Fig. 6: Metal-to-metal charge transfer many-electron diagrams.

Contour plots of selected potential energy surfaces in the two-dimensional configurational space (d_Eu–S, d_Tb–S) (a, b) and configurational coordinate diagrams along the electron transfer reaction coordinate (Q_et) (c, d) for Eu–Tb pairs in CaS (a, c) and SrS (b, d). The configurational coordinate diagrams display the following levels for the Eu²⁺–Tb³⁺ configuration: 4f⁷(⁸S)–4f⁸(⁷F) (black), \(4{f}^{6}5d{t}_{{\rm{2}}g}^{1}-4{f}^{8}(1\,{A}_{{\rm{1}}g})\) (green for high-spin states, red for low-spin states) and 4f⁷(⁸S)–4f⁸(⁵D, L, G) (gold), and following levels for the Eu³⁺–Tb²⁺ configuration: 4f⁶(⁷F)–4f⁸5d¹(1Γ_7g) (black), 4f⁶(1A_1g)–4f⁸5dt_2g (green for high-spin states, red for low-spin states), and 4f⁶(1A_1g)–4f⁹ (gray). The quasi-continuum formed by all other levels of the Eu–Tb pair, where none of both ions is in its ground state are represented by the green-colored background. The contour plots display the isolines corresponding to the Eu²⁺4f⁶5d¹(1Γ_8g)–Tb³⁺4f⁸(1A_1g) and Eu³⁺4f⁶(⁷F)–Tb²⁺4f⁸5d¹(1Γ_7g) (lowest metal-to-metal charge transfer (MMCT) state) surfaces. The reaction coordinate is indicated in red, along with the intersection between both surfaces (dashed red line).