The first, and still one of the most important investigations along these lines, is the discovery of an electronic shell structure by de Heer and coworkers when he was a graduate student at UC Berkeley 25 years ago. This discovery stands out as a prime example of the power of the technique and the information that can be obtained from it. Conside figure 1, which shows a typical mass spectrum for sodium clusters.

This spectrum was obtained by heating a sodium metal target in an enclosed source chamber equipped with a pinhole nozzle. The resulting sodium vapor is then mixed with cooled He gas and is ejected out of the nozzle. In the process, the metal vapor condenses into extremely small droplets, what we call clusters. The clusters then enter a high vacuum chamber where they are ultimately detected as follows: The clusters are ionized using a UV light source and the cluster ions M_N⁺ are detected with mass sensitivity in a quadrupole mass analyzer that is supplied with a sensitive ion detector, resulting in a mass spectrum as shown in figure 1 above.

Another advance in the cluster field was the measurement of the magnetic moments of Fe, Co, and Ni clusters. Producing a beam of these clusters required new innovation in cluster sources. These metals have very high melting temperatures, so their cluster forms cannot be produced with the same gas aggregation methods used to produce beams of Na clusters. Instead of thermal evaporation, a laser pulse can be used to produce the metal vapor by ablation. A nanosecond pulse of light from a Q-switched laser is focused onto a tiny spot of a metal target rod. The focused laser pulse has high enough intensity to instantly vaporize the metal at the focal point. If this vapor is quenched by a stream of cooled, inert gas, then the metal will condense into clusters. The development of this method enabled the production of beams of Fe, Co, and Ni clusters, to name a few.

One of the defining properties of a metal is the screening of electric fields. Because of these screening properties, the internal electric field of a metal is exactly zero, and thus there is no electric dipole moment. At low temperatures (~30 - 50 K), however, it has been observed that Nb, V, and Ta clusters acquire large electric dipole moments. The fact that these dipole moments vanish at such a low transition temperature suggests a mechanism that acts at a very low energy scale.

These materials are good conductors in the bulk, so their screening must be inhibited when they are formed into clusters. Deflection experiments reveal that the ferroelectric polarization is weakly coupled to the cluster body. The most curious effect of all is the strong odd-even alternation of the ferroelectric fraction which begins around N = 20, and persists consistently until N = 100 (see figure 3). The even-N clusters show a larger ferroelectric fraction than the odd-N clusters. This odd-even alternation depends only on the total number of valence electrons in the cluster. The electronic origin of this effect has been demonstrated by a series of experiments where Nb clusters were doped with an impurity - impurities which donate or oxidize an odd number of valence electrons invert the odd-even effect, while impurities such as oxygen which oxidize an even number of valence electrons leave the odd-even effect unchanged. Doping a cluster with a magnetic impurity like manganese quenches the dipole moment.