ELECTRONIC SHELLS AND SHELLS OF ATOMS IN METALLIC CLUSTERS

Intensity anomalies (magic numbers) have been observed in the mass spectra of sodium clusters containing up to 22 000 atoms. For small clusters (Na(n), n less-than-or-equal-to 1500) the anomalies appear to be due to the filling of electronic shells (groups of subshells having the same energy). The shells can be characterized rather well by a pseudoquantum-number, indicating the possible existence of a symmetry higher than spherical. The mass spectra of larger clusters (1500 less-than-or-equal-to n less-than-or-equal-to 22 000) are well explained by the completion of icosahedral or cuboctahedral shells of atoms. The fact that the two types of shells (electron and atom) occur in distinct and non-overlapping size intervals might indicate the existence of a "liquid" to "solid" transition in going from small to large clusters.