Photoionization yields in the doubly dopedSrF2:Eu,Sm system

The doubly doped ${\mathrm{SrF}}_2$:Eu,Sm system was examined as a model system for the photoionization process. These crystals were codoped with 0.01 mol % ${\mathrm{Eu}}^{2+}$ and 0.02 mol % ${\mathrm{Sm}}^{3+}$. From optical-absorption measurements, the absolute ionization efficiency of ${\mathrm{Eu}}^{2+}$ was found to be 0.65% at 295 K and 1.6% at 310 K upon irradiation with 4.9-eV light in this system. The ${\mathrm{Sm}}^{3+}$⋅${\mathrm{F}}^{\mathrm{-}}$ centers were found to be the principal traps for the generated electrons. The temperature dependence of the ionization efficiency of ${\mathrm{Eu}}^{2+}$ with irradiation at 5.2 and 5.7 eV led to the measurement of an activation barrier of 0.34 eV. These experiments also show that only cubic ${\mathrm{Eu}}^{3+}$ is initially formed upon photoionization of ${\mathrm{Eu}}^{2+}$ and that subsequently an interstitial fluoride-ion charge compensates the ${\mathrm{Eu}}^{3+}$ ion. The photoconductivity does not show a temperature dependence corresponding to that of the ionization yield. This result leads us to propose that the trapping of electrons at the majority ${\mathrm{Sm}}^{3+}$⋅${\mathrm{F}}^{\mathrm{-}}$ sites is followed by a reverse ${\mathrm{F}}_{\mathit{i}}^{\mathrm{-}}$ current from trap to donor, which cancels the electron current. The measured photocurrent arises from the small minority of other traps present. A mechanism is proposed to explain the observed yields and temperature dependence.