Creation and Structure Study of Vacuum Isolated Clusters of Argon, Krypton and Xenon

摘要

Homogeneous clusters of Ar, Kr or Xe are formed in adiabatic, high Mach number expansions through very small, diverging Laval nozzle sources. A He carrier gas is employed and the mole fraction of condensable species varied from 1.0 to 0.06 in an effort to control the cluster size distribution and temperature. The nozzle source is part of a multi-stage molecular beam apparatus. Electron diffraction, using a 40 keV beam, is used to study cluster structure. Cluster sizes are varied from g-bar=50 to 1,500 atoms per cluster and temperatures range from 20 to 60 K. The structure for g-bar>1,000 is bulk f.c.c. and transforms gradually into a non-crystalline structure as size decreases. The smallest structures, for all three noble gases, are consistent with the icosahedral structures which are theoretically favored energetically in this regime. For a fixed mole fraction of 0.06, the cluster temperature, T sub c, are systematically higher going from Ar to Xe with dimensionless temperatures, kTc/epsilon, systematically lower. There is evidence that P sub 0 D sub 0/(kT sub 0/epsilon) is the unifying parameter for predicting the onset of massive clustering for rare gases in these nozzle-type sources. (Author)