Upconverted hot luminescence of Cr^4+ ions in Mg_2GeO_4

Nonequilibrium excited vibronic states of Cr⁴⁺ ions in Mg₂GeO₄ were studied with the upconverted hot-luminescence technique. From the discrete spectral peaks observed, information on the nonradiative relaxation of the Cr⁴⁺ ions was obtained. The phonon modes involved in the nonradiative relaxation were measured to be ${\omega }_1=451\pm 20 {\text{cm}}^{-1}$, ${\omega }_2=342\pm 20 {\text{cm}}^{-1}$, and ${\omega }_3=252\pm 20 {\text{cm}}^{-1}$. The experiment also shows the presence of an electronic bottleneck centered at $\sim 2.1$ eV. These findings are compared with results previously obtained in Cr⁴⁺-doped Mg₂SiO₄ and provide information on the nonradiative relaxation of the same ion in different host crystals.