The photoabsorption intensities of intra-4fN transitions (f-f transitions) in lanthanide systems have been extensively studied with the semiempirical Judd-Ofelt theory. The oscillator strengths of most f-f transitions are insensitive to a change of surrounding environment because 4f electrons are shielded by closed-shell 5s and 5p electrons from outside. However, there are some exceptional transitions, so-called hypersensitive transitions, whose intensities are very sensitive to a change of surrounding environment, and the reason for this hypersensitivity has not been clarified. In this study, we calculated the oscillator strengths of lanthanide trihalides (LnX3; Ln = Pr, Tm; X = Br, I) with the multireference spin-orbit configuration interaction method and obtained reasonably accurate values. To clarify the cause of hypersensitivity, we examined various possible effects on the oscillator strengths, such as molecular vibration, f-d mixing, ligand to metal charge transfer (LMCT), and intraligand excitation, and concluded that the effect of molecular vibration is very small and that the oscillator strengths of most f-f transitions including hypersensitive transitions arise from both the LMCT and dynamic-coupled intraligand excitations through their configuration mixings with the dominant configurations of 4fN.
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