Energy gaps in the 4f35d1 manifold 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.