Strong Static And Dynamic Jahn-Teller And Pseudo-Jahn-Teller Effects In Niobium Tetrafluoride

We present a first-principles study of the static and dynamic aspects of the strong Jahn-Teller (JT) and pseudo-JT (PJT) effects in niobium tetrafluoride, NbF4, in the manifold of its electronic ground state, E-2, and its first excited state, T-2(2). The complex topography of the full-dimensional multi-sheeted adiabatic JT/PJT surfaces is analyzed computationally at the complete-active-space self-consistent-field (CASSCF) and multireference second-order perturbation levels of electronic structure theory, providing a detailed characterization of minima, saddle points, and minimum-energy conical intersection points. The calculations reveal that the tetrahedral (T-d) configuration of NbF4 undergoes strong JT distortions along the bending mode of e symmetry, yielding tetragonal molecular structures of D-2d symmetry with T-d -> D-2d stabilization energies of about 2000 cm(-1) in the X2E state and about 6400 cm(-1) in the A2T2 state. In addition, there exists strong X2E-A2T2 PJT coupling via the bending mode of t(2) symmetry, which becomes important near the crossing seam of the X2E and A2T2 potential energy surfaces. A five-state five-mode JT/PJT vibronic-coupling Hamiltonian is constructed in terms of symmetry-invariant polynomial expansions of the X2E and A2T2 diabatic potential energy surfaces in the e and t(2) bending coordinates. The parameters of the Hamiltonian are determined by a least-squares fit of its eigenvalues to the CASSCF ab initio data. The vibronic spectra and the time evolution of adiabatic electronic population probabilities are computed with the multi-configuration time-dependent Hartree method. The complexity of the spectra reflects the effects of the exceptionally strong E x e and T-2 x e JT couplings and (E + T-2) x (e + t(2)) PJT coupling. The time evolution of the populations of the adiabatic electronic states after the initial preparation of the A2T2 state reveals the femtosecond nonadiabatic dynamics through a multidimensional seam of conical intersection. These results represent the first study of the static and dynamical JT/PJT effects in the X2E and A2T2 electronic states of NbF4.