Indium-sodium (InnNam) bimetallic clusters were produced by two independent laser vaporization methods. Ionization potentials (IPs) of the InnNam clusters were measured up to m = 2 using a tunable ultraviolet laser combined with a time-of-flight (TOF) mass spectrometer. At small sizes (n = 3-15), the ionization potentials decrease by 0.1-0.8 eV with the addition of Na atom(s), whereas the IPs of larger Inn (15 ≤ n ≤ 27) clusters do not decrease with Na addition. Moreover, IPs of In7Na1 and In13Na1 clusters are higher than those of In7 and In13, and the IP increments can be explained by electronic shell closings of the 1p (8e) and 2p shell (40 e), where In atoms in the clusters are monovalent and trivalent, respectively. The electronic shell structure was also examined by a magic number in mass distributions of InnNam- cluster anions; the In12Na3- cluster can be observed as magic numbers, corresponding to the 2p shell closing. In contrast, no electronic shell structure is observed in pure Inn clusters around n = 13. These results indicate that Na atom addition can induce s/p hybridization in the Inn clusters. We also present mass distributions of aluminum-sodium cluster anions, AlnNam-, whose feature can be understood by the electronic shell model.
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