Electronic shell structure of indium-sodium (In_nNa_m) bimetallic clusters examined by their ionization potentials and mass distributions

Indium-sodium (In_nNa_m) bimetallic clusters were produced by two independent laser vaporization methods. Ionization potentials (IPs) of the In_nNa_m 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 In_n (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 In_nNa_m^- 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 In_n clusters around n = 13. These results indicate that Na atom addition can induce s/p hybridization in the In_n clusters. We also present mass distributions of aluminum-sodium cluster anions, Al_nNa_m^-, whose feature can be understood by the electronic shell model.