Metastable optical absorption of relaxed electronic excitations in BeO-Zn crystals

Spectra of metastable optical absorption and its relaxation kinetics have been studied in zinc-doped BeO crystals by time-resolved pulsed absorption spectroscopy. A comparison of the observed induced optical absorption of self-trapped excitons and small-radius excitons bound to the zinc impurity suggests that their hole components have similar structures and reveals distinctive features of “forbidden” optical transitions in the electronic components. Metastable optical absorption in Zn⁺ centers has been discovered. It is shown that the small-radius excitons bound to the zinc impurity form in the hole stages of thermally stimulated tunneling recombination processes involving Zn⁺ electronic centers. It has been found that the high recombination probability of the electronic and hole centers created in BeO-Zn crystals by an electron beam may be due to the high degree of their spatial correlation. Fiz. Tverd. Tela (St. Petersburg) 41, 601–605 (April 1999)     

Transient optical absorption of excited F -centres in BeO

Time-resolved spectra, decay kinetics and polarization of the transient optical absorption induced by irradiation of additively colored BeO crystals with electron pulses have been studied. It has been established that the two bands at 3.8 and 4.3 v eV of the transient optical absorption are due to the transitions between triplet and singlet excited states of F -centres in BeO. The polarization of excited F -centres absorption is discussed on the basis of analysis of the splitting of singlet and triplet states in crystalline field of the C 3v symmetry.     

Effect of hydroxyl and oxygen ions on optical stability of F-centers in KBr crystals

The effect of the anionic impurities OH^–, O ₂ ^§- , and O^2– on the optical stability of F-centers in KBr crystals was investigated. It is shown that the stability of F-centers is independent of the presence of hydroxyl ions (OH^–) and molecular oxygen ions (O ₂ ^– ) in the crystal lattice. It is suggested that the low stability of F-centers in KBr-KOH crystals is due to doubly charged oxygen ions.Translated from Izvestiya Vysshikh Uchebnykh Zavendenii, Fizika, No. 10, pp. 107–112, October, 1972.     

Cathodoluminescence in ion-implanted BeO crystals

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 51, No. 2, pp. 218–224, August, 1989.     

Origin of F-centers in X-ray irradiated NaCl crystals

Etching of NaCl crystals X-ray irradiated at room temperature does not reveal a displacement of dislocation lines such as might be expected according to various mechanisms suggested in the literature for the generation of F-centers at dislocations.     

Point-ion-lattice calculations on F-centers in alkali halide crystals

It is pointed out that the wave functions for the point-ion model of an F-center in an ionic crystal can be obtained by numerical solution of the Schrödinger equation for the approximation in which the solutions are constrained to pure s and pure p-functions. Results are given for the F-center optical transition energy in the following crystals: LiF, NaF, LiCl, NaCl, KCl, and RbBr.     

Scintillation properties of BeO single crystals

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 49, No. 2, pp. 286–291, August, 1988.     

Exciton mechanism of energy transfer to F-centers in dosimetric corundum crystals

The optical absorption (OA) and thermoluminescence of oriented -Al₂O₃ crystals colored in aluminum vapor have been studied. Optical absorption measurements of an X-irradiated sample at the orientation EC₃ during isochronal annealing revealed mutual transformations of F- and F⁺-centers within the dosimetric peak from 380 to 500 K. The transformations occurred simultaneously with emptying of the electron trap whose OA is at 2.8 eV. The predominant F→F⁺ conversion, which could be explained by delocalization of holes, was observed in the low-temperature part of the peak near 380–440 K. From the available experimental data it was inferred that the simultaneous release of electrons and holes in the low-temperature part of the peak facilitated formation of excitons bound on F-type centers. The excitons transferred part of the energy stored in the irradiated crystal to F⁺- and F⁻-centers during thermal stimulation, leading to luminescence of these centers.     

Formation of F-centers in KCL crystals grown with vibration of the melt

Russian Physics Journal -     

Simulation of nonlinear optical absorption in ZnSe:Co crystals

The paper presents theoretical approach to simulation of nonlinear optical absorption in zinc selenide crystals doped with cobalt (II) ions (ZnSe:Co²⁺), which was reported by us earlier (Opt. Laser Technology, V. 35, (2003), 169). We used ZnSe:Co²⁺ crystals as saturable absorbers for generation of giant-pulse eye-safe laser radiation. It was found that minimal optical losses (maximal final transmission) occurred for ZnSe samples containing 1.6×10¹⁹ cm⁻³ of Co²⁺ ions. Band structure and photoinduced molecular dynamics simulations were performed to explain the parabolic dependence of optical losses versus Co²⁺ concentration. The minimum was shown to be the result of photoinduced anharmonic electron–phonon interaction.     

Time-resolved spectroscopy of natural and synthetic BeO crystals

The results of comparative luminescence investigation of natural and synthetic BeO crystals are presented. Time-resolved luminescence (2.5–8 eV) and luminescence excitation spectra, and the kinetics of glow decay were measured using ultraviolet-vacuum-ultraviolet (VUV) synchrotron radiation (5–22 eV) or x-radiation (50–620 eV or 3–62 keV) ranges. X-ray and thermostimulated luminescence of natural BeO crystals were compared to the glow of additively colored synthetic crystals. The characteristic luminescence of F and F ⁺ centers was found in natural crystals. In synthetic crystals similar luminescence is observed only after additive or radiation coloration by virtue of the creation of F ⁻ and F ⁺ centers on anion vacancies. The defects found in the crystal lattice of a natural BeO crystal testify to the degree of mineral metamictization of the given deposit.     

Time-resolved optical absorption in YAlO3 crystals

The present work is devoted to the investigation of transient absorption (TA) induced by a pulsed electron beam (E=250 keV) in pure and doped YAlO₃ (YAP) single crystals. The nature of centers responsible for TA is discussed.     

Short-wavelenght luminescence and thermostimulated processes in single crystals of BeO

The results of a study of thermostimulated luminescence and recombination processes in pure and impure single crystals of BeO are reported. The origin of the trapping and recombination centers, their parameters (activation energies, frequency factors), the recombination schemes and the TL spectra were determined. It was shown that, in the BeO single crystals during electron and hole recombinations, the excitation of the intrinsic luminescence band at 4.9 eV proceeds and its characteristics inherit no properties from the recombination centers.     

Multiphoton absorption controlled by the resonance optical stark effect in crystals

Probabilities of n-photon transitions between upper valence and lower conduction bands under conditions in which the frequency of light is in resonance with the frequency of adjacent transition between two conduction bands are calculated for arbitrary integer n. The case of n = 3 is investigated in detail. Effects caused by transformation of the electronic band spectrum due to the resonance optical Stark effect are analyzed. It is shown that the rate of photogeneration of electron-hole pairs as a function of radiation intensity j is nonmonotonic and includes areas of extremely fast rise and/or decrease due to the appearance of new van Hove singularities in the modified band spectrum.     

Shift of the optical absorption edge in C60 clathrate single crystals

We measured the transmission between 1.6–2.2 eV of pure C₆₀ and three clathrate single crystals containing different aliphatic guest molecules. The absorption edge in the clathrates shows a blue shift of 0.1 eV irrespective of the guest molecule, indicating that C₆₀ is electronically isolated in these compounds.     

Metastable F2 center states in LiF crystals

Using time-resolved pulsed absorption and luminescence spectroscopy we have studied the creation ofF ₂ centers during electron pulses of nanosecond duration in the temperature range 80 to 300 K. We show that the formation ofF ₂ centers in long-lived metastable states is thermally activated and competes withF ₂ center relaxation to the singlet state. It is shown that the S-T crossing in the set of states which describe theFC₂ center lies above the¹ _u level. We propose that when the crystal is excited with the lowest energy quanta which produce a photocurrent, the formation of tripletF ₂ centers takes place by localization of a conduction electron on anF ₂ ⁺ center.State Academy of Architecture and Construction, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 17–22, November, 1994.     

Transfer matrix theory for the optical absorption spectrum of mixed crystals

We study the density of states and optical absorption spectrum of linear mixed crystals in the one- dimensional alloy transfer matrix approximation. Local modes, which are present in the single impurity limit, develop into collective modes by merging into a band as the impurity concentration increases. The composition dependence of local and collective like optical absorption peaks is discussed, and we show that qualitatively different behaviours are obtained for single band and split bands type density of states.