Aluminum-sodium bimetallic clusters (AlnNam, n = 2-26) were produced by two independent laser vaporization methods. To investigate the electronic structures, ionization potentials of the Al nNam clusters were measured up to m = 3 using a tunable ultraviolet laser combined with time-of-flight (TOF) mass spectrometer. In general, the ionization potentials monotonically decrease with the number of sodium atoms, and the ionization potentials of AlnNa1 generally decrease by 0.2-0.6 eV compared to those of the corresponding Al n cluster. In contrast, the ionization potentials of Al 13Na1 and Al23Na1 are higher than (or equal to) those of Al13 and Al23. The anomalous change in the ionization potentials of Al13Na1 and Al 23Na1 can be explained by the electronic shell model: By the addition of one sodium atom to the bare aluminum clusters, the total number of their valence electrons strictly satisfies the shell closing of 2p and 3s shells, respectively, and therefore, the clusters can be stabilized electronically.
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