Light-induced coloration and transformation process in ${\mathrm{PWO}}_4$ crystals and the effects of the defect pair $V_{\mathrm{Pb}}{-V}_{\mathrm{O}}$

The optical absorption spectrum of the as-grown ${\mathrm{PWO}}_4$ crystal mainly exhibits a weak band peaking at 350 nm, overlapping with the absorption edge of the crystal. The ultraviolet-irradiated crystal emits a strong band peaking at 420 nm and a broad band in the range of 500–700 nm almost in pairs along with a decreasing of the 350-nm band. It is found that the 420- and 500–700-nm bands can be reduced, with a reirradiation of the 411-nm monochromatic light, while the 350-nm band is strengthened simultaneously. In this paper the mechanism is studied. It is demonstrated that the increasing and/or decreasing of both the 420-nm band and the 500–700-nm band are caused by the creation and/or the annihilation of $V_{\mathrm{Pb}}^{2-}$ and $V_{\mathrm{O}}^{2+},$ respectively. The creation and annihilation of $V_{\mathrm{Pb}}^{2-}$ and $V_{\mathrm{O}}^{2+}$ are found to be related to the separation and recombination of the vacancy pair $V_{\mathrm{Pb}}^{2-}-V_{\mathrm{O}}^{2+},$ which plays an important role in the formation and transformation process of light induced color centers in the PWO crystal.