Qiren Zhang

A New Absorption Band and the Decomposition of the 350 nm Absorption Band of PbWO4

Single crystal PbWO 4 has been grown by the improved Bridgman method. The absorption spectra in polarized light of the sample with the crystal c-axis parallel to its surface have been measured. The absorption spectra of the as-grown crystal show a 350 nm weak band with dichroism in polarized light. Subtracting the polarized light absorption spectrum with the electric vector E parallel to the c-axis by that one with E perpendicular to the c-axis, the polarized light difference spectrum is obtained. The polarized light difference spectrum indicates that the 350 nm band has two peaks and can be decomposed into two bands peaking at 330 and 360 nm, respectively. In order to determine the detailed structure of the 350 nm band, annealing experiments in air condition of the as-grown crystal at different temperatures were performed Difference spectra of the annealed crystal have been obtained by subtracting the absorption spectra of the crystal annealed at different annealing temperatures by that one of the as-grown crystal. The absorption spectra features also indicate that the 350 nm absorption band is composed of two bands peaking at 330 and 360 nm, respectively. The annealing properties of the 330 nm and the 360 nm band are obviously different. We come to the conclusion that the 350 nm band is a composed band and can be decomposed into two bands peaking at 360 and 330 nm, respectively. The two bands are suggested to belong to different color centers.

Study of electronic structures and absorption bands of BaMgF4 crystal with F colour centre

The electronic structures of BaMgF4 crystals containing an F colour centre are studied within the framework of the fully relativistic self-consistent Direc—Slater theory, using a numerically discrete variational (DV-Xα) method. It is concluded from the calculated results that the energy levels of the F colour centre are located in the forbidden band. The optical transition energy from the ground state to the excited state for the F colour centre is about 5.12 eV, which corresponds to the 242-nm absorption band. These calculated results can explain the origin of the absorption bands.