Magic Numbers, Reactivity, and lonization Mechanisms in ArnXm Heteroclusters

Abstract

During the past decade, there has been an enormous increase in experimental and theoretical studies directed toward obtaining a fundamental understanding of the properties of van der Waals (vdW) clusters (Castleman 1990; Castleman and Keesee 1986b, 1988b; Garvey et al. 1991; Goyal et al. 1993; Janda 1985; Jortner 1984; Levy 1981; Märk 1987; Märk and Castleman 1985; Ng 1983; Stace 1992). As the term "cluster" is frequently used in the scientific community with different connations, it would be appropriate to define "cluster" in the context of this field. Gas phase clusters are defined as finite gas phase aggregates composed of two to several million components (i.e., atoms or molecules). These species are held together by different types of forces, ranging from the weak van der Waals forces all the way up to strong electrostatic forces. A techique of classifying clusters based on the type of the binding forces has been developed. Another convenient basis for the classification of clusters is according to the size, such that clusters with n = 2-10 or 13 have been termed microclusters, clusters with n = 10-102 have been referred to as small clusters, and aggregates with n ≥ 102 are called large clusters (Jortner 1984). Clusters composed of a few molecules can be treated, to a first approximation, as isolated gas phase species, while clusters with sizes n > 103 begin to exhibit properties resembling those of condensed or bulk materials. Between these two extremes lies the regime where cluster systems cannot be adequately treated by either the molecular concepts or the conventional solid-state approach. Thus, clusters have previously been described as the conceptual bridge linking the gas and the condensed phases (bulk liquids or solids) (Castleman 1990; Castleman and Keesee 1986b, 1988b; Garvey et al. 1991; Goyal et al. 1993; Janda 1985; Jortner 1984; Levy 1981; Märk 1987; Märk and Castleman 1985; Ng 1983; Stace 1992).