Observation of polarized luminescence from Jahn-Teller split states of self-trapped excitons in PbWO₄ by time-resolved spectroscopy

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Time-resolved luminescence spectra of oriented PbWO₄ crystals have been measured for polarizations with the electric vector parallel to the crystallographic c axis ( E ∥ c) and perpendicular to it (E ⊥ c). All the three emission bands (I, II and III) due to the Jahn-Teller split states of a self-trapped exciton (STE) located on the (WO₄)²⁻ complex anion are found to exhibit significant polarizations in the ac plane. The main band II peaking at 425 nm is polarized with E ∥ c at low temperatures (T < 100 K), but changes its polarization from E ∥ c to E ⊥ c at T > 150 K. Band III at 450 nm is polarized with E ⊥ c at T = 230 and 300 K. The degree of polarization of these two bands does not depend on the polarization of excitation light. On the other hand, band I at 407 nm is polarized with E ⊥ c ( E ∥ c) under the photoexcitation polarized parallel (perpendicular) to the c axis, in the temperature range of T = 150-300 K. The observed polarization characteristics of three emission bands invoke further lowering of symmetry of tetrahedral (WO₄)²⁻ ions due to the uniaxially compressive crystal field, in addition to the Jahn-Teller distortion. The sublevels of the STEs responsible for the PbWO₄ luminescence are assigned on the basis of a group-theoretical consideration.
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