Repulsion energy, charge-transfer transitions

Fig. 10. A schematic representation of the one electron (d electron) energy levels in Prussian blue, and the visible charge transfer transitions, I and II. As a result of electron repulsion which is not accounted for in this one electron diagram, transitions I and II are not spontaneous.

This observed pattern of charge transfer and switching processes is consistent with the vertical-transition model (Franck-Condon principle) as discussed by Bearman et al. (1976), who interpreted the cross sections for ionic excitation in low-energy charge-transfer collision between HeJ and some diatomic neutrals. In analogy to that, in the cases of KrJ reactions, it is not the total recombination energy RE(KrJ) = 12.85 eV that is available, but only the effective recombination energy Reeff(KrJ) = 11.91 eV, which is determined, as shown in Fig. 6, by the vertical transition from KrJ to the repulsive state of JCr-Kr at the equilibrium distance f o(Kr2 ) ... 

There is considerable repulsion energy involved in this transition, since an electron must be moved from a delocalized ir orbital into an occupied b2 orbital which is localized on the vanadium. This repulsion energy is estimated as ca. 11,700 cm.-1 by comparing the positions of the first charge transfer band of VOa+ with the first band of VO-Cla.21 This 11,700 cm.-1 estimate is included in the predicted energy for the B2 - 2E(II) transition. Both the predicted energy and the predicted / value agree wdl with the experimentally observed values. 

Figure 7.2. The onsite d-d Coulomb repulsion energy (left) and the charge transfer excitation energy (right) for the 3d transition metal oxides. Plots are from data by Bocquet et al. (1992, 1996).

---