The word "clusters" evokes images that differ greatly depending on one's scientific discipline. The structural chemist may envision the balls and sticks of coordination polyhedra, whereas the astrophysicist might see spiraling galaxies.
Figure 1. Clusters are weakly bonded assemblages of atoms or molecules that cannot readily be characterized as solid, liquid or gas. Because of the facile control of composition and extent of clustering, these isolated nanoscale aggregates are particularly amenable to the study of solvation effects in reaction dynamics. In particular, when rapidly ionized by intense laser light, clusters readily undergo a process called Coulomb explosion?a rapid and forceful repulsion caused by the close proximity of like charges. The authors have used time-of-flight mass spectrometry combined with femtosecond (10?15 second) laser pulses to Coulomb explode, and therefore arrest the development of, intermediate products during reactions. Shown here is a comparison of a large and a small cluster subject to laser ionization that strips four electrons from each, resulting in a charge of +4. In this illustration, the smaller cluster is unable to sustain the ionization and Coulomb explodes, whereas the larger one retains sufficient cohesive force to remain intact.
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