Energy-transfer processes between ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Pr}}^{3+}$ Ions in ${\mathrm{CsCdBr}}_3$

The property of preferential pair formation for trivalant ions in ${\mathrm{CsCdBr}}_3$ has been utilized to isolate and investigate energy cross-relaxation and upconversion processes of thulium and praseodymium. The principal pair centers formed by ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Pr}}^{3+}$ ions in ${\mathrm{CsCdBr}}_3$ have been identified, including the heterogeneous ${\mathrm{Tm}}^{3+}$-${\mathrm{Pr}}^{3+}$ pair center. Their energy levels and fluorescence lifetimes have been measured at 10 K. Many efficient upconversion processes were observed, involving both intrapair and interpair energy transfer. Red ${(}^3$ ${\mathrm{F}}_3$ ${\to }^3$ ${\mathrm{H}}_6$ ${\mathrm{Tm}}^{3+}$,${\mathrm{}}^1$ ${\mathrm{D}}_2$ ${\to }^3$ ${\mathrm{H}}_4$ ${\mathrm{Pr}}^{3+}$), blue ${(}^1$ ${\mathrm{G}}_4$ ${\to }^3$ ${\mathrm{H}}_6$ ${\mathrm{Tm}}^{3+}$, ${}_{}{}^3{}_{}{}^{}\mathrm{P}_0^{}{}_{}{}^{}$ ${\to }^3$ ${\mathrm{H}}_4$ ${\mathrm{Pr}}^{3+}$), and ultraviolet ${(}^1$ ${\mathrm{D}}_2$ ${\to }^3$ ${\mathrm{H}}_6$ ${\mathrm{Tm}}^{3+}$) upconversion fluorescence is produced when pumping the infrared ${}_{}{}^3{}_{}{}^{}\mathrm{H}_6^{}{}_{}{}^{}$ ${\to }^3$ ${\mathrm{H}}_4$ ${\mathrm{Tm}}^{3+}$ transitions in the mixed-dopant crystal. Dual-pulse laser techniques were used to identify directly the intermediate states involved in the energy-transfer upconversion and mechanisms are proposed to explain the processes observed. Excitation energy is localized in the pair centers, which prefer to relax via sequential energy transfers between the two ions, when these involve smaller energy mismatches than multiphonon decay. © 1996 The American Physical Society.