Lead microclusters in the vapor phase as studied by molecular beam electron diffraction: Vestige of amorphous structure

Abstract

The metal microclusters are formed in the gas phase by an isobaric cooling due to argon carrier gas, followed by adiabatic expansions through nozzle apertures. Electron diffraction patterns of these condensates show the typical Debye–Scherrer rings for the fcc Pb metal. The average size of clusters obtained, depending on the experimental conditions, ranges from 30 to 100 Å in diameter, and the cubic lattice parameters for these clusters are essentially the same, within an experimental error of 0.3%, as that for the bulk crystal. The analysis of the Bragg‐peak intensities is carried out based on the usual kinematic theory and the dynamical (two‐beam approximation) theory by Blackman. The former theory fails completely in accounting for small angle reflections (s≲5 Å⁻¹), whereas the latter explains successfully the whole observe intensities, except for the second and third order reflections (400), (440), (333), and (600). From the damping of peak intensities the temperatures of these clusters are estimated to be of the order of 150 °K. A close inspection on the comparison between the observed and theoretical intensities, however, reveals periodic fluctuations in the observed peak heights as a function of the scattering angle, indicating a superposition of some diffuse scattering. The amplitudes of the fluctuations are more marked for the smaller size clusters. The characteristics of the fluctuations are accounted for by introducing liquidlike random atomic configurations into the microcrystalline. This model explains also the abnormal Bragg‐peak intensities of Ar clusters reported in the literature.