4.1 Experimental Observation of Electronic Shells

Sodium vapor, or other alkaline vapors, can be expanded supersonically from a hot stainless steel oven with a fine exit nozzle, resulting in well focused cluster beams. Clusters form as a result of collisions between the atoms in the tiny expansion zone, terminating some tenths of a millimiter beyond the nozzle. The clusters warm up because the condensation is an exothermic reaction, so there is also a tendency for evaporation. As the expansion proceeds, collisions between Na atoms end, and the tendency to evaporate atoms from the hot clusters dominates. Each cluster loses mass and cools down. In the evaporation chains, clusters with low evaporation rates, i.e., with strong binding energies, tend to become abundant. In 1984, Knight and his coworkers made a remarkable discovery [1, 2]. They found that the abundance distribution shows a nonmonotonic variation as a function of cluster size, with prominent maxima and/or steps at cluster sizes N = 8, 20, 40, 58 and 92. Their original results are shown on Fig. 4.1, which has become one of the best known and most influential results in cluster physics. The arguments given above indicate that clusters of those sizes are especially stable. Similar experiments confirmed the same magic numbers in the mass spectra of other alkaline elements (Li, K, Rb, Cs). Furthermore, measurements of the ionization potential, IP, as a function of cluster size shown in Fig. 4.2, indicate that the value of IP drops abruptly between