Near-IR absorption transition of H4 to F2 for UCl6 complex in M2ZrCl6 host crystals (M = K, Rb, and Cs): an experimental and theoretical study

The near-IR electronic transition from ground-state Г₁ of ³H₄ to the excited-state Г₃ of ³F₂ for the UCl₆²⁻ complex in three M₂ZrCl₆ host crystals (M = K⁺, Rb⁺, and Cs⁺) has been investigated experimentally and theoretically. The vibration frequency of the excited state is found to be sensitive to the alkaline ion substitution and the frequency variation follows the trend predicted by the ‘counterion effect’. In contrast to previous studies, the apparent anomaly in the intensity ratio of the two vibronic peaks from the prediction of a Boltzmann distribution relationship is attributed to the vibration frequency difference between the ground and the excited state. This new interpretation is supported by computer simulation of the experimental optical spectra.