Laser excitation of Pr3+ sites in ThCl4:Pr3+

Use of site selection spectroscopic techniques combined with low temperature absorption spectral studies has enabled the identification and characterization of ten optically important charge-compensated crystallographic sites for Pr³⁺ in the β-ThCl₄:Pr³⁺ system. Fluorescence decay time measurements, variation of site distribution with dopant concentration and fluorescence spectra were used to characterize the sites. Distant charge compensation is found to be one of the dominant modes of charge compensation of Pr³⁺ in ThCl₄, and its predominance over other sites increases as Pr³⁺ concentration decreases. The sinusoidal distortion of the crystal field symmetry around Pr³⁺ in the incommensurate phase of β-ThCl₄ is believed to be mainly responsible for the splitting and continuous shift in fluorescence spectral lines which occur when ³P₀→³F₂ fluorescence scans are made with excitation at different points on the ³P₀←³H₄ absorption line belonging to different sites.