Crystal-field splitting of core excitons in ionic crystals

High-resolution and temperature-dependent photoabsorption measurements at the potassium $L_2$,3 edges in KF, KCl, KBr, KI, and ${\mathrm{KMnF}}_3$ are used to study the crystal-field splitting of potassium 2$p^{\mathrm{-}1}$3d core excitons. The direct observation of two split states allows one to explore the dependence of the crystal-field potential and of the 3d orbital spatial extent on thermal expansion, atomic vibrational motion, and the halide atom. Using the simple 10Dq theoretical model, we measured the size of the 3d orbitals and found that they collapse toward the potassium 2p core hole. The dynamical character of this process is demonstrated by the correlation between the core-exciton size and the optical dielectric constant.