VK Centres Research Articles

Photo- and thermo-stimulated luminescence of CsI—Tl crystal after UV light irradiation at 80 K

Thermo- and photo-stimulated luminescence are studied for CsI—Tl crystal after the irradiation with the UV light at 80 K. Creation spectrum of the photostimulated luminescence coincides with the D absorption band of Tl+ ions. Nature of the defects created by UV light at low temperatures is discussed basing on the correspondence between the thermostimulated glow curve peaks and thermal evolution of the photostimulation spectra observed after irradiation in the D absorption band. Three bands at 1400, 950 and 580 nm have been observed in the stimulation spectrum at 80 K. The 1400 and 950 nm stimulation bands are presumably explained as the optical transitions in the Tl0 centre forming the spatially correlated defect pair with Vk centre while the 580 nm stimulation band is connected with the unperturbed Tl0 centres. It is concluded that the Tl+ luminescence at low temperature is connected with the electron recombination with the Tl2+ centre.

Investigation of Radiation-Induced Defects in Cs2NaYF6

We present a detailed study of the radiation damage centres produced in Cs2NaYF6 upon X-irradiation at 10 and 300 K. The defects were investigated using magnetooptical and magnetic resonance spectroscopy. As hole trap centres at low temperatures VK centres and HA-type centres were identified by EPR and their optical transitions by optical absorption and the magnetic circular dichroism of the absorption. As electron trap centres F centres were investigated in detail by ENDOR.

Local structure of the trigonal defect center for Cr3+ ions in KMgF3 crystals

The zero-field splitting D, the anisotropic g-factors g ∥, Δg(=g ∥ − g ⊥) and the first excited state splitting Δ(2 E) for the trigonal Cr3+–VK center in KMgF3: Cr3+ crystals have been studied from Macfarlane's high-order perturbation formulas. From the studies, the local structure of the trigonal center is obtained. The local lattice distortions (i.e., the displacement directions of the ions in the center) are consistent with the expectation based on the electrostatic interaction.

Hole-trapping sites and the mechanism of the photostimulated luminescence of the x-ray storage phosphor RbI:Tl+

Radiation damage centers generated by x irradiation at 80 and 300 K in the storage phosphor RbI:Tl+ were investigated with optically detected magnetic resonance and optical measurements. It is shown that the intensity of the photostimulated luminescence depends on the concentration of Tl2+ centers. Bleaching into any of the seven identified Tl2+ absorption bands destroys the Tl2+ centers and leads to a Tl+ emission as well as to VK centers. Proportionally to photodestruction of Tl2+ centers, the photostimulated luminescence is decreased. The traps for information storage in RbI:Tl+ are F centers and Tl2+ centers.

Peculiarities of interstitials in a simple cubic CsCl crystal

Abstract The processes of the thermal annealing of VK centres (160–190 K) and anion interstitials (I and II centres) have been studied in a CsCl crystal by means of luminescent and EPR methods. The thermal annealing of I centres in a CsCl crystal takes place in a narrow region of 16–20 K, in contrast to the multistage annealing of I centres in KCl. The tunnel recharge of a primary F, H pair leads to the formation of an α centre and a localized (not crowdion-type) split I interstitial.

Magneto-optical properties of the UV absorption bands of VK centres in alkali halides

A new UV absorption band below the original UV band has been discovered through the magnetic circular dichroism of the VK centre absorption (MCDA) in seven alkali halide crystals. This new band is attributed to the transition Sigma u to Pi u, which becomes dipole allowed as a result of a loss of inversion symmetry. We attribute this lowered symmetry to the slow axial oscillation along the VK axis which results in a momentarily 'frozen' geometry of the species at the classical turning point. An approximate estimation of the coupling between the even and odd parity molecular orbitals of Sigma and Pi symmetry is made in KBr. From this it appears that the coupling between the even and odd parity states is not negligible. Observed experimental trends in the seven crystals are explained from this model.

Vkcentre of LiH crystal surface

The effects of Vk centres on the binding energy of Li2+ 2H2- 2 in LiH ionic crystals are investigated theoretically. In constructing model potentials, it is shown that the contribution of many body effects is confirmed and that basis sets with diffuse functions are required to treat the anions accurately.

Investigation of thermoexoemission and thermoluminescence of KCl: Ag by the fractional glow technique

The examined KCl:Ag crystal, X-rayed at ≈︁80K, exhibits two distinct well-correlated thermo-stimulated electron emission (TSEE) and luminescence (TSL) peaks at ≈︁210 and ≈︁380 K. induced by Vk centre delocalization and Ag° centre thermoionization, respectively. The revealed close coincidence of the activation energies (≈︁0.7 eV for the 210 K and ≈︁1.15 eV for the 380 K peak) shows that the observed TSEE and TSL are induced by the thermal decay of identical (bulk type) centres, and the TSEE run is not controlled by the electron affinity, but it is either an emission of “hot” electrons accelerated in the X-ray produced electric field in the sample or/and facilitated by the electron escape spots at the surface. The shifts between temperature and TSEE/TSL runs within individual heating oscillations demonstrate that Vk centre hopping diffusion controls the rate of TSEE/TSL acts in the 210 K peak. The results resemble the ones formerly obtained by the same method for LiF and NaF and encourage the using of fractional heating TSEE for the spectroscopy of traps in the surface layers of solids.

Thermoluminescence emission of X-irradiated Eu2+doped KBr single crystals

Abstract In this paper we discuss the results of thermoluminescence (TL) studies carried out on freshly quenched crystals of KBr doped with ∼50ppm of Eu2+ ions which were X-irradiated at room temperature. The TL glow curve of this phosphor material consists of three glow peaks at 355, 376 and 398K whose intensities increased as a function of X-irradiation time. The TL glow peaks were analyzed by the total curve-fitting method in order to obtain the characteristic parameters; activation energy, pre-exponential factor and kinetic order. The spectral character of the emission recorded during thermoluminescence was found to be the same for all glow peaks and consists of a broad band centered at -420 nm. This band is composed of two overlapping bands peaking at 419 and 460 nm. The former band is due to the well known fluorescence of Eu2+ ions corresponding to the transition 4f6 5d (T2g) → 8 S7/2, while the latter may be associated with the recombination of F-centers with the radiation induced interstitial chlorine atoms which are thermally released from traps. It is proposed that the model of the TL process most consistent with our experimental results is one in which the Eu2+ impurity acts as an electron trap during the irradiation process and that the radiation induced center (partner of an F center) and the Vk center (partner of an Eu+ ion) are trapped together in the form of a Cl− 3 molecular ion at the Eu2+ -cation vacancy dipoles and their smaller aggregate complexes such as dimers, trimers, etc. During the TL run, the H and Vk centers escape from the trap and recombine with the complementary center giving rise to the observed TL emission spectra. The dosimetric characteristics of the KBr:Eu2+ phosphor material are also reported.

Effects of X-ray irradiation on BaFBr single crystals at LHeT

Color center formation and luminescence excitation in BaFBr single crystals by X-ray irradiation was studied at low temperatures. F(Br−) centers formed by X-irradiation at LHeT showed an absorption band at 2.09 eV at LHeT. This peak energy is different from those of isolated F(Br−) centers, suggesting formation of F centers perturbed by nearby impurities or defects. A dichroic absorption band due to self-trapped holes, VK(Br2−) centers, was observed at 3.35 eV. The dichroism of the VK(Br2−) band diminished above 110 K. Three luminescence bands were observed at 5.2, 3.9, and 2.5 eV when BaFBr single crystals were X-irradiated at LHeT. Among these luminescence bands, the 5.2 eV band can be attributed to the recombination of VK centers with electrons.

Excitonic mechanism of athermal halogen atom desorption from NaBr, KBr and RbBr

Energetic halogen atom desorption from the (100) surface of NaBr, KBr and RbBr crystals under electronic excitation is studied from the point of view of excitonic instability. It is shown that the triplet self-trapped exciton (STE) state undergoes an instability on and near the surface (the first and second layer below the surface) similar to that in the bulk and results in the expulsion of neutral Br atom in all three bromides studied. The relaxation energy which could be converted to kinetic energy of the ejecta is estimated to be about 0.5 eV. From the third layer, only in KBr and RbBr such ejection seems possible. The trajectory of the ejected Br atom which is originally along the halogen row in the 〈110〉 axis direction undergoes a deflection resulting in a trajectory at the end which is found to deviate from the normal by only about ≅ 20° at a distance of twice the lattice parameter. It is also observed that a localized hole on the surface and below is efficiently converted into a Vk center without leading to a halogen atom desorption. Excited alkali atom desorption preceded by that of a halogen atom is also briefly studied, with the indication that it is energetically possible if the F centre is in an excited state.

Theoretical simulations of the radiation-induced defect processes in insulating materials

The results of two basic kinds of computer simulations of radiation-induced processes in insulating materials, one based on quantum-mechanical and pair-potential (atomistic) approaches, and the other a phenomenological theory of diffusion-controlled reactions, are presented. It is shown that, by combining different techniques (atom-atom potentials and semi-empirical quantum chemical methods) the optimized geometry and the electronic structure of a family of hole centres in crystalline corundum (α-Al2O3) could be found. Their energetics are analyzed; V2−, V−V−Mg hole centres all have a common basic element, namely the diatomic molecule O23−, which is responsible for their similar absorption energies. Our calculations provide evidence for the existence of a small-radius two-centre polaron (self-trapped hole, or STH) with an optical absorption energy of around 2.9 eV. The strong covalent bonding of the two O atoms sharing a hole makes this centre analogous to the VK centre in alkali halides. The calculated activation energy for the STH hops (≈ 0.9 eV) between different O triangles is close to the experimental.

The nature of the X-ray-induced luminescence and the hole centres in CsI:Tl studied by optically detected electron paramagnetic resonance

In the X-ray-induced luminescence band peaking at 560 nm we have detected the electron paramagnetic resonance (EPR) spectrum using optical detection (ODEPR) Of VK centres, which have their molecular axes perpendicular to the magnetic field, an EPR was only detected as a microwave-induced change in the magnetic circular polarization of the emission, and not as an emission intensity change. No parallel centres were seen, nor any trapped electron centre. The observations are explained as a recombination luminescence of a kind of Tl-bound exciton. Furthermore, the ODEPR via the magnetic circular dichroism of the absorption of VKA centres generated by X-irradiation at 4.2 K and trapped near Tl+ and near Na+ were measured. These centres are room-temperature stable for a few minutes and responsible for afterglow effects when using CsI:Tl, Na as X-ray-scintillator crystals.

Femtosecond spectroscopy of self-trapping processes of holes and excitons in alkali halides

By pump and probe experiments we investigated self-trapping processes of holes and excitons in KI and RbI crystals. It is found that a free hole is relaxed initially into a one-center self-trapped hole (small polaron) and subsequently a two-center self-trapped hole (Vk center) is formed. The self-trapping of electron-hole pairs involves a one-center self-trapped hole correlated with an electron as a precursor state for a self-trapped exciton.

Excitonic instability and athermal halogen atom desorption from NaBr, KBr and RbBr

Abstract Energetic halogen atom desorption observed from certain alkali halide crystals under electron or photon stimulation is being investigated. The relaxed structure of the localized one-center (Frenkel-type free exciton) and two-center (self-trapped exciton: STE) electronic excitations are studied. It is shown that the triplet STE state undergoes an instability on and near the surface (the first and second layer below the surface) similar to that in the bulk and results in the expulsion of neutral Br atom in all three bromides studied. The relaxation energy which could be converted to kinetic energy of the ejecta is estimated to be about 0.5–1.0 eV. From the third layer, only in KBr and RbBr such ejection seems possible. The trajectory of the ejected Br atom is originally along the halogen row in <110> axes. However, as the ejecta approaches the (100) surface, it undergoes a deflection resulting in a trajectory outside the crystal which is found to deviate from the vertical by only about ≌20° at a distance of twice the lattice parameter. It is also observed that a localized hole on the surface and below is efficiently converted into a Vk center without leading to a halogen atom desorption. These results are presented within a broader context of the STE relaxation in the alkali halide crystals.

Magnetic circular dichroism and optically detected electron paramagnetic resonance of trapped hole centres in KBr and KI

In KBr and KI the magnetic circular dichroism of the optical absorption (MCDA) of VK and H centres created by X-irradiation at 4.2 K in situ was investigated as well as the MCDA-detected electron paramagnetic resonance (EPR) spectra. MCDA extrema were found at the spectral positions of the known absorption band peaks for VK centres including a second band in the ultraviolet (UV), which had been reported previously for KI but not for KBr. Only one MCDA band was found for H centres, corresponding to the infrared (IR) transition. No MCDA-detected EPR spectra were observed for VK and H centres with their molecular axes parallel to the magnetic field. Using perturbation theory to describe the effect of spin-orbit coupling and magnetic field mixing of the X2- molecular orbitals used to describe VK and H centres previously, the experimental results could be explained except for the new UV MCDA band, which cannot be explained assuming inversion symmetry of the VK centres. It is suggested that this band arises because of the low-frequency (1012 s-1) axial displacement mode of VK centres, breaking the inversion symmetry, which is a quasi-static distortion for the optical transition, but fast with respect to EPR, and therefore not seen there.

Dissociation of Electronic Excitations Localized near Vacancy Defects in KI Crystal

AbstractOn the example of a KI crystal it is shown that, together with the radiative decay of electronic excitaions localized near anion vacancies (cα) and vacancy quartets (cq), their nonradiative decay also occurs at 4.2 K. VK and F centers are created at eα decay and VK centers and electron vacancy complexes are created at cq decay. These defects arise as a result of eα and eq dissociation that occurs in the electric field of an anion vacancy or of a vacancy quartet. Some details of such a process are discussed.

Quantum chemical simulations of hole self-trapping in corundum

Microscopic quantum chemical calculations and simulations based on atom-atom potentials have been undertaken for hole self-trapping in pure corundum ( alpha -Al2O3) crystals. A comparison of different modes of ionic relaxation during hole trapping has shown that the inward Jahn-Teller 40% displacement of two O ions accompanied by the 20% outward displacement of the two nearest Al ions is energetically the most favourable. Eighty per cent of the hole density is concentrated on these two O ions, thus confirming that a small-radius two-site polaron model similar to that for alkali halides (the VK centre), is applicable here. The calculated absorption energy of the STH (2.9 eV) is close to that observed experimentally.

Theoretical simulation of VK-centre migration in KCl. I. A quantum-chemical study

Using embedded quantum cluster models, the authors have studied the mechanism of VK-centre 60 degrees reorientation in KCl and LiCl, and have considered the possibility that both one-centre 'dielectric' polarons and 'molecular' polarons (VK centres) coexist in KCl. These studies have shown that lattice polarization lowers the activation energy for VK reorientation since the VK centre strongly polarizes near the barrier point. The calculated adiabatic barrier for VK reorientation is 0.63 eV, in reasonable agreement with the available experimental data. From the self-trapping energy calculations and the analysis of the structure and mechanism of the diffusion of the one-centre 'dielectric' polaron, it is apparent that it transforms adiabatically into a VK centre without an energy barrier.

Theoretical simulation of VK-centre migration in KCl. II. Phenomenological theory

For pt.I, see ibid, vol.4, p.7417 (1992). An adequate theoretical basis is developed and presented for a nonstationary technique for the experimental detection of reactions in insulating crystals controlled by tunnelling recombination of mobile defects (hole centres) with electron centres. The effect of finite hop lengths of point defects in a crystalline lattice is carefully analysed. This method is based on measuring the inertial increase or decrease in the recombination luminescence intensity after a stepwise change of temperature (defect mobility). Theoretical analysis of the transient process observed experimentally for VK centres in KCl:Tl crystals argues for their motion through 60 degrees rotations with the frequency factor St=4*1013 s-1, very close to that obtained earlier in the optical dichroism studies.