The possibility of ellipsoidal distortions in free-electron metal clusters, analogous to the shape variations among atomic nuclei, is investigated with the use of a modified Nilsson Hamiltonian. In most cases, the predicted equilibrium shape is ellipsoidal rather than spherical, so that the spherical shells are divided into ellipsoidal subshells. A strong correlation is observed between the energy-level sequence of these subshells and the sequence of peaks in alkali-metal cluster mass spectra, indicating that metal clusters generally assume approximately ellipsoidal shapes.

• Received 13 March 1985

DOI:https://doi.org/10.1103/PhysRevB.32.1359