility of energy loss. These processes are (a) the coupling to low-lying excitations which is manifested in a finite imaginary part of the self energy, (b) the interaction with roton-like excitations and the broadening of the roton minimum due to finite-size effects, and (c) the discreteness of low-lying excitations in helium clusters.