  The paper presents the results of the experimental study of polarization
bremsstrahlung profile halfwidth performed on free xenon clusters with various
atom numbers. We used 0.7- and 0.3-keV electrons to excite mainly the core and
the surface of the clusters, respectively. The halfwidth vs. atom number
dependencies were found to be quite different for the 0.7- and 0.3-keV
electrons. Analysis of the observed difference allowed us to conclude that the
clusters with N = 2000-8000 atoms per cluster (R being about 3-5 nm) consisted
of a crystalline fcc core covered by a noncrystalline shell. Our data provide
evidence that the surface shell is in the pseudocrystalline state, which is its
first observation in rare gas clusters. The pseudocrystalline state observed in
metallic clusters is a volume effect, the clusters in this state are
structurally unstable and fluctuate continuously between liquid and solid
phases.
