Cluster, defined as a group or bunch in Webster's dictionary, has different meanings depending on the given discipline. In medicine, for example, a cluster can refer to a severe headache that can occur several times in a day, whereas, in astronomy, clusters are usually associated with stars and galaxies. Clusters also are associated with bombs, music, and computers. However, to physicists and chemists, the word cluster has come to mean a group of atoms or molecules formed by interactions ranging from very weak van der Waals contacts to strong ionic bonds. Although reference to the formation of aggregates and related nucleation phenomena can be found in literature dating from the 1930s and earlier, studies of clusters in mass spectrometer ion sources and later in molecular beams (1) began to emerge in the 1950s and developed rapidly as a subject of considerable interest in the 1970s and 1980s. The advent of the laser vaporization technique (2) enabled researchers to produce clusters of virtually any element in the periodic table and spawned wide-ranging interest in the studies of clusters of various compositions, beyond systems of volatile materials, which had been the focus in the beginning. For this community, in the past 30 years clusters have come to symbolize a new embryonic form of matter that is intermediate between atoms and their bulk counterpart. Clusters bridge phases as well as disciplines.
