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

Bimetallic niobium-doped aluminum clusters, NbAl n -/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 NbAl n - anions with n = 3-8 can be described by gradually adding Al atoms to the NbAl 3 - core. Starting from n = 9, the lowest-energy geometric structures of NbAl 9-12 - transform into bilayer structures. In particular, NbAl 12 - has a C 3v symmetric structure, which can be viewed as a NbAl 6 regular hexagon over a bowl-shaped Al 6 structure. More detailed analyses indicate that NbAl 9 and NbAl 12 - possess unusual stability, which may be attributed to their closed-shell electron configurations with superatomic features.