Electron diffraction experiments on argon clusters nucleated in helium using Laval nozzle beams

Abstract

A cluster beam-electron diffraction machine has been modified with a low-temperature ( T 0 = 100 K), Laval nozzle source to nucleate noble gases as a small mole fraction (χ 0 to 0.01) in helium carrier gas. The use of specific nozzle contours, plus the variables above, allows one to nucleate clusters which have as narrow a size distribution as possible and which are colder (10 K) than clusters formed in pure-gas expansions. These features simplify, in principle, the interpretation of diffraction patterns and tend to form cluster structures which are in minimum-energy configurations. Electron diffraction experiments are carried out on argon clusters and show a transition from fcc to a non-crystalline structure over the size range from 3500 to 1500 atoms per cluster. A relaxed icosahedral cluster model produces diffraction patterns consistent with the experiments. Model diffraction patterns reveal that the height of the knee on the (111) peak is an excellent parameter for the size determination of icosahedral clusters.