The classical integral cross sections of large superfluid 4He_N droplets and the number of atoms in the droplets (N=10^3-10^4) have been measured in molecular beam scattering experiments. These measurements are found to be in good agreement with the cross sections predicted from density functional calculations of the radial density distributions with a 10-90 % surface thickness of 5.7\AA. By using a simple model for the density profile of the droplets a thickness of about 6-8\AA is extracted directly from the data.
@article{arxiv.cond-mat/9705070,
title = {Density of Superfluid Helium Droplets},
author = {Jan Harms and J. Peter Toennies and Franco Dalfovo},
journal= {arXiv preprint arXiv:cond-mat/9705070},
year = {2009}
}