Elucidating the Electronic Structures of the Ground States of the VO2(-/0) Clusters: Synergism between Computation and Experiment.

Abstract

Electronic structures of VO2 and its anion were investigated with density functional theory (DFT), complete active space second-order perturbation theory (CASPT2), and restricted coupled-cluster with single, double, and perturbative triple excitations (RCCSD(T)) computational quantum chemical methods. The results show that there is a near-degeneracy of the lowest (3)B1, (3)A1, and (1)A1 states of the anion. Therefore, the 532 and 193 nm photoelectron spectra of VO2(-) are interpreted by exploring these states as possible initial states. The anionic ground state was identified at the highest computational level, that is, RCCSD(T), as (3)B1 allowing the X band to be assigned to the (3)B1 → (2)B1 transition, while the lower intensity and lower binding energy X' and X″ features are ascribed to the (1)A1 → (2)A1 and (3)A1 → (2)A1 ionizations, respectively. The latter assignment is different from the recently proposed assignment of the corresponding slow electron velocity-map imaging (SEVI) spectra. Further, the A band is suggested to be mainly the result of an ionization from (3)B1 to 2(2)A1. For all these ionizations an electron is removed from a predominant metal orbital. The higher energy bands B and C on the contrary can be ascribed as electron detachments out of molecular orbitals largely located on the oxygen centers. More precisely, the B band is attributed to the ionizations from (3)B1 to (4)A2 and (2)A2, while the C band is proposed to originate from the (3)B1 → (4)B1 and (3)B1 → 2(2)B1 ionizations. The proposed novel assignment is further corroborated by calculating the Franck-Condon factors, which largely agree with the experimental vibrational progressions of the SEVI spectra.