Clusters are aggregates of loosely bonded molecules, in which each of the units retains the structure that it has as a free molecule. Because of the weak interactions among the molecules, clusters are stable only in cold environments such as are found in molecular beams. The weak intermolecular bonds provide an interesting testing ground for theories of intramolecular vibrational energy redistribution (IVR) and thus for theories of unimolecular dissociation. In addition, clusters constitute the bridge between the gas and liquid phases. Such phenomena as solvation, heat capacity, and phase transitions, which are ill defined for small clusters, become progressively more precise as the cluster size increases. Typical binding energies for neutral clusters are below 1000 cm-1. Ionic clusters, because of their ion-induced dipole forces, tend to be more strongly bonded with binding energies in excess of 5000 cm-1. Not infrequently, a neutral van der Waals dimer such as Ar2 with its binding energy of about 100 cm-1 (Tang and Toennies, 1986) changes its character upon ionization. The equilibrium bond distance is reduced from about 4 Å to 2.43 Å (Huber and Herzberg, 1979; Ma et al., 1993) and the binding energy increases to 10,000 cm-1 (Norwood et al., 1989; Furuya and Kimura, 1992). Clearly, the Ar2+ ion no longer meets our definition of a dimer. Rather, the neutral dimer is converted into a stable ion with a bond order of 1/2. A molecule that is frequently referred to as a cluster is C60. However, it is held together neither by weak bonds, nor is it composed of a collection of monomers. It is thus better classified as a large covalently bonded molecule. Table 10.1 summarizes some binding energies for various classes of dimers. When clusters comprise several loosely bound molecules, the atoms within each molecule are held together by strong bonds while the molecules themselves are attracted to neighboring molecules by weak bonds. This discrepancy in forces translates into disparities in the respective vibrational frequencies.
