Ultra‐Violet Excited Paramagnetic Centres in Calcium Tungstate

Abstract

Two sets of paramagnetic species having a spectroscopic splitting factor slightly higher than the free electron value are produced in CaWO, doped with copper after irradiating the crystal with ultra-violet light. One set of lines is identified as being due to a hole trapped a t a single tungsten site and is found to bleach out on warming the crystal through the thermoluminescence glow peak a t 160 OK. The other set of lines increases in intensity first with the emission of light in the 160 "K peak and then again with the 260 O K peak and bleaches out on heating the crystal through the thermoluminescence glow peak a t 400 OK. The copper does not take part in the excitation process directly. It is suggested that the low temperature paramagnetic species arises from the loss of an electron from the tungstate complex.Zwei Gruppen paramagnetischer Zentren mit einem spektroskopischen Aufspaltungsfaktor, der etwas grol3er ist als der Wert fur ein freies Elektron, werden in kupferdotierteni CaWO, nach Bestrahlung der Kristalle mit ultraviolettem Licht erzeugt. Die eine Gruppe von Linien wird einem Loch zugeschrieben, das an ein einzelnes Wolframatom gebunden ist. Diese Gruppe verschwindet beim Erwarmen des Kristalls iiber das Thermolumineszenz-Emissionsmaximum bei 160 "K. Die Intensitat der anderen Gruppe steigt mit der Lichtemission des 160 "K-Maximums und dann nochmals mit der des 260 OK-Maximums an. Die Linienintensitat verschwindet bei weiterem Erwarmen des Kristalls iiber das Thermolumineszenz-Emissionsmaximum bei 400 O K . Die Kupferatome nehmen nicht direkt am AnregungsprozeLi teil. E s wird vermutet, dal3 das paramagnetische Zentrum bei tiefer Temperatur durch Verlust eines Elektrons des Wolframatkomplexes entsteht.

Luminescence decay and delayed spectra of CaWO4 single crystals

“…Deviations a t the beginning of the decay were either attributed to high excitation densities [4] or, to the influence of trapping levels [ 5 ] . On the other hand the spectral composition of the thermoluminescence (TL) emission was shown to be different from the spontaneous luminescence [2, 6,7]. It was found [S] that the glow spectra change during heating, so that the maximum of the emission band shifts towards longer wavelengths below T = 265 O K with a backshift for higher temperatures.…”

An EPR study of an S = 3/2 defect in CaWO4

“…Zeldes and Livingston [3] first reported a spectrum which they implied was associated with a hole localized between two oxygens on neighbouring (W0J2tetrahedra. This centre was later referred t o as the Vk-centre by Born et al [4, 51 and Sayer and Lynch [6] in analogy with the Vk-centre in alkali halides [7]. The centre was created by 1) Now at: Physics Department, Liverpool University.…”