Clusters of atoms and ions are widely studied in physics because they represent a form of matter that is intermediate between single atoms and bulk matter. Now Hans-Joachim Guntherodt and co-workers at the University of Basel in Switzerland have shown that clusters of tin atoms have different shapes, depending on the number of atoms in the cluster (T Bachels et al. 2000 Phys. Rev. Lett. 84 4890).
Smaller clusters tend to have elongated shapes, and the energy of formation is essentially independent of the number of atoms in the cluster. Larger clusters are spherical and the formation energy is inversely proportional to the cube root of the number of atoms. Such detailed knowledge of the energetics of cluster formation is needed to understand how the condensed phase of a material can form from the vapour phase.
Guntherodt and co-workers used a molecular beam apparatus in their experiment, and a “shutter” technique allowed the average size of the clusters in the beam to be varied between 95 and 975 atoms. The team then used a combination of micromechanical and pyroelectric calorimeters to measure the heat released when the clusters were deposited on a polymer foil. The heat released can be related to the formation energy of the cluster.
The Swiss team conclude that “the existence of elongated clusters with significantly reduced formation energies compared to the compact spherical clusters can strongly influence the kinetics of the formation of the condensed phases.”
