Multiplication of anion and cation electronic excitations in luminescent wide-gap ionic crystals

The spectra of intrinsic luminescence excitation by synchrotron radiation (6–32 eV) at 8 K have been analyzed for NaCl, KCl, RbCl, KBr, RbBr, CsBr, MgO, CaO and YalO₃ crystals. In all crystalsv (except MgO and CaO) the process of multiplication of electronic excitations (MEE) causes a sharp increase of the intensity of self-trapped exciton emission, but leads (in KBr and NaCl) to the decrease of intra-band luminescence efficiency. The analysis of the intensity ratio spectra for two components of exciton emission allows us to separate the process of secondary exciton creation by hot photolectrons (NaCl, KBr, YAlO₃). The threshold energies of excitonic and electron-hole mechanisms of MEE are compared for a number of alkali halides.     

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)     

Multiplication of electron excitations and the creation of stable radiation-induced defects in ionic crystals

The paper discusses the phenomenon of electron-excitation multiplication that arises when alkali halide crystals are exposed to 12–32 eV photons, with absorption of one photon followed at 8 K by the appearance of two or three electron excitations. Measurement of the spectrum of F-center generation by 6.5–32 eV photons showed that at 200–300 K, when vacancies and interstitials are mobile, the formation of stable defects is especially efficient when radiation creates a mixture of electron-hole pairs and excitons. Certain mechanisms of creation and stabilization of defects at various stages in electron-excitation multiplication are discussed. Institute of Physics of Estonia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 30–44, November, 1996.     

Luminescence decay in AgCl crystals

The kinetics of luminescence decay of single crystal plates of AgCl was measured at the temperature of liquid nitrogen. Luminescence decay first takes place (fort≦2·5× ×10^?3 sec) according to a hyperbole and then according to an exponential. The constantsa anda of the hyperbolic andt of the exponential dependence were measured for different intensity of the exciting radiation in normal and deformed samples and in samples irradiated withb-particles during measurement.     

Etch pits in AgCl crystals

Single crystal sheets have been prepared by the capillarity method. The dislocations can be revealed with a sodium thiosulphate solution at surface orientations from two regions in the basic triangle. These are: region A around 001 pole up to ≈25°, and region B around 111 pole up to ≈pa 10°. In the remaining region C of the basic triangle the dislocations do not etch. In region A the etch pits have the shape of pyramids. In region B they are triangular pyramidal in shape. Evidence is given that the edge and screw components of annealed and fresh dislocations are revealed by etching. At surface orientations near either the {00l} or {111} planes the slip can be activated in such zones 〈110〉 that are nearly parallel to the surface of the sheets. The slip bands are then straight and predominantly of the edge type.     

Nonradiative heterofission of guest molecule electronic excitations in doped organic molecular crystals

Formulae for the rate constants of the nonradiative heterofission of a guest molecule electronic excitation into one host molecule electronic excitation and one guest molecule electronic excitation in the doped organic molecular crystals are derived. The coherent and incoherent host exciton heterofission channels are considered. Formulae derived for the singlet host exciton heterofission channels are related to the measurable optical properties of the host and guest molecule of the crystal. The effects of the intramolecular vibration are discussed.     

Luminescence quantum yield and multiplication of electronic excitations in the corundum crystals

The excitation spectra of luminescence in irradiated and nonirradiated corundum crystals are investigated by means of highly polarised synchrotron radiation in 5 to 30 eV region. In the fundamental absorption region the double-exciton peaks are observed in the region 8.5-9.2 eV for irradiated (especially by neutrons) crystals at temperature 90 K. At 9.5 eV sharp drops appeared, in the luminescence spectrum space which were interpreted as nonirradiative near-surface recombination, the probability of which sharply increased at high absorption coefficients. The enhancement of the luminescence efficiency in the high-energy region was connected with the decay effect of electron excitations as well as with the influence of volumetric excitations leading to the enhancement of recombination glow. Received 16 October 1998 and Received in final form 20 January 1999     

Relaxation of electronic excitations in wide-gap crystals studied by femtosecond interferometry technique

Time-resolved interferometry with a 100-fs temporal resolution was applied for the first time to studying the relaxation of electronic excitations in complex oxides, namely, tungstates CDWO₄ with a crystal lattice of the wolframite-type and CaWO₄ with a scheelite-type lattice. Two stages of charge carrier relaxation, namely, very fast carrier trapping in 200 fs resulting in self-trapped exciton formation and a relatively slow picosecond relaxation process probably due to configurational relaxation within the oxyanion molecule and modification of the surrounding lattice, are revealed in tungstate crystals. Corresponding models of self-trapped exciton creation in tungstate crystals are discussed. The text was submitted by the authors in English.     

Luminescence centres in pure AgCl crystals

The temperature dependence and the influence of pressure on band width of blue-green luminescence were measured in pure AgCl crystals. The comparison of results obtained with other published data support the existence of two luminescence centres (in the temperature ranges between 130–160°K and between 20–30°K).     

Indirect optical cooperative excitation of two-particle electronic excitations in doped organic molecular crystals

A theory on the indirect optical cooperative excitation of a two-particle electronic excitation composed of one host exciton and one guest-molecule one-particle electronic excitation through one electric dipole-moment active singlet guest molecule one-particle electronic excitation is developed by considering the nonradiative coupling bstween one- and two-particle electronic excitations.The complex dielectric tensor and the corresponding absorption coefficient are found out, which are expressed in terms of the Fourier-transform of the retarded Green function: The derived formulas are discussed in an explicit form for the one-dimensional model of the doped organic molecular crystal. Within the framework of ths one-dimensional model also the conditions for the existence of the host exciton-guest molecule one-particle electronic excitation bound states are established and discussed.     

Pulse measurements of thermostimulated currents in AgCl crystals

The course of thermostimulated currents in AgCl crystals was measured in a temperature region between 200 and 300K. Maxima at 240, 275 and 290K were observed on the curves obtained.     

Interaction of long-wavelength excitations in crystals

The conditions of resonance interaction of phonons for a scalar model of a crystal in the region of the initial linear section of the dispersion curve of acoustic phonons are investigated using two-particle Green's functions. Fiz. Tverd. Tela (St. Petersburg) 41, 130–133 (January 1999)     

Luminescence of AgCl/Mn++ crystals

The temperature dependence of the luminescence spectra and kinetics, the excitation spectra, luminescence polarization, quantum yield of the integral luminescence and paramagnetic resonance of AgCl crystals doped with three concentrations of manganese ions at 77°K were measured and the results are discussed.The author expresses his gratitude to associate professor K. Vacek, Ph.D. for suggesting the work on the above problem and for his help. Thanks are also due to Dr. Krutina and Dr. Sroubek, of the Research Institute of Radio Engineering of the Czechoslovak Academy of Sciences, for enabling the measurement of the spectra of paramagnetic resonance to be carried out in their laboratory.     

On slip in AgCl crystals at room temperature

At moderately wavy and branched slip bands on the surfaces of sheets of AgCl crystals the distribution of slip lines has been observed by means of the electron microscope. From the results it can be deduced that the cross slip of screw dislocations takes place. The divergence of the ends of slip bands has also been observed, which can be as well explained by the cross slip. From the active slip direction and the directions of straight slip lines in the cross-slip regions the microscopic slip planes have been determined. They lie in the region roughly limited by the {113} and {331} planes and the problem is discussed whether these planes are low-index crystallographic planes.     

Electromodulation spectroscopy of localized excitations in crystals

Electromodulation spectroscopy has become a powerful tool for the study of electronic transitions of point defects with strong electron-phonon coupling, i.e., broad absorption and emission bands. The potential and physical significance of the method will be illustrated using a few selected examples of well characterized defect structures in ionic crystals. Starting with a simple symmetric F-center it is shown that electromodulation in absorption and emission allows the investigation of excited states with parity-forbidden optical transitions, and is important for the study of the electron-phonon interaction. For more complex defects with lower symmetry, more detailed information (like the presence and magnitude of internal electric fields and relative displacements of the wave-functions of different electronic states) can be obtained using both first and second harmonic modulation techniques. The potential of field-modulation spectroscopy in the study of point-defects with a central instability (“off-center defects”) and of tunneling molecular defects in crystals will be briefly discussed.