Onset of metallic behavior in magnesium clusters

We have measured the photoelectron spectra of mass-selected magnesium cluster anions, Mg- n, over the size range, n=3-35. Their s-p band gaps were observed to close at n=18, signaling the onset of metallic behavior. Electronic shell structure was implicated by gap "reopenings" and mass spectral magic numbers. Complementary calculations are presented in the companion paper P. H. Acioli and J. Jellinek, Phys. Rev. Lett. 89, 213402 (2002)]].