Triad spectroscopy via ultraviolet up-conversion inPr3+:LaF3

The first observation of isolated isoionic triads in solids is reported. A three-ion energy transfer according to ${}_{}{}^1{}_{}{}^{}\mathrm{D}_2^{}{}_{}{}^{}$+${\mathrm{}}^1$ ${\mathrm{D}}_2$+${\mathrm{}}^1$ ${\mathrm{D}}_2$→ ${\mathrm{}}^1$ ${\mathrm{S}}_0$+${\mathrm{}}^3$ ${\mathrm{H}}_4$+${\mathrm{}}^3$ ${\mathrm{H}}_4$ was observed in dilute ${\mathrm{Pr}}^{3+}$:${\mathrm{LaF}}_3$ where triads of random distributed impurities can be considered as isolated. The ultraviolet luminescence following this process allows the spectroscopic observation of different kinds of triads.