Coexistence of the Eu 2+ and Eu 3+ centers is confirmed for two kinds of CaO : Eu phosphors prepared in N 2 and H 2 by observing electron spin resonance (ESR) spectra, photoluminescence and optical excitation spectra and luminescence decay curves. The ESR signal intensity due to Eu 2+ in CaO : Eu prepared in H 2 was about 7 times that in CaO : Eu prepared in N 2 . Details of the luminescence spectra of the Eu 2+ centers in CaO are reported. The emission band of CaO : Eu 2 + is located at 733 nm at 300 K, and the emission and excitation bands exhibit phonon structures at low temperatures. The luminescence lifetime of the Eu 2+ center is about 1 μs at 300 K.
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N. Yamashita (1993) studied this question.