No-PhononT24g−A24gTransitions ofCr3+in TiO2

Abstract

Absorption and emission at 4 and 77 K are observed in Cr-doped Ti${\mathrm{O}}_2$ consisting of two sharp no-phonon lines at 12 685 and 12 732 ${\mathrm{cm}}^{-1\mathrm{}}$, and vibronic sidebands with an integrated intensity ${10}^4$ times that of the no-phonon lines. In undoped Ti${\mathrm{O}}_2$, identical spectra are observed but are a factor of 100 less intense. The center responsible for these spectra is identified as substitutional ${\mathrm{Cr}}^{3+}$ by observing, in emission, a splitting of the 12 685-${\mathrm{cm}}^{-1\mathrm{}}$ line of 1.4 ${\mathrm{cm}}^{-1\mathrm{}}$, a value previously determined by EPR measurements. The vibronic sidebands have some structure superposed on a broad band. The vibronics are different in emission and absorption and have a lifetime of 45 μsec, equal to that of the no-phonon line. They are shown to be principally due to defect-induced phonons arising from the ${\mathrm{Cr}}^{3+}$ defect. We argue that the spectra observed are not characteristic of the usual ${}_{}{}^2{}_{}{}^{}E_g^{}{}_{}{}^{}-{}_{}{}^4{}_{}{}^{}A_{2g}^{}{}_{}{}^{}$ transition, and propose that the ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{}$ lies lower in energy than the ${}_{}{}^2{}_{}{}^{}E_g^{}{}_{}{}^{}$ state. If so, the new results of this work are (1) no-phonon line emission for the ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{}-{}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{}$ transition and (2) defect phonon states which are different for the ${}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{}$ and ${}_{}{}^4{}_{}{}^{}T_2^{}{}_{}{}^{}$ states. The excitation spectrum is also presented. It shows that the excitation of the ${\mathrm{Cr}}^{3+}$ emission occurs mainly by the transfer of energy from excited trapping centers rather than by direct excitation.