Atom scattering at ⁴He droplets

We present theoretical calculations of the structure and excitations of helium clusters. Our calculations include the multi-particle excitations and lifetime effects and, thus, bring the description to a level that is comparable to the one previously reached only in the bulk liquid. We calculate both the low-lying, discrete excitations and the dynamic structure function and demonstrate that it shows strength for both, a phonon-roton branch and a ripplon branch. Equipped with a plausible description of the pure droplets, we then consider ³He impurities and their excitations, and finally the scattering of ⁴He and ³He off ⁴He droplets and determine the dependence of scattering cross sections on the energy and the particle species.