Observation of polarized luminescence from Jahn–Teller split states of self-trapped excitons inPbWO4 by time-resolved spectroscopy

Abstract

Time-resolved luminescence spectra of orientedPbWO4 crystals have been measured for polarizations with the electric vector parallel to the crystallographicc axis and perpendicular to it . All the three emission bands (I, II and III) due to the Jahn–Tellersplit 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 mainband II peaking at 425 nm is polarized with at low temperatures (T<100 K), but changes its polarization from to at T>150 K. Band III at 450 nm is polarized with at T = 230 and 300 K. The degree of polarization of these two bands does not depend on thepolarization of excitation light. On the other hand, band I at 407 nm is polarized with () under the photoexcitation polarized parallel (perpendicular) to thec axis, in thetemperature range of T = 150–300 K. The observed polarization characteristics of three emissionbands invoke further lowering of symmetry of tetrahedral(WO4)2− ions due to the uniaxially compressive crystal field, in addition to theJahn–Teller distortion. The sublevels of the STEs responsible for thePbWO4 luminescence are assigned on the basis of a group-theoretical consideration.