Shell structure of magnesium and other divalent metal clusters

Abstract

Clusters of the divalent metals magnesium, cadmium, and zinc have been grown in ultracold helium nanodroplets and studied by high-resolution mass spectrometry, with a special emphasis on magnesium. The mass spectra of all materials show similar characteristic features independent of the chosen ionization technique---i.e., electron impact ionization as well as nanosecond and femtosecond multiphoton excitation. In the lower-size range the abundance distributions can be explained by an electronic shell structure. The associated electron delocalization---i.e., metallic bonding---is found to set in at about $N=20$ atoms. For ${\mathrm{Mg}}_{N}$ we have resolved crossings of electronic levels at the highest-occupied molecular orbital which result in additional magic numbers compared to the alkali metals: e.g., ${\mathrm{Mg}}_{40}$ with 80 electrons. This specific electronic shell structure is also present in the intensity pattern of doubly charged ${\mathrm{Mg}}_{N}$. For larger clusters $(N\ensuremath{\geqslant}92)$ a coexistence of electronic shell effects and geometrical packing is observed and a clear signature of icosahedral structure is present beyond $N\ensuremath{\geqslant}147$.