THEORETICAL INVESTIGATION OF FINE-STRUCTURE EFFECTS IN THE BENDING AND SYMMETRICAL STRETCHING VIBRONIC SPECTRUM OF FEH2 AND FED2

Two-dimensional potential energy surfaces were determined for the 25 s patial and spin components of the low-lying electronic (5)Delta(g), (5 )Pi(g), and (5)Sigma(g)(+) states of iron dihydride along the bending and symmetric stretching coordinates. Spin-free electronic energies an d electric dipole moments were obtained by means of an averaged couple d-pair functional employing a one-component relativistic Hamiltonian. Diagonal and off-diagonal spin-orbit coupling matrix elements were eva luated at the ab initio level for a variation of the symmetric stretch ing coordinate while the dependence on the bending angle vias estimate d from the variation of the angular momentum matrix elements. Vibronic energy levels were calculated separately for each multiplet component : for the treatment of Renner-Teller coupling in the large amplitude b ending motion an effective Hamiltonian was used in which the symmetric stretching motion is separated off and integrated over. We find that the Renner-Teller coupling is negligible in the X(5)Delta(g) state and that its vibronic energy level scheme is dominated by spin-orbit coup ling effects. The spatial components of the excited (5)Pi(g) state, on the other hand, exhibit a considerable energy separation upon bending . Close to the (5)Pi(A2) component we locate the (5)Sigma(g)(+) electr onic state which has large spin-orbit coupling matrix elements with bo th (5)Pi(g) components.