Neutral and charged excimer complexes in cathodoluminescence spectra from substrate-free icosahedral and crystalline clusters of argon

Abstract

We studied cathodoluminescence spectra from substrate-free argon clusters produced in a supersonic jet expanding adiabatically into a vacuum. The average cluster size varied from 500 to 8900 atoms per cluster. As clusters grew bigger, their structure changed from a quasicrystalline icosahedral structure with a 5-fold symmetry axis to a crystalline fcc one. Clusters were excited by a 1-keV electron beam. Luminescence spectra were measured in the 8.1-11.8 eV energy range containing emission bands of the neutral and charged excimer complexes (Ar2)* and (Ar4+)*. An analysis of the intensity of radiation from their relaxed vibrational states was performed on the basis of a new approach that allows us to take into account the fraction of jet substance condensed into clusters. It was shown that in crystalline clusters with an fcc structure the emission from neutral (Ar2)* molecules comes from within the bulk of a cluster, while the radiation from charged (Ar4+)* complexes originates in its near-surface layers. We found the cluster size range in which the jet is dominated by quasicrystalline clusters with the structure of multilayer icosahedron and demonstrated that the transition from icosahedral clusters to fcc clusters occurs when the average cluster size is 1400±400 atoms.