PHOTOIONIZATION YIELDS IN THE DOUBLY DOPED SRF2-EU,SM SYSTEM

The doubly doped SrF2:Eu, SM was examined as a model system for the photoionization process. These crystals were codoped with 0.01 mol% Eu2+ and 0.02 mol% Sm3+. From optical absorption measurements, the absolute ionization efficiency of Eu2+ 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 Sm3+.F- centers were found to be the principal traps for the generated electrons. The temperature dependence of the ionization efficiency of Eu2+ 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 Eu3+ is initially formed upon photoionization of Eu2+ and that subsequently an interstitial fluoride-ion charge compensates that Eu3+ 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 Sm3+.F- sites is followed by a reverse F(i)- 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.