Fragmentation and Structure of Silicon Microclysters
In the past several years there has been growing interest in the properties of small clusters of atoms.1–7 Inert gas clusters are known to form predominantly 13-, 55-, etc. atom clusters explained by stable icosahedral packing.7 More recently there has been an effort to determine the geometrical arrangements and electronic configurations for Group IV elements, C, Si, Ge and Sn.8–11 Silicon and germanium have both been found to form unusually stable 6- and 10-atom clusters. Si4 and Ge14 are also found to be particularly stable. These unusually stable clusters are referred to as the magic numbers. Unlike Si and Ge, carbon exhibits chain-like structures due to strong π bonding with their own set of magic numbers. Bloomfield, Freeman, and Brown8 (BFB) performed a photofragmentation experiment on ionized silicon, measuring relative cross sections and individual fragmentation channels for clusters containing upto 12 atoms. Neutral clusters were created by vaporizing bulk silicon in the presence of a stream of inert gas. These clusters were then ionized by a laser and mass selected by means of electrodes. Fragmentation occurred upon exposing the cluster to an intense beam of laser radiation. The size of the resulting charged fragment was then determined. The temperature of the cluster prior to dissociation reaches the order of the melting temperature of bulk silicon (T ~ 2000 K). The fragmentation channels were found to depend critically on the overlap between the laser beam and the cluster. Bloomfield et al. report relatively low total photofragmentation cross sections for Si+ 4, Si+ 6, and Si+ 10 in addition to the unusually common occurrence of Si+ 6 and Si+ 10 from the fragmentation of Si+ 7−11 and Si+ 12, respectively. The magic numbers for silicon have been determined to be 4, 6, and 10 from these experimental results.
KeywordsMagic Number Potential Minimum Fragmentation Energy Trigonal Bipyramid Fragmentation Temperature
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