Reactivity-structure correlations in ion clusters: A study of the unimolecular fragmentation patterns of argon ion clusters, Ar+n, for n in the range 30–200

Abstract

A high resolution double focusing mass spectrometer in association with a pulsed supersonic nozzle has been used to study the unimolecular fragmentation of Ar+n clusters for n in the range 30–200. All ion clusters within this range are observed to undergo the reaction Ar+n→Ar+n−1+Ar; with some ions also exhibiting further fragmentation involving the loss of two and three argon atoms. A systematic study of the above reaction as a function of n reveals that the relative intensities of the fragment ions fluctuate considerably. In many cases, these fluctuations coincide with depleted or enhanced ion intensities in the normal ion cluster mass spectrum. Through the use of a model based on unimolecular reaction rate theory (RRKM), it has been possible to reproduce many of the features observed in both the fragmentation spectra and the normal mass spectrum. Of the two principle variables involved in the model, reaction symmetry number and dissociation or binding energy, we have been able to show that only the binding energy is important in determining the fragmentation pattern. Through the calculations, the experimental data are interpreted in terms of ‘‘stable’’ and ‘‘unstable’’ ion clusters. Stable ion clusters have enhanced intensities in the normal mass spectrum, whereas unstable ions produce both depletions in the normal mass spectrum, and enhanced fragment ion intensities. It is proposed that unstable argon ion clusters consist of an atom weakly bound to an underlying stable cluster. A series of hard-sphere structures are presented to account for some of the observed stable ion clusters. Each of the structures has icosahedral symmetry and accommodates a central Ar+2 ion. It is concluded that, because the clusters are forced to accommodate a charge-carrying unit, the positive charge appears to have a structural influence which extends well beyond the electrostatic limit.