Anion photoelectron spectroscopy and quantum chemical calculations of bimetallic niobium-aluminum clusters NbAln-/0 (n=3-12): identification of a half-encapsulated symmetric structure for NbAl12-

Bimetallic niobium-doped aluminum clusters, NbAln-/0 (n = 3-12), are investigated through a synergetic combination of size-selected anion photoelectron spectroscopy and theoretical calculations. It is found that the dominant structures of NbAln- anions with n = 3-8 can be described by gradually adding Al atoms to the NbAl3- core. Starting from n = 9, the lowest-energy geometric structures of NbAl9-12- transform into bilayer structures. In particular, NbAl12- has a C-3v symmetric structure, which can be viewed as a NbAl6 regular hexagon over a bowl-shaped Al-6 structure. More detailed analyses indicate that NbAl9 and NbAl12- possess unusual stability, which may be attributed to their closed-shell electron configurations with superatomic features.