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

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.