Electronic shell structure in clusters as reflected in mass abundance spectra

The electronic shell structure in metal clusters was discovered through mass abundance spectrometry of sodium clusters from a supersonic expansion source. Here we review the stepwise approach towards a quantitative understanding of the measured variations in relative abundances in terms of the variations in atomic separation energy produced by the shell structure. Experimental results for sodium clusters with 50-240 atoms are compared with theoretical model predictions. The comparisons suggest that clusters outside the magic regions assume permanent non-spherical shapes. At the same time, the temperature-induced motion of the metal ions gives rise to dynamic shape changes, resulting in a roughening of the cluster surface that tends to soften the shell oscillations around the magic numbers.