E. Vasil’chenko

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).

Thermo- and Photostimulated Luminescence in LiF : Mg,Ti Single Crystals Irradiated by Ions and VUV Light

A coordinated study of the relaxation of optical absorption induced by vacuum ultraviolet radiation, x-rays, and α-particles, as well as of photo- and thermostimulated luminescence (TSL) of LiF : Mg, Ti crystals (TLD-100) in the 295–750-K interval, has revealed that TSL regions characterized by activation energies Ea = 2.2–2.4 eV and anomalously high frequency factors p0 = 1021–1022 s−1 alternate with regions where Ea = 1.5 eV and p0 = 1012–1014 s−1. The relative intensities of the TSL peaks produced by UV illumination (10–17 eV) differ strongly under the conditions of selective photon-induced generation of anion excitons, free electrons and holes, or near-impurity electronic excitations. The latter are responsible for the high efficiency of tunneling radiative (involving titanium centers) or nonradiative (involving hydroxyl ions) recombination. The analysis of TSL peaks of LiF: Mg, Ti and LiF took into account two-step processes, namely, thermal dissociation of three-fluorine F3− molecules and recombination of the products of their decay (VK and VF centers, H interstices).

Formation and stabilization of F centers following direct generation of self-trapped excitons in KCl crystals

The spectrum of luminescent F centers generated in high-purity KCl crystals by 7–10.2-eV photons has been measured at 230 K. The pulsed annealing of these centers (250–550 K), as well as the dependence of the efficiency of stable F-center generation on irradiation temperature (80–500 K) has been studied. The efficiencies of F− and Cl3−-center generation are maximum under direct optical creation of self-trapped excitons in the region of the Urbach intrinsicabsorption tail. Besides the exciton decay with formation of F centers and mobile H centers, a high-temperature exciton decay channel which involves creation of cation defects stabilizing the H centers has been revealed.

Low-temperature investigation of electronic excitations in wide-gap materials doped with RE3+ or Cr3+ ions

The processes of photon multiplication in insulators have been considered. The luminescence of Tb3+ ions (5D3 → 7FJ, 5D4 → 7FJ transitions) upon intracenter excitation, the optical excitation of oxyanions, or the formation of separated electrons and holes has been studied for CaSO4 doped with Tb3+ and Na+ ions at 6–9 K. An increase in Tb3+ concentration from 0.2 to 4 at % and transition from single Tb3+-Na+ states to centers that contain two or three terbium ions leads to the redistribution of the luminescence intensities in favor of the 5D4 → 7FJ transitions and increase in their efficiency due to the possibility of the cooperative 5D3 → 5D4 and 7F6 → 7FJ transitions and the 4f75d1 → 5D3 and 7F6 → 5D4 transitions in the two- and three-terbium centers. Based on the example of MgO single crystals with highly mobile excitons, holes, and electrons, the migration of free excitons and holes toward Cr3+ ions in the crystal bulk and their exit from the bulk to the surface have been revealed at 9 K. Surface losses limit the luminescence quantum yield of MgO:Cr3+, CaSO4:Tb3+, and many other materials.

Color center formation by synchrotron radiation in the Na6Al6Si6O24(Nal)1.6 optical ceramic

The spectrum of F-center excitation by 5-to 27-eV photons in the Na6Al6Si6O24(NaI)2x sodalite optical ceramic (x=0.8) was measured at 80 K by high-sensitivity photoexcited luminescence techniques. The F-centers are created by photons with an energy of 5.6-to 8.5 eV through the excitation and ionization of iodine centers of two types; in the 8.2-to 27-eV region, through the generation of electronic excitations in the aluminosilicate framework of alternating Al3+ and Si4+ ions, each coordinated tetrahedrally by oxygen ions. At the low irradiation doses used, the F centers are created primarily through photoelectron capture by the iodine vacancies which exist before irradiation. In the 23-to 25-eV region, the efficiency of F-center formation doubles as a result of the multiplication of electron-hole pairs.

Influence of uniaxial compression at 80 K on the formation of radiation defects in KCl, KBr, and KI crystals

The influence of uniaxial compression at 80 K on the efficiency of formation of stable radiation defects in KCl, KBr and KI crystals is investigated by absorption spectroscopy. It is found that compression along the 〈100〉 directions does not alter the efficiency of radiation defect formation in KCl and KBr crystals, but in KI the efficiency drops by more than an order of magnitude. It is concluded from a semiquantitative analysis that the observed difference is attributable to the impossibility of H centers fitting into the compression-reduced interstitial voids in KI, whereas several multiples of ten-percent compression is required to produce the analogous effect in KCl and KBr.

Hole-electron mechanism of F-H pair generation in RbCl crystals with impurity electron traps

Production of F, Cl3−, Ag0, and Tl0 centers in RbCl:Ag and RbCl:Tl crystals by photons having energies ranging from 5 to 10 eV has been studied at 295 and 180 K. It is shown that creation of near-impurity excitations is accompanied by formation of F centers localized in the vicinity of Ag+ and Tl+ ions. F centers are produced in direct optical generation of self-trapped excitons. In addition to the well-known mechanism of F-H pair production in nonradiative recombination of electrons with self-trapped holes, a hole-electron process has been revealed for the first time to operate in RbCl:Ag having deep electron traps. By this mechanism, F-H pairs appear in the following sequence of stages: thermally stimulated unfreezing of hopping diffusion of self-trapped holes (VK centers), tunneling electron transfer from Ag0 to the approaching VK centers, and subsequent nonradiative decay of triplet self-trapped excitons near Ag+ ions.

Complex terbium luminescence centers in spectral transformers based on CaSO4

The blue and green terbium luminescence excitation spectra (5–15 eV) at a temperature of 9 or 300 K have been measured for a set of CaSO4 : Tb3+ phosphors with calcium and terbium ions characterized by fundamental features, which were synthesized by the solid-state reaction method with variations in the composition of the raw material, the type of terbium compounds, and the concentration of introduced terbium. A comprehensive analysis of these data, emission spectra, and decay kinetics (with the rising stage) of the terbium luminescence has revealed that the main luminescence centers in an efficient phosphor with 4 at % Tb3+ each contain four closely spaced Tb3+ ions in calcium positions, as well as Na+ ions and hydrogen-containing radicals. In such complex luminescence centers, cooperative nonradiative resonant transitions in neighboring terbium ions leads to a nearly complete concentration of the terbium luminescence in the green spectral region (5D4 → 7FJ). The quantum yield of the green luminescence in CaSO4 : Tb3+ (4 at %) exceeds unity, QY > 1, in the region of direct excitation of Tb3+ into the 5d state (~5.9 eV) or excitation of oxyanions near the terbium ions (~8.1 eV).