Ground and Low-Lying Excited States of NbC3-/0 Clusters: Assignment of the Anion Photoelectron Spectra from Multiconfigurational Calculations.

The BP86 DFT, CCSD(T), and CASPT2 methods were employed to study the low-lying states of NbC3-/0 clusters. The anionic and neutral ground states were determined to be the 1A1 and 12A1 of the cyclic-NbC3-/0 isomers. All bands in the photoelectron spectra of the NbC3- cluster were interpreted based on electron detachment processes from the 1A1 of the cyclic-NbC3- isomer. The X, A, B, and C bands in the spectra are, respectively, ascribed to the transitions from 1A1 to the 12A1, 2B1, 22A1, and 2B2 states. The wave function of the initial 1A1 anionic state exhibits a pronounced multireference character that plays to some extent a role in the 1A1 → 12A1 and the 1A1 → 22A1 ionizations. Only the 1A1 → 2B2 transition appears as a clear one-electron detachment process. The Franck-Condon factor simulations for the 1A1 → 12A1 and 1A1 → 2B1 transitions are consistent with the observed vibrational progressions of the X and A bands in the spectra.