Between isolated atoms or molecules and bulk materials there lies a class of unique structures, known as clusters, that consist of a few to hundreds of atoms or molecules. Within this range of ''nanophase,'' many physical and chemical properties of the materials evolve as a function of cluster size, and materials may exhibit novel properties due to quantum confinement effects. Understanding these phenomena is in its own rights fundamental, but clusters have the additional advantage of being controllable model systems for unraveling the complexity of condensed-phase and biological structures, not to mention their vanguard role in defining nanoscience and nanotechnology. Over the last two decades, much progress has been made, and this short overview highlights our own involvement in developing cluster dynamics, from the first experiments on elementary systems to model systems in the condensed phase, and on to biological structures.I n the early 1980s, our research group at Caltech began exploring real-time dynamics of clusters (1-5) by studying time-resolved spectroscopy, with picosecond (ps) and then femtosecond (fs) resolution (6-8), in molecular jets and beams (9, 10). These first results (ref. 11; for commentary, see ref. 12) opened the door for many more applications as they clearly elucidated in real time the influence of microsolvation and stepwise solvation effects on reactivity and photophysical processes in clusters (12). The ultimate goal we had in mind was to ''bridge'' the gap between the isolatedmolecule behavior and that in the bulk phase, and to understand at a microscopic level the influence of solvent on reactions.The scope of phenomena studied varied, from the influence of one-atom solvation and caging (13)(14)(15)(16)(17)(18)(19), to stepwise solvation of acid-base (20-22) and isomerization reactions (23, 24) in clusters of up to 30 solvent molecules. Both electron (25-28) and proton transfer (20)(21)(22)29) processes were also studied, elucidating for the former one of the classics for Mulliken's dative bonded complexes, that of benzene-iodine, and for the latter the dynamics of single and double proton transfer. With mass selection, we investigated ionic clusters, the premier being solvated electrons (30) and dioxygen anions (31-33); both are important in biological activities. For chemical reactions, we used this approach of complexation to study bimolecular reactive (34-37) and nonreactive (38) scattering, and, by analogy, biomolecular structures exhibiting dative bonding and involved in molecular recognition (39-41). A descriptive outline of the work over two decades is given in Fig. 1, including relevant references (refs. 6-8 and 13-49; ref. 50 and references therein). In what follows, we only highlight some prototypical examples.
Bimolecular ReactionsEarly in the development of fs transitionstate spectroscopy (51) it was possible to directly probe the transition state by realtime clocking of a unimolecular reaction (half collision): ABC* 3 A⅐⅐⅐B⅐⅐⅐C* ‡ 3 AB ϩ C. For a bimolecular reacti...
