Shell structure in photoionization spectra of large aluminum clusters

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Abstract
Photoionization mass spectrometry experiments, performed on an extensive size range of aluminum clusters produced by laser vaporization technique, are reported. Ionization potential values are deduced from individual photoionization efficiency curves for the smaller Al(N) Clusters (N = 36-112). Our results confirm and complete those previously published. The mass spectra of larger clusters (N almost-equal-to 250-1400) reveal a regular signal oscillation. Several additional experiments give proof that this striking pattern originates from size-dependent ionization threshold effects. This structure exhibits exact periodicity as a function of N1/3 or N(e)1/13 (N(e) the number of valence electrons).