Vibronic Transitions of Eu³⁺ Ions in CaWO₄ and Other Tungstates

Abstract

Absorption and excitation spectra associated with the 4 f -4 f transitions of Eu 3+ ions in CaWO 4 :Eu, Na, NaEu(WO 4 ) 2 and KEu(WO 4 ) 2 were studied at low temperatures. Vibronic transitions are observed as bands with fine structure composed of sharp lines. The observed vibronic spectra were compared with the k =0 phonon spectra of CaWO 4 . Fairly good agreement is found between the both spectra, and the existence of a distinct gap in the spectra is noted. The intensity of the vibronic transition associated with 7 F 0 → 5 D 1 transition is weaker by one order of magnitude than those associated with 7 F 0 → 5 D 0 and 5 D 2 transitions. These results are well interpreted by use of Judd and Ofelt's theory. Namely, the 7 F 0 → 5 D 2 and 5 D 2 vibronic transitions are allowed by the first-order electric-dipole, while the 7 F 0 → 5 D 1 vibronic transition is only allowed by the second-order electric-dipole and first-order magnetic-dipole.