I. Kudryavtseva

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

Creation of F centres and multiplication of electronic excitations in Na6Al6Si6O24(NaBr)2x optical ceramics under VUV irradiation

The reflection spectra, the excitation spectra of various emissions (2.4–4.5 eV) and the creation spectra of F centres have been measured in Na6Al6Si6O24(NaBr)2x optical sodalite ceramics (x = 0.94 and x = 0.81) using VUV radiation of 6–35 eV. An analysis of the spectra allowed us to separate several groups of electronic excitations: the photons of 6.7–8.3 eV excite or ionize Br− centres in the β-cages, 8.5–32 eV photons generate electronic excitations of the aluminosilicate carcass, while 33 eV photons excite Na+ ions up to the 2p53s state. The creation mechanism of F centres, connected with the trapping of conduction electrons by pre-irradiation bromine vacancies and localization of holes at Br− centres in β-cages, has been revealed in sodalites at 80 K. The efficiency of F centre creation triples in the multiplication region of electronic excitations of the aluminosilicate carcass (20–30 eV) in a sample with x = 0.94. Using methods of thermoactivation spectroscopy, the creation of thermally stable F centres (up to 480 K) has been detected in sodalites irradiated by x-rays or 27 eV photons. However, such F centres cannot be created by 7.6–11 eV photons, i.e. when there is no multiplication of excitations and an exciting photon forms only one electronic excitation.

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).

Photon multiplication in wide-gap BAM and SAM aluminates

Processes of various intrinsic and impurity luminescence excitation by 4-32 eV photons or 18 and 300 keV electrons have been studied in pure and doped BaMgAl10O17 (BAM) and SrMgAl10O17 (SAM) phosphors at 6-300 K. In BAM:Eu (l0%), the quantum yield of Eu2+ center emission is QY = 1 in the region of exciting photon energies of hνec = 7-12 eV, the value of QY reaches 2 at 14-21 eV and sharply increases at hνec = 22-32 eV, where secondary electron-hole pairs are created by hot conduction electrons. The processes connected with the rise of QY for various types of emission in the region of 14-21 eV have been thoroughly studied for BAM and SAM phosphors. It has been suggested that such exciting photons cause the ionization of oxygen ions and form hot valence holes, the energy of which is partially used for the excitation of Eu2+ ions ->(4f7->4f65d1 transitions) due to nonradiative Auger transitions. The intensity of the Eu2+ emission increases after a single nanosecond electron pulse with a rise time of 50-150 ns. This rise is connected with the energy transfer from spinel blocks to Eu2+ ions located at cation planes of the β-alumina-type materials.

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.

Electronic excitations and self-trapping of electrons and holes in CaSO4

A first-principles study of the electronic properties of a CaSO4 anhydrite structural phase has been performed. A theoretical estimation for the fundamental band gap (p →s transitions) is Eg=9.6 eV and a proper threshold for p →d transitions is Epd=10.8 eV. These values agree with the data obtained for a set of CaSO4 doped with Gd 3+ ,D y 3+ ,T m 3+ and Tb 3+ ions using the methods of low-temperature highly sensitive luminescence and thermoactivation spectroscopy. The results are consistent with theoretical predictions of a possible low-temperature self-trapping of oxygen p-holes. The hopping diffusion of hole polarons starts above ∼40 K and is accompanied by a ∼50–60 K peak of thermally stimulated luminescence of RE 3+ ions caused due to the recombination of hole polarons with the electrons localized at RE 3+ . There is no direct evidence of the self-trapping of heavy d-electrons, however, one can argue that their motion rather differs from that of conduction s-electrons.

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.

Recombination luminescence of CaSO4:Tb3+ and CaSO4:Gd3+phosphors

A comparative study of the excitation of luminescence by VUV radiation as well as of thermally and photostimulated luminescence has been carried out for CaSO4:Tb3+ and CaSO4:Gd3+ phosphors, where Na+ or F− ions are used for charge compensation. The distinction in hole processes for the phosphors with Na+ or F− compensators is determined by the differing thermal stability of the holes localized at/near Tb3+Na+ and Gd3+Na+ (up to 100–160 K) or at/near Tb3+F−VCa and Gd3+F−VCa centers involving also a cation vacancy (up to 400–550 K). Tunnel luminescence in the pairs of localized electrons and holes nearby Tb3+ or Gd3+ has been detected. The mechanisms of electron-hole, hole-electron and tunnel recombination luminescence as well as a subsequent released energy transfer to RE3+ ions are considered.

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.

Multiplication of electronic excitations and formation of radiation defects in alkali halides

The spectrum of F center creation has been measured for KBr:Tl using synchrotron radiation (12–32 eV). The creation efficiency of a stable F center is especially high at 15–17 and 28–30 eV where the absorption of one photon leads to the formation either both of an electron-hole pair and an exciton or two electron-hole pairs and an exciton. In these cases there are favourable conditions for stabilization of mobile radiation defects due to the association of interstitial halogen atoms, holes and cation vacancies.