Mass spectra are presented for sodium clusters of $N$ atoms per cluster ($N=4-100\mathrm{}$) produced in a supersonic expansion with argon carrier gas. The spectra show large peaks or steps at $N=8, 20, 40, 58, \mathrm{and} 92$. These can be understood in terms of a one-electron shell model in which independent delocalized atomic $3s$ electrons are bound in a spherically symmetric potential well.

• Received 12 April 1984

DOI:https://doi.org/10.1103/PhysRevLett.52.2141