Pseudocrystalline state of the surface of free xenon clusters

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.