Phosphor with Fluorescence Larger Than the Energy Gap

Under certain conditions of excitation a ${\mathrm{Pr}}^{+3\mathrm{}}$-activated SrS phosphor exhibits an emission which is energetically greater than the energy gap. This emission is observed at 77°K and not at room temperature. It is proposed that the excitation and short-wavelength emission take place between levels of the ${\mathrm{Pr}}^{+3\mathrm{}}$ center. At 77°K the reduction of lattice vibrations effects the isolation of the ${\mathrm{Pr}}^{+3\mathrm{}}$ site from the lattice and hence the probability of hole transfer from the activator to the lattice is diminished. The nature of the energy levels of the Pr ion is discussed.