Matter Wave Diffraction from an Inclined Transmission Grating: Searching   for the Elusive He-4 Trimer Efimov State
  The size of the helium trimer is determined by diffracting a beam of He-4 clusters from a 100 nm grating inclined by 21 degree. Due to the bar thickness the projected slit width is roughly halved to 27 nm, increasing the sensitivity to the trimer size. The peak intensities measured out to the 8th order are evaluated via a few-body scattering theory. The trimer pair distance is found to be 1.1+0.4/-0.5 nm in agreement with predictions for the ground state. No evidence for a significant amount of Efimov trimers is found. Their concentration is estimated to be less than 6%. 
R. Bruehl, A. Kalinin, O. Kornilov, J. P. Toennies, (MPI fuer   Stroemungsforschung, Goettingen), G. C. Hegerfeldt, M. Stoll...