Dynamics of atom scattering from He-4 nanoclusters

We present a microscopic theory and results of atom scattering calculations to determine the dispersion of surface modes (ripplons) of superfluid helium-4 nanodroplets, expanding previous work [J. Chem. Phys. 115, 10161 (2001)]. A quantum transport formalism is adapted to the many-body scattering problem, yielding both elastic and inelastic fluxes. We demonstrate that, in analogy to the dynamic structure function S(k,omega) obtained from neutron scattering, a dynamic structure function sigma(k,omega) can be obtained from He-3 scattering. The He-3 dynamic structure function sigma(k,omega) is sensitive to surface dynamics, whereas the neutron dynamic structure function S(k,omega) is dominated by bulk-like excitations, in particular by rotons. Unlike for neutron-scattering, the total inelastic cross section for atom-scattering on He-4 nanodroplets is large which we believe makes experimental detection feasible. We also show that scattering identical particles, i.e. He-4 atoms, does not provide information about the dispersion of surface modes. Instead, inelastically scattered He-4 atoms preferably lose roughly half their energy.