Spin-forbidden and spin-enabled 4f(14)-->4f(13)5d(1) transitions of Yb(2+)-doped CsCaBr3.

The lowest part of the 4f-->5d absorption spectrum of Yb(2+)-doped CsCaBr(3) crystals has been calculated using methods of quantum chemistry and it is presented here. A first, low-intensity band is found on the low energy side of the spectrum, followed by several strong absorption bands, in agreement with experimental observations in trivalent and divalent lanthanide ions of the second half of the lanthanide series, doped in crystals. Based on Hund's rule, these transitions are usually interpreted as "spin-forbidden" and "spin-allowed" transitions, but this interpretation has been recently questioned in the literature. Here, a two-step relativistic method has been used which reveals the spin composition of the excited state wave functions. The forbidden band is found to be due to spin-forbidden transitions involving "high-spin" excited states because their 1 (3)T(1u) character is 90%. However, the allowed bands cannot be described as spin-allowed transitions involving "low-spin" excited states. Rather, they correspond to "spin-enabled" transitions because they get their intensity from limited (smaller than 45%) electric dipole enabling low-spin (1)T(1u) character. Calculations using a spin-free Hamiltonian revealed that the difference in their electronic structures is related to the fact that the 4f(13)5d(t(2g))(1) manifold is split by an energy gap which separates the lowest (high-spin) 1 (3)T(1u) from the rest of terms, which, in turn, lie very close in energy from each other. As a consequence, the lowest spin-orbit components of 1 (3)T(1u) are shown to remain 90% pure when spin-orbit coupling is considered, whereas a strong spin-orbit coupling exists between the remaining 4f(13)5d(t(2g))(1) terms, among which the 1-3 (1)T(1u) enabling ones lie. As a result, there is a widespread electric dipole enabling (1)T(1u) character, which, although never higher than 45%, leads to a number of spin-enabled absorption bands.