Thermal Effects on the Fluorescence Lifetime and Spectrum of MgO:V2+

Abstract

The vibronic spectrum associated with the ${}_{}{}^2{}_{}{}^{}E\to {}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{}$ transition at 8700 Å of MgO: ${\mathrm{V}}^{2+}$ has been investigated as a function of temperature. The position and the width of the purely radiative ($R$) line associated with this transition were measured from 4 to 460°K, and the total intensity of the fluorescence spectrum from 77 to 650°K. At low temperatures vibrational satellites are observed only on the low-energy side of the pure electronic transition, corresponding to spontaneous phonon emission; at higher temperatures they also appear on the high-energy side, corresponding to phonon absorption. With increasing temperature the vibronic bands become more intense relative to the pure electronic transition, the peaks broaden, and multiphonon processes enter and form a fluorescence continuum extending from ≈7000 to 10 500 Å. The fluorescence lifetime of the ${}_{}{}^2{}_{}{}^{}E$ level, which is 50 msec and nearly temperature-independent at 77°K, decreases rapidly above 200°K to ≈0.03 msec at 800°K. The enhancement of the transition probability of the vibronic bands, determined from the relative increase in the integrated area after correction for the temperature dependence of the absorption strength, can account for the major temperature dependence of the fluorescence lifetime, without requiring the presence of any purely nonradiative transitions.