Photoluminescence studies ofSm2+in the stimulable phosphor SrS:Eu,Sm

Abstract

The doubly rare-earth doped phosphor SrS:Eu,Sm is representative of a class of stimulable phosphors that may find application in optical data storage and related areas of photonic technology. The infrared photoluminescence (PL) from the ${\mathrm{Sm}}^{2+}$ ions is a potentially useful probe of the “written” state of this phosphor, i.e., the right-hand side of the charge-transfer reaction $\left({\mathrm{Eu}}^{2+}{+\mathrm{S}\mathrm{m}}^{3+}{\rightleftarrows \mathrm{Eu}}^{3+}{+\mathrm{S}\mathrm{m}}^{2+}\right).\mathrm{}$ In the present study, several properties of the ${\mathrm{Sm}}^{2+}$ PL from bulk ceramic SrS:Eu,Sm were examined, including the emission spectrum, temperature dependence from 14 to 300 K, electron-phonon coupling, excitation spectrum, and excitation intensity dependence. The full width at half maximum of the PL emission spectrum, $W\left(T\right),$ was found to increase with temperature in a manner consistent with a linear electron-phonon coupling model, ${W\left(T\right)=W}_0\left[\tanh \right(\Omega {/(2k}_{B}T\left)\right){]}^{-1/2},$ with $W_0=0.190\mathrm{}\mathrm{eV}$ and coupled phonon energy $\Omega =0.026\mathrm{}\mathrm{eV}.$ Periodic oscillations in the PL spectrum at $T<\mathrm{}60\mathrm{}\mathrm{K}$ provide direct evidence for coupling to two phonon modes, with phonon energies ${\Omega }_1=0.0155\mathrm{}\mathrm{eV}$ and ${\Omega }_2=0.028\mathrm{}\mathrm{eV}.$ The peak of the emission spectrum occurs at 0.709 eV at 14 K and increases with temperature at a rate $1.9\times {10}^{-4}\mathrm{e}\mathrm{V}/\mathrm{K}.$ The electron-phonon coupling model predicts that the corresponding optical absorption peak, which is anti-Stokes shifted relative to the PL peak, should occur at 1.17 eV at low temperature, or 1.23 eV at room temperature. Previous measurements of the charge-transfer excitation spectrum in SrS:Eu,Sm showed a peak at 1.25 eV, in good agreement with the model prediction. The PL excitation spectrum from 1.7 to 6.2 eV was measured at $T=14\mathrm{}\mathrm{K}.$ Excitation peaks were observed at 2.08 eV, ascribed to absorption to a higher excited ${\mathrm{Sm}}^{2+}$ state; at 2.65 eV, ascribed to ${\mathrm{Eu}}^{2+}$ absorption followed by $\mathrm{E}\overrightarrow{u}\mathrm{Sm}$ charge transfer; at 2.92 eV; and at 4.49 eV, ascribed to interband excitation of the SrS.