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Temperature Dependence of Silicon Cluster Isomerization

The thermally induced isomerization of neutral silicon clusters has been studied by a molecular beam experiment, investigating velocity slip distributions with a time-of-flight method. The clusters dispersed in helium flow through a heated tube and, at its end, expand into vacuum. The observed velocity distributions can be explained by the existence of a cylindrical and a spherical class of isomers, whose relative intensities can be examined in dependence of the tube temperature. Assuming that the isomerization can be described by a unimolecular gas-phase reaction, one is able to derive a rate constant for the isomerization. The results are analyzed within the activated complex theory and give evidence, that entropic effects play a crucial role for the cluster isomerization reaction.