A. Lushchik

Excitonic and recombination processes in CaWO4 and CdWO4 scintillators under synchrotron irradiation

Excitation spectra of intrinsic (IL) and extrinsic luminescence (EL) from CaWO 4 and CdWO 4 crystals, measured using synchrotron radiation (4-30 eV) at 8 K, have been analysed. In CaWO 4 the IL (2.9 eV) is efficiently excited by photons of 5.5-7.0 eV at the creation of molecular excitons. The EL is efficiently excited in interband transitions (hv>6.8 eV). The energy threshold for the multiplication of electronic excitations is E t 15 eV in CaWO 4 and E t 10 eV CdWO 4 . In CdWO 4 , a fraction of the molecular excitons undergoes autoionization due to an overlap with the continuum of the interband transitions. Fast emission (<2 ns) is found at 80 K under the pulse excitation by electrons with an energy of 300 keV. The continuous temperature-independent emission of CaWO 4 crystals in the region of 2-5 eV is interpreted as an intraband luminescence. The emission at 5.5 eV is ascribed to a metastable molecular excitons.

Energy transfer in ZnWO4 and CdWO4 scintillators

A systematic spectroscopic study of oriented single ZnWO4 crystals was performed using UV and synchrotron excitation. Emission and phosphorescence spectra in the energy region 1-4 eV as well as reflection spectra and excitation spectra of various emissions and phosphorescence in the energy region 3.5-32eV were studied at 4.2-300K using synchrotron radiation. The results are compared to those available for CdWO4. The peculiarities of the band structure and those of excitonic and electron-hole processes in the systems investigated are discussed.

Separation of excitonic and electron-hole processes in metal tungstates

Absorption, reflection and emission polarization spectra of CdWO4 crystals have been studied in the region 3.5-30 eV in order to distinguish the excitonic and electron-hole processes in the vicinity of the band gap. The following parameters have been defined from the Urbach tail study at 6-300 K: E-0 = 5 eV, sigma(0) = 0.31, homega = 70 meV (565 cm(-1)). Excitonic processes have been shown to dominate at excitation of tungstate crystals in the lower part of the conduction band. Excitons are formed due to the transitions into the tungstate W5d states hybridized with O6p and possess a very strong tendency for self-trapping. Free electrons and holes can be created at the energies 1-2 eV (depending on the crystal) higher than the bottom of the conduction band due to the transitions into cationic states

Multiplication of electronic excitations in CaO and YAlO3 crystals with free and self-trapped excitons

Using synchrotron radiation of 6-32 eV the reflection spectrum and excitation spectra for 5.7, 4.6 and 2.8 eV emissions have been measured for a freshly cleaved CaO crystal at LHeT. In CaO, indirect band-to-band transitions take place in the region of hv>or=6.4 eV. The excitation spectra for intrinsic emissions of 5.9 and 4.2 eV were measured in a YAlO3 crystal at LHeT as well. As in many other dielectrics the electron-hole mechanism of multiplication of electronic excitations connected with the creation of secondary electron-hole pairs by hot photoelectrons also occurs in CaO and YAlO3 crystals. The creation of secondary excitons by hot photoelectrons has been detected in YAlO3 for the first time. The excitonic mechanism of multiplication of electronic excitations has a high efficiency in crystals with self-trapping excitons of small radii.

Dependence of the efficiency of various emissions on excitation density in BaF2 crystals

Abstract The dependence of the intensity of the emission of self-trapped excitons, crossluminescence (CL) and intraband luminescence on the excitation density in BaF 2 by electron pulses (300 keV , 3 ns ) has been studied. The energy input above 2×10 19 eV cm −3 causes quenching of 5.5 eV CL due to the recombination of conduction electrons with the outermost-core holes resulting in the formation of cation excitons and the subsequent energy transfer from a cation exciton to a valence electron of a neighbouring anion. The excitation spectra for various fast (τ ns ) and inertial emissions have been measured using synchrotron radiation at 9 K .

Low-temperature excitonic, electron–hole and interstitial-vacancy processes in LiF single crystals

The emission spectra and the excitation spectra of various emissions have been measured in LiF crystals at 9 K using VUV radiation of 10–33 eV. In contrast to the luminescence of self-trapped excitons (3.4 eV), the efficiency of several extrinsic emissions (4.2, 4.6 and 5.8 eV) is very low in the region of an exciton absorption (12.4–14.2 eV). A single exciting photon of 28–33 eV is able to create a primary electron–hole (e–h) pair and a secondary exciton. The tunnel phosphorescence has been detected after the irradiation of LiF by an electron beam or x-rays at 6 K, and several peaks of thermally stimulated luminescence (TSL) at 12–170 K appeared at the heating of the sample. It was confirmed that the TSL at 130–150 K is related to the diffusion of self-trapped holes (VK centres). The TSL peak at ∼160 K is ascribed to the thermal ionization

Excitonic and electron-hole mechanisms of the creation of Frenkel defect in alkali halides

Abstract Excitonic and electron–hole (e–h) mechanisms of stable F centre creation by VUV radiation in alkali halide crystals are discussed. In KCl at 4.2 K, the efficiency of stable F–H pair creation is especially high at the direct optical formation of triplet excitons with n =1. At 200–400 K, the creation processes of stable F centres in KCl are especially efficient at the formation of one-halide exciton in the Urbach tail of an exciton absorption. In KCl and KBr, the decay of a cation exciton (∼20 eV) causes the formation of two e–h pairs, while in NaCl a cation exciton (33.5 eV) decays into two e–h and an anion exciton. An elastic uniaxial stress of a crystal excited by VUV radiation decreases the mean free path of excitons before their self-trapping (KI) and increases the mean free path of hot holes before self-trapping (NaCl).

Creation of groups of spatially correlated defects in a KBr crystal at 8 K

The creation spectrum of triplets of spatially correlated defects (F centre + self-trapped hole, centre + interstitial halogen ion, I centre) was measured for the first time in a KBr crystal at 8 K, using synchrotron radiation of 6-25 eV and highly sensitive luminescent methods. The spectrum of spatially correlated F and H centre creation by synchrotron radiation at 8 K was measured as well. The efficiency of triplet creation is especially high at crystal irradiation by photons under the conditions of multiplication of electronic excitations when the absorption of one photon leads, respectively, to the formation of an electron-hole (e-h) pair and a secondary exciton (15.0-16.5 eV), or two e-h pairs (16.7-19.0 eV), or to the formation of a cation exciton (19.5-20.5 eV). It is shown that the formation of an triplet is caused by the recharging of a primary F-H pair by an e-h pair or by a free exciton.

Excitonic and electron-hole processes in NaCl and NaCl:Ag crystals under conditions of multiplication of electronic excitations

The excitation spectra of and emissions of self-trapped excitons (3.4 eV and 5.4 eV, respectively) as well as the excitation spectra of 5.17 eV luminescence of Ag+ impurity centres were measured in NaCl and NaCl:Ag crystals using synchrotron radiation of 5 - 38 eV. Fast and slow components of these emissions were detected. An analysis of the differences in the excitation spectra measured at 8 and 295 K allowed us to separate the excitonic and electron-hole (e-h) mechanisms of the multiplication of electronic excitations. A photon of 17 - 19 eV forms an e-h pair and a secondary exciton, while the absorption of a 21 - 27 eV photon causes the creation of two e-h pairs. Using luminescent and photoelectric methods, it was shown that a 2p3s Na+ cation exciton, formed at the absorption of a 33.4 eV photon, decays with the creation of an anion exciton with a 3p hole component and two e-h pairs. Three e-h pairs are formed after the absorption of a 31 eV photon by a chlorine ion.

Multiplication of anion and cation electronic excitations in wide-gap KCl and CsCl crystals

Abstract Luminescence of self-trapped excitons, extrinsic recombinational luminescence and crossluminescence on the excitation by synchrotron radiation (7.7–30eV) have been investigated for KCl and CsCl crystals at 6–80 K. The following mechanisms of multiplication of electronic excitation are discussed: (i) decay of cation excitons with the formation of a double amount of anion excitons (KCl); (ii) photoionization of anions and cations with the subsequent formation of secondary excitons and electron-hole pairs by fast electrons.