Direct observation of free excitons in luminescence spectra of xenon   clusters
  Luminescence of surface and free bulk excitons is detected in xenon for the first time for substrate-free rare-gas clusters. Xenon clusters were produced by the method of gas condensation in a supersonic jet emitted into vacuum. Optical study was accompanied by electron diffraction measurements to determine the structure of clusters. 
O.G. Danylchenko, Yu.S. Doronin, S.I. Kovalenko, M.Yu. Libin, V.N.   Samovarov, V.L. Vakula