Atomic origin of 3d⁹4 f ¹ configuration in La³⁺ solids

We have studied the excited electronic structure of LaBr₃(Ce) scintillator by soft x-ray spectroscopy such as x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES). The La 3d XAS and XES spectra of LaBr₃(Ce) are compared with those of other La³⁺ solids (LaF₃, La₂O₃, and La metal). From this comparison, it turns out that the La 3d XAS and XES spectra from all the La³⁺ solids considered here appear at almost the same energy, even though the corresponding binding energies of the 3d core holes determined by XPS (x-ray photoelectron spectroscopy) are very different. As a result, we argue that the atomic nature of the $3d^{9}4f^1$ configuration created by $3d^{10}4f^0\to 3d^{9}4f^1$ x-ray absorption process in La³⁺ solids is maintained via the localized 4 f ¹ state, which screens the 3d core holes differently from one La³⁺ solid to another. (paper)