Absorption and luminescence in PbCl 2 : Tl + crystals

Abstract

Absorption and luminescence due to Tl + doped in PbCl 2 crystals have been investigated by varying temperature and the content of Tl + ions. The low-energy tail of the lowest excitonic absorption band of PbCl 2 shifts gradually toward the low-energy side as the Tl + content increases. No specific impurity absorption peaks were observed, which means that the 6s level of the Tl + ion to merge into the valence band mainly composed of the 6s orbit of the host Pb 2+ ion. Besides the well-known intrinsic luminescence, namely, ultraviolet (UV), blue (B) and blue–green (BG) emission bands, a new green emission band (G 1) appears at 2.48 eV with excitation in the shifted low-energy absorption tail region. The G 1-emission shows similar temperature dependence to the intrinsic UV-emission and is related to the [Tl 0(Cl −) 3+hole] center corresponding to the [Pb +(Cl −) 3+hole] center for the UV-emission in PbCl 2. On increasing the doped TlCl concentration up to 1 mol% another emission band (G 2) is observed at 2.39 eV under excitation in the band-to-band region of the doped crystal, PbCl 2 : Tl +. Because of its temperature dependence being similar to that of the BG-emission, the recombination luminescence at the [(Pb 2) 3+-STEL+hole] center in PbCl 2, the G 2-emission is connected to the [(PbTl) 2+-STEL+hole] center in PbCl 2 : Tl +.