Theoretical analysis of orbital and correlation effects on the electronic absorption spectrum of theMnS4center in zinc-blende crystals

Optical excitation energies ascribed to the ${\mathrm{Mn}}^{2+}$ center in ZnS:Mn are calculated from Gaussian-based self-consistent-field (SCF) molecular orbitals (MO’s) obtained for the tetrahedral ${\mathrm{MnS}}_4^{6\mathrm{-}}$ cluster at three Mn-S distances in a static external potential, followed by a detailed investigation of two levels of electron correlation effects. The first level includes only configuration interaction (CI) among the ligand field (LF) $d^5$ states plus other intra-d-shell contributions by the empirical correlation-energy-correction method (CEC), which is formally equivalent to the intermediate-crystal-field (CF) model. The second level extends the CI to include a large manifold of cluster S→Mn relaxed charge-transfer (RCT) states. At the LF-CI+CEC level, the relative separations among, and the overall width of, the six lowest quartets agree closely with the four bands observed in the spectrum. However, contrary to usual assignments, b ${}_{}{}^4{}_{}{}^{}T_1^{}{}_{}{}^{}$ ${(}^4$P) is found below b ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{}$ ${(}^4$D), and all quartets are obtained about 0.5 eV too high relative to ${}_{}{}^6{}_{}{}^{}\mathrm{A}_1^{}{}_{}{}^{}$. The delocalization effects in the MO’s are noticeably smaller than obtained from empirical CF fits to the spectrum. At the second level, CI mixing with the RCT states introduces important changes and interactions not properly encompassed by the orbital-based LF parametrization schemes. This CI depresses the quartet levels almost uniformly down by 0.2 eV relative to ${}_{}{}^6{}_{}{}^{}\mathrm{A}_1^{}{}_{}{}^{}$. The order of b ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{}$ and b ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{}$ and $b^4$ $T_1$ is reversed and now agrees with the usual assignments. Finally these various results are discussed in comparison to less quantitative models in which extended interactions with the host are considered. A degree of similarity between the two formulations is established which gives further justification to the model as used herein.