J. Phys. Chem. A, 113, 12591, 2009

 DOI:10.1021/jp904868b

G. Sánchez-Sanz, L. Seijo, and Z. Barandiarán.

Energy Gaps in the 4f 13 5d1 Manifolds and Multiple Spontaneous Emissions in Yb2+ -doped CsCaBr3

Multiple spontaneous 4f135d1 → 4f14 emissions are predicted in Yb2+-doped CsCaBr3 crystals by ab initio quantum chemical calculations. Four emission bands are found at 23 900, 26 600, 34 600, and 43 900 cm−1 that should be experimentally observable at low temperatures. The first, third, and fourth bands are slow, electric dipole forbidden emissions that can be described as spin-forbidden. The second band is a fast, electric dipole-allowed emission that cannot be described as spin-allowed, but as spin-enabled; its radiative emission lifetime is 400 ns. Large energy gaps (23 900, 4600, 4000 cm−1, respectively), relative to the maximum local phonon energies calculated (around 185 cm−1), are found below the emitting levels of the slow bands, which indicates that these states should be significantly stable and multiphonon relaxation to the lower states should be negligible. A smaller gap (2600 cm−1) separates the states of the fast band, which should result in a temperature dependent competition between radiative and nonradiative decay. Differential correlation between 4f−4f and 4f−5d pairs, splitting of the 5d shell by interactions with the host, and spin−orbit effects within the 4f13 subshell, are found to be responsible for the existence of the gaps, which, in turn, split the absorption spectrum into four groups of separate bands, three of which could lie below the host absorption threshold. The quantum chemical methods employed make use of explicit wave functions expanded in terms of flexible basis sets, multiconfigurational self-consistent-field and multireference second-order perturbation methods to account for nondynamic and dynamic electron correlation, scalar and relativistic terms in the (YbBr6)4− defect cluster Hamiltonian, and quantum mechanical embedding potentials to represent the host crystal.

J. Chem. Phys., 131, 024505, 2009

DOI:10.1063/1.3171567 

G. Sánchez-Sanz, L. Seijo, and Z. Barandiarán.

Spin-forbidden and spin-enabled 4f 14 → 4f 13 5d1 transitions of Yb2+ -doped CsCaBr3.  

The lowest part of the 4f5absorption spectrum of Yb2+-doped CsCaBr3 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 statewave functions. The forbidden band is found to be due to spin-forbidden transitions involving “high-spin” excited states because their 13T1u 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 1T1u character. Calculations using a spin-free Hamiltonian revealed that the difference in their electronic structures is related to the fact that the 4f135d(t2g)1manifold is split by an energy gap which separates the lowest (high-spin) 13T1u 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 13T1are shown to remain 90% pure when spin-orbit coupling is considered, whereas a strong spin-orbit coupling exists between the remaining 4f135d(t2g)1 terms, among which the 131T1u enabling ones lie. As a result, there is a widespread electric dipole enabling 1T1u character, which, although never higher than 45%, leads to a number of spin-enabled absorption bands.

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