(b) Emission and excitation spectra of BaS : Bi3+ phosphors are measured a t 300, 80, and 6 K. I n the excitation spectra the A and C excitation bands are observed. I n the emission spectra only one band is observed which is assigned to the transition 3A1U + lAlg on the basis of the measurement of the time-resolved emission spectra and the decay constant of the phosphorescence. The diffused reflection spectra and the magnetic effect on the phonon structure of the emission band are also measured a t 80 and 4.2 K, respectively. The former is compared with the excitation spectra.The reason why the emission band 3T1, -lAlg cannot be observed a t low temperatures is explained by the fact that the probability of the tunnel effect 3T1u -3A1, is larger than that of the luminescent transition 3T1, + lAig.
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