The optical properties of intra-4fN transitions (f–f transitions) in lanthanide compounds are usually insensitive to the surrounding environment due to the shielding effect of the outer 5s and 5p electrons. However, there are exceptional transitions, the so-called hypersensitive transitions, whose oscillator strengths change sensitively to a small change of the surrounding environment. The mechanism of the hypersensitive transitions was explained mostly with the dynamic-coupling (DC) model. In this study, the oscillator strengths of hypersensitive transitions in lanthanide trihalides (LnX3; Ln = Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm; X = Cl, Br, I) were calculated by the multi-reference spin–orbit configuration interaction (CI) method, and the origin of the hypersensitive transition intensities was examined. To compare the intensities derived from the DC model and from the ab initio CI computations, we evaluated two Judd–Ofelt intensity parameters: τ2(dc) by the DC model and τ2(ab) by the CI computations. Although these two parameters showed similar overall behaviors, their Ln dependences were different, suggesting the involvement of other mechanism(s) in τ2(ab). Close examination of the spatial distributions of the transition densities and the integrand of the transition dipole moments (TDMs) suggested that the Judd–Ofelt theory contributions were also involved in τ2(ab) with the opposite sign relative to the TDMs with the DC model in all the hypersensitive transitions of LnX3. Moreover, the different Ln dependences in τ2(dc) and τ2(ab) were related to the different amount of the mixing of ligand-to-metal charge transfer configurations into the dominant 4fN configurations, especially for Eu and Tb.
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