Interpretation of the optical and EPR spectra of the Cr<Superscript>3+</Superscript> ion in a lithium niobate crystal

The optical and EPR spectra of the Cr³⁺ γ-center in a lithium niobate crystal are interpreted, and the energy levels of the ground-state spin quadruplet and all the experimentally revealed doublet states are described. The parameters of the Coulomb Hamiltonian for the spin-orbit interaction of electrons and their interaction with an electrostatic field of the crystal are determined. It is found that the crystal field acting on the paramagnetic ion is relatively strong and has trigonal symmetry. The Cr³⁺ ion embedded in the crystal is characterized by considerable changes in the interactions associated with the excited configurations.     

Proton-induced F-centers in LiF and MgF2

The growth of F-centers in LiF irradiated at room temperature with 40- and 85-MeV protons and with ⁹⁰Sr electrons was found to be proportional to the square root of the absorbed energy over the range 0.5 to 2.3 Mrad which corresponds to an F-center density range of 1 × 10¹⁶ to 1.5 × 10¹⁷ per cm³. The production efficiency was 5 × 10³eV per F-center at an absorbed energy of 2.3 Mrad. The density of F-centers produced in MgF₂ by 40- and 85-MeV protons was measured over an absorbed energy range of 0.2 to 29 Mrad which corresponds to a maximum F-center density of 2 × 10¹⁶ per cm³. The production efficiency for MgF₂ was 4 × 10⁵eV per F-center at an absorbed energy of 16 Mrad.     

Z1 and Z2 centers in KCl: Eu

It is shown that the Z₁ and Z₂ centers in Eu-doped KC1 have optical absorption bands at 600 and 678 nm, respectively, at liquid nitrogen temperature. A new EPR spectrum due to the Z₂ center was found and explained with the Ohkura's model that the Z₂ center is an F'-center associated with Eu²⁺ at a nearest neighbor site.     

Characterization of the E1′ center in quartz: Role of aluminum hole centers and oxygen vacancies

The growth of E₁′ centers in a variety of natural and synthetic quartz crystals has been investigated by employing the electron spin resonance technique. It has been reported that the growth of E₁′ centers, formed by irradiation and subsequent annealing at 300 °C for 15 min, scale with the concentration of the aluminum hole center, [AlO₄]⁰, till a limit. Later, the E₁′ centers show saturation even when the [AlO₄]⁰-center continues to grow. For quartz with low efficiency of the formation of [AlO₄]⁰-center or irradiated with low accumulated doses, the intensity of E₁′ center was small where not all oxygen vacancies are converted to the E₁′ center. Thus, besides the availability of a number of oxygen vacancy sites, the number of holes released from the [AlO₄]⁰-center plays an important role in the formation of E₁′ centers in quartz.     

Electronic structure of F,-center in MgO

The embedded-cluster numerical variational method has been developed to calculate the electronic structure of perfect MgO, F and F⁺-centers in MgO. The energy band, bulk density of states has been calculated by cluster Mg₁₄O₁₃, Mg₁₄O₁₂F⁺ and Mg₁₄O₁₂F. The calculated absorption energy for F⁺ and F centers is in good agreement with experimental data. In our calculated defect energy levels, that the first excited state of F⁺-center is at CB-3.46 eV, indicates the necessity of a large photoelectron yielding energy. We also calculate the radius of color center electron, and plot the map of charge-density distribution of valence electrons in which the structure of the color center is shown directly. Received 22 May 1998     

Paraelastische Defektelektronen-Zentren in Alkalihalogeniden

We have investigated the stress-induced alignment of Cl₂ ^? and Br₂ ^? molecular ions substituting for Cl^? and Br^? in KCl and KBr, respectively (so calledH-centers). By means of the dichroism induced in the optical absorption (H-band) by uniaxial stress we determined the static coupling to the lattice and the kinetics of the reorientation. Contrary to the O₂ ^?-center, which also substitutes for an anion, the reorientation-process of theH-center can be described by an Arrhenius-relation. TheH-centers freeze in between 10 and 13°K. In KBr a new center was found which is closely related to theH-center, but differs in the kinetics of the reorientation. It is presumably anH-center associated with another lattice defect. Further we have studied the paraelastic properties of the defects which in KCl and KCl:NaCl give rise to theV ₁-band. We found that depending upon the doping of the crystal different defects produce theV ₁-band.     

Relativistic corrections and very small g-shifts in solids

The relativistic Breit-Margenau correction to the Zeeman-interaction has been calculated for a group of atoms in the periodic table with Herman Skillman wave functions and potentials. The results are applied to the S-state ions Mn²⁺ and Gd³⁺, to the F-center and V_k-center, and to the shallow donors in Si.     

Electron-beam irradiation effects on luminescence properties in subsurface regions of single-crystalline sapphires treated with and without hydrogen plasma exposures

Electron irradiation effects on various insulating sapphires treated with and without hydrogen plasma have been investigated mainly by means of cathodoluminescence (CL) measurements. The samples examined included Be-diffusion-treated natural sapphire (BNS) and two types of synthetic sapphires grown by Verneuil and Czochralski methods. For all the samples examined, on one hand, their CL intensities of the F⁺-center-related emission peaked at ≈3.8 eV rapidly increased with increasing the fluences of keV electrons, and were represented roughly by exponentially saturating curves. There occurred slight blue-shifts of the F⁺-center luminescence other than the intensity increases for some of the electron-irradiated specimens, suggesting possible presence of two components for the F⁺-center luminescence. On the other hand, a hydrogen plasma exposure to these sapphires resulted in sample-dependent changes in the optical property and in the beam-irradiation effect on the F⁺-center CL emission. Such variations were induced most strongly in the BNS sample, whose color changed from orange to pink due to substantial decreases in the absorbance after the hydrogen plasma treatment. Furthermore, the energy positions of both the Cr³⁺-center luminescence peaked at ≈1.8 eV and its satellite peaks were found to slightly shift for the untreated and H-plasma-treated BNS samples after the electron beam irradiations. Possible origins of these observations are discussed.     

Electronic excitation dynamics and energy transfer in lithium-gadolinium borates doped by rare earths

This paper reports on luminescence studies of lithium borate Li₆Gd(BO₃)₃ doped with Eu³⁺ and Ce³⁺ and Li₆Eu(BO₃)₃ crystals upon selective excitation by synchronous radiation in the pump energy region 3.7–27 eV at temperatures of 10 and 290 K. The effective energy transfer between the rare-earth ions Gd³⁺ → Ce³⁺ and Gd³⁺ → Eu³⁺ is found to operate by the resonant mechanism, as well as through electron-hole recombination. A study is made of the fast decay kinetics of the Ce³⁺-center activator luminescence under intracenter photoexcitation and excitation in the interband transition region. The mechanisms underlying luminescence excitation and radiative relaxation of electron states of rare-earth ions are analyzed and energy transfer processes active in these crystals are discussed.     

Effect of Br− ions on the F-center formation in KCl crystals under u.v. light irradiation

The relative efficiency of the F-center formation, Y_F in KCl crystals has been studied as a function of photon energy of u.v. light between 5.7 and 10.0 eV. Y_F is maximum at the peak of the absorption band due to the localized excitons at Br^? ions, and increases with Br^? concentration. Results suggest an important role of the localized excitons in the F-center formation.     

Effect of spin of F3-center on changes in its charge under the action of radiation

Methods of pulse spectroscopy with time resolution were used to investigate creation and annihilation processes of F₂ ⁺-, F₂-, F ₂ ^– -centers, which are noninertial with respect to the duration of an electron pulse of nanosecond duration acting on the crystal. It is concluded on the basis of these studies that the effect of radiation on an F₂-center is a change in its charge and spin states. It is established that the direction of change of an F₂ -center charged state is determined by the value of its spin: localization occurs of a band hole on an F₂-center in the singlet ground state, and of an electron in the on an F₂-center in the triplet state. An F₂-center triplet state is formed during the electron localization process on an F₂ ⁺-center or subsequent localization of band holes and an electron on an F₂-center as a result of a thermally activated intercombination transition from a higher state than the radiating singlet.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 109–120, October, 1991.In conclusion the author is grateful to V. A. Kravchenko for participation in the experiments, and to V. M. Lisitsyn and V. Yu. Yakovlev for interesting discussions.     

Zur Bildungskinetik der F-Folgezentren in KCl

Following the same procedure as in part I the formation of Z₁-centers under Flight irradiation was investigated in additively colored KCl crystals doped with Ca⁺⁺ and Sr⁺⁺ ions. The measurements show that again F′-centers and empty anion vacancies are the first reaction product and determine the rate of Z₁-center formation. The temperature dependence and rate of the Z₁-center formation is in agreement with the corresponding results on F _A -center formation, demonstrating that Z₁-centers are formed as the consequence of an ionic process and not a mere electronic transfer. It is concluded that again empty anion vacancies or F′ centers diffuse randomly in the lattice (activation energy 0.6 eV) until they are captured at an impurity site. Arguments are presented that these impurities are divalent cation/vacancy complexes, so that the model of the Z₁-center should be a combination of the F-center with this complex. The independence of the Z₁-band of the nature of the divalent cation indicates the assoziation of the F-center to the vacancy-side of the complex. A characteristic delay period in the Z₁-center formation can be accounted for by a non-statistical distribution of F-centers and divalent cations to each other. This is a necessary consequence of the high temperature dynamical equilibrium between these imperfections which is frozen in by the quenching of additively colored crystals. A direct proof for this interpretation is the absence of the delay period in crystals where the F-centers are formed by x-irradiation at room temperature.     

Picosecond configuration relaxation in the1 T 1u -state of Ag−-centers in KI

The relaxation of the optically excited Ag^–-center in KI is investigated by measuring the risetime of the spontaneous emission from the relaxed excited state. The energy and number of phonons created in the relaxation process are determined.     

Dislocation motion in photomechanical effects and the lifetime of photochemical products of the F-center in alkali halides

A simple theory describing photomechanical effects in colored alkali halides was proposed based on statistical dislocation dynamics. From the present theory together with empirical data on photomechanical measurements, the lifetime of the F^--center in KCl is found to be about 2 s at around RT and the relevant activation energy is 0.29 eV.     

H-center interactions during thermal annealing in KBr

Formation of the V₄-center and the H_A-center during thermal annealing in pure and Na⁺- and Li⁺-doped KBr, X-irradiated at low temperatures, is studied. It is found that these centers are produced accompanied with the annealing of the H-center at the second stage, of which the reaction order is two. The dependence of the concentration of Na⁺ impurity on the formation yield of the V₄-center by thermal annealing is studied and it is shown that the yield of the V₄-center formation is proportional to the square of the concentration of the H-center annealed at the second stage. This result confirms the former di-H-center model of the V₄-center.     

Farbzentren in Ammoniumhalogeniden

Color centers produced by X-rays in ammonium halides at various temperatures between 20°K and room temperature have been investigated by means of paramagnetic resonance and by optical methods. Two kinds of paramagnetic defects were found to be predominant, the self-trapped hole (V _K-center) and another electron deficiency center involving a NH₃ ⁺-radical. The electronic structure of theV _K-center is the same as in the alkali halides, except that the orientation of the molecular axis is along [100] instead of [110]. The kinetics of the thermally activated motion of theV _K-centers and of their recombination with electrons has been studied. The electronic structure of the second center was derived from the hyperfine spectrum of the paramagnetic resonance. The rotation of the NH₃ ⁺ ion and its connection with the order-disorder transition in NH₄Cl has been studied.     

Energy spectrum of the A + + e complex in a quantum dot in the adiabatic approximation

The energy states of an A ⁺ + e complex in a quantum dot described within the hard-wall potential model are considered using the zero-range potential method in the adiabatic approximation. These complexes can be formed under nonequilibrium conditions (for example, under photoexcitation). A relationship describing the dependence of the binding energy of a hole located at a neutral acceptor on the parameters of the zero-range potential of the system and the quantum state of the electron is obtained analytically. It is demonstrated that, in quantum dots of small radius, the binding energy of a hole in the A ⁺ + e complex can be considerably higher than the ground-state energy of the A ⁺ stationary center.     

The Hd-center interaction with a Rb+ ion during irradiation and thermal annealing in kBr

The role of Rb⁺ ions on defect formation in KBr has been studied. The impurity suppresses colorability due to X-raying at 6 K, but does not result in the formation of any centers characteristic to Rb⁺ ions at this temperature. A new optical absorption band peaked at 3.19 eV is produced only by thermal annealing of irradiated KBr:Rb. This band is annealed in parallel to the annealing of the F band in a stage at 55 K, obeying second-order rate equation with an activation energy of 0.102 eV. This band is ascribed to the H_A(Rb⁺)-center. Calculation is made on the elastic interaction energy between the H-center and a Rb⁺ ion, to show that the interaction along 〈110〉 is repulsive, whereas that along 〈001〉 is attractive. Based on this result of calculation, the structure and the formation mechanism of the H_A(Rb⁺) center, and origin of suppression of colorability at 6 K are discussed. The difference in the interaction of the interstitial atom with Rb⁺ during its dynamical motion and thermal motion is emphasized.     

Zur Bildungskinetik der F-Folgezentren in KCl

The process of F-center aggregation under light irradiation, which involves ionic movement at low temperatures (observable down to — 60°C), is not at all understood in its mechanism. It is the aim of this work to evaluate quantitatively the kinetics of this process for different F-aggregate centers. In part I the assoziation of F-centers in KCl crystals with isolated Na⁺ or Li⁺ ions was thoroughly investigated as the clearest model case of F-center-aggregation. The reaction product in these crystals after light irradiation, an F-center associated to a Na⁺ or Li⁺ ion as nearest (100) neighbor (F _A -center), is well established in its model and can be detected by its double peak absorption structure. By optical measurements of the rate of F _A -center formation in dependence on light-intensity, time, Na⁺ or Li⁺-concentration, F→F′ conversion rate and temperature, the kinetics of this reaction could be evaluated in a simple equation of bimolecular type. The analysis leads to the conclusion, that either the anion vacancy or the F′-center must be regarded as a unit of high thermal mobility (activation energy 0·6±0·05 eV, jump frequency about 10² sec^?1 at room temperature) which diffuses randomly in the lattice and can be captured by a Na⁺ or Li⁺ ion.     

Bipolar Alq3-based complexes: Effect of hole-transporting substituent on the properties of Alq3-center

Two bipolar Alq₃-based complexes, tris{5-[(carbazole-9'-yl)methyl]-8-hydroxyquinoline} aluminum (Al(CzHQ)₃) and tris{5-[(phenothiazine-9'-yl)methyl]-8-hydroxyquinoline} aluminum (Al(PHQ)₃), involving an Alq₃-center and three hole-transporting substituents (carbazole or phenothiazine), were prepared and characterized. Effects of hole-transporting substituent on the properties of Alq₃-center were investigated in detail. It is found that the two complexes have improved hole-transporting performance and appropriate thermal stability (the 5%-weight-loss temperatures T_5%>260 ^oC). Photoluminescence (PL) spectra indicate that both energy transfer and electron transfer can take place simultaneously in the PL process of these complexes. Both thermodynamics and dynamics of the electron transfer were studied and corresponding parameters were calculated. Energy transfer is favorable for the PL of Alq₃-center, while electron transfer is unfavorable for the PL of Alq₃-center. These results will be useful to explore novel OLEDs material with increased efficiency.