Ionization potentials of aluminum-sodium bimetallic clusters (Al _nNa_m)

Aluminum-sodium bimetallic clusters (Al_nNa_m, n = 2-26) were produced by two independent laser vaporization methods. To investigate the electronic structures, ionization potentials of the Al _nNa_m 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 Al_nNa₁ 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.