Structural Studies of Isolated Small Particles Using Molecular Beam Techniques

Abstract

Especially designed Laval nozzles are presently used as continuous sources of Ar, Kr, or Xe clusters in a crossed molecular beam/40-keV electron beam experiment. Because the clusters have random orientation, the electron diffraction patterns generated are of the Debye-Scherrer type and reveal that the clusters are crystalline, with temperatures in the 15-60 K range and a mean size of 50-1000 atoms/cluster. As the mean size decreases below 500, a progressive structure change from bulk face-centered cubic is observed. Similar structure changes occur in all three species. Because particles in this size range can be present in the atmosphere, due to photochemical reduction, they may possess nonbulk properties in general, and in particular, catalytic activity.