Color Center Production in Ca${\mathrm{F}}_2$${\mathrm{Sm}}^{3+}$ (${\mathrm{Y}}^{3+}$) by Ultraviolet Irradiation

Color centers have been produced in certain Ca${\mathrm{F}}_2$ ${\mathrm{Sm}}^{3+}$ crystals by ultraviolet irradiation. The color centers are attributed to reduction of ${\mathrm{Y}}^{3+}$ impurities. Although no ${\mathrm{Sm}}^{3+}$ reduction was observed, its presence was found to be indispensable for ultraviolet coloration. A complex is proposed, within which charge transfer takes place.