Spectroscopy, chemistry and catalysis of metal atoms, metal dimers and metal clusters

Abstract

This paper starts with the experimental realization of trapped silver atoms in various matrix supports, with emphasis placed on optical absorption, emission and e.s.r. spectroscopic properties. Special attention is devoted to ground- and excited-state silver atom interactions with the surrounding matrix cage of atoms, particularly with respect to the recently discovered phenomenon of light-induced diffusion and aggregation of silver atoms to small, precisely defined silver clusters. Various aspects and applications of photodiffusion methods are highlighted and some pertinent comparisons are made with metal concentration and bulk annealing approaches as alternatives to controlled metal nucleation. This will lead to the embryonic clusters Ag₂ and Ag₃ for which there now exists considerable spectroscopic, photochemical and theoretical information. Silver-containing bimetallic clusters, generated either by metal concentration deposition or photoaggregation methods, will be a natural extension to the Ag_2,3 presentation; their relevance to bimetallic cluster catalysts will be briefly contemplated. The concept of generating anionic silver clusters by Na/Ag photoionization methods will be briefly described, with reference to the parent Ag^– anion. In considering the higher silver clusters, one is now in a position to evaluate experimentally the genesis of silver nucleation from the stage of an isolated silver atom, through to a six-atom array and to higher, less well defined aggregates. With these data one can attempt to track the evolving optical and e.s.r. properties by reference to the build-up of cluster electronic states calculated by way of SCF-Xα-SW molecular-orbital procedures for assumed cluster geometries. The observation of rudimentary interband transitions of silver clusters in the range 6-13 atoms, that absorb in a similar energy region to the collective electronic excitations associated with plasmon absorption of silver microcrystallites, simultaneously with the evolution of conduction electron spin resonance absorption, whose observed linewidths and g-shifts conform to the theoretical predictions of Kubo and Kawabata, can in principle provide a valuable criterion on which to judge the atomic composition at which optical–electronic characteristics of silver aggregates transform from those of the molecular to the bulk state. Future directions in diatomic-metal and metal-cluster chemistry are briefly contemplated in the light of recent break-throughs with ambient-temperature metal-vapour–liquid-polymer techniques and the discovery of polymer-supported, very low nuclearity metal cluster, generated and stable at, or very close to, room temperature.