Electronic and Vibronic Spectra of Ce³⁺ in YPO₄ and LuPO₄

Abstract

Rather sharp zero-phonon lines accompanied by many vibronic lines are observed in the near-ultraviolet luminescence and excitation spectra due to the 4f-5d transitions of Ce³⁺ ion in YPO₄ and LuPO₄ crystals at low temperature. Comparing the vibrational energies of the vibronic lines with the results of infrared and Raman measurements of YPO₄ crystals, it is shown that these vibronic lines can be separated into two parts; the one is due to the lattice (external) vibration of Y³⁺ and PO₄³⁻ ions and the other is due to the coupled molecular (internal) vibration of PO₄ complexes. The coupling scheme of these vibrations with the 4f or 5d electron of Ce³⁺ ion is discussed.