A metal cluster generator for gas‐phase electron diffraction and its application to bismuth, lead, and indium: Variation in microcrystal structure with size

摘要

An oven has been designed and built for the production of metal microcrystals via nucleation and growth of the metal vapor in an inert‐gas atmosphere. It has a flowing‐argon double‐orifice sampling system producing a supersonic free‐jet mixture which crosses a 40‐keV electron beam. Debye‐Scherrer diffraction patterns are obtained from the metal cluster samples which range in size from 40 to 95 Å in diameter with estimated concentrations of 1012–1013 cm-3 at the electron beam location. The average cluster size produced in the oven for all three metals studied correlated well with the product of oven pressure times metal evaporation temperature p0T0m. As p0T0m increases, average size increases and cluster concentration decreases, in qualitative agreement with other metal evaporation research and with nucleation studies of vapor–inert‐gas expansions in supersonic nozzles. Analyses of the diffraction patterns reveal changes in crystal structure from that of the bulk in the neighborhood of 50–60 Å in diameter (2000–4000 atoms per cluster). The differences are most pronounced in indium which changes from tetragonal to face‐centered cubic as the size decreases. Cluster temperatures at the electron beam location have been estimated from Debye‐Waller factors, from extrapolation to bulk structure, and from calculated values of the argon‐gas temperature in the free jet. The three methods yield consistent results, and the hotter the clusters are prior to the free‐jet expansion, the greater is the temperature difference between the clusters and the argon gas when the mixture becomes collisionless (i.e., no further change in the temperature of gas or custer).