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

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