Dynamics of atom scattering from 4He 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,ω) obtained from neutron scattering, a dynamic structure function σ(k,ω) can be obtained from 3He scattering. The 3He dynamic structure function σ(k,ω) is sensitive to surface dynamics, whereas the neutron dynamic structure function S(k,ω) is dominated by bulk-like excitations, in particular by rotons. Unlike for neutron-scattering, the total inelastic cross section for atom-scattering on 4He nanodroplets is large which we believe makes experimental detection feasible. We also show that scattering identical particles, i.e. 4He atoms, does not provide information about the dispersion of surface modes. Instead, inelastically scattered 4He atoms preferably lose roughly half their energy.