Luminescence and the migration of the energy of electron excitation in stilbene crystals

The dependence of the quantum luminescence yields on the concentration of impurities in crystals of stilbene with admixtures of anthracene is investigated. On the basis of the experimental results a diffusion length is calculated for the excitons as well as the probability of their capture by impurity molecules.     

Luminescence and migration of electron-excitation energy in diphenyl crystals

The absorption spectra and luminescence of solutions and crystals of diphenyl are studied. According to the dependence of quantum yields of fluorescence on impurity concentration, the probabilities of capture of exitons by molecules of different impurities as well as the diffusion-displacement length of singlet exitons in diphenyl crystals are determined. It is established that the probabilities of capture of exitons by the impurity depend on the overlap integral of the impurity's absorption spectra with the fluorescence spectrum of the crystalline matrix, as well as on the dimensions and configuration of the impurity molecules.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 15, No. 6, pp. 69–73, June, 1972.     

On the mechanism of electronic excitation energy migration in organic liquids

The rate constant for electronic excitation energy transfer from toluene toa fluorescent indole measured as a function of temperature indicates unequivocally that the excitation energy migration is entirely due to resonance interaction and not due to excimer formation-dissociation.     

Luminescence and energy transfer mechanisms in CaWO4 single crystals

The processes of the excitation energy transfer to the emission centers have been investigated for calcium tungstate crystals taking into account features of the electronic structure of valence band and conduction band. The calculations of the electronic structure of host lattice CaWO₄ were performed in the framework of density functional theory. The underestimation of the bandgap value in the calculations has been corrected according to the experimental data. Luminescence of two samples grown using Czochralski (cz) and hydrothermal (ht) techniques were studied. Intrinsic emission band related to excitons, self-trapped on WO₄ complexes has been observed for the both samples while the additional low-energy emission band related to the defects of crystal structure has been observed only for (ht) sample indicating the enhanced concentration of the defects in the sample. It was shown that the features of the conduction band electronic structure are reproduced in the excitation spectrum of intrinsic luminescence only for the (ht) sample while for (cz) sample the correlation is absent. The enhanced role of the competitive channels in the process of excitation energy transfer to intrinsic emission centers in (ht) sample is responsible for the observed difference.     

The propagation of excitation energy in perturbed crystals

The process of the propagation of excitation energy in perturbed dielectric crystals with a weak bond between the atoms having zero permanent dipole moment is studied. It is shown that on certain assumptions this process of propagation can be regarded as the motion of a Frenkel exciton in the electrostatic field of the defect. Frenkel's exciton can be characterized in this case as a neutral polarizable particle having induced dipole moment equal to the change in the induced dipole moment of the crystal during the excitation of one of its atoms.

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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     

Luminescence of AgCl crystals at room temperature after ruby laser excitation

Emission spectrum of pure AgCl crystals at room temperature after ruby laser excitation was measured. The emission consists of a continuous fluorescence band with the maximum at about 460 nm which is at sufficiently high excitation intensities covered with characteristic line structure emission. The appearance of continous emission at this temperature was in accordance with experimental results explained by the increase of the radiative transition probability in AgCl crystals.     

Luminescence of mixed crystals of silver halides at high excitation densities

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 55, No. 5, pp. 826–831, November, 1991.     

The diffusion of the excitation energy in molecular crystals

A theory of the coefficient of diffusion of an exciton in molecular crystals is elaborated, based on the conception of the exciton as the excited state of the molecule of a crystal propagating through the crystal by chance transition from one molecule to another. The theoretical results are compared with the experimental data for an anthracene crystal.     

Migration transfer of electronic excitation energy in activated solids

An analytic expression for the kinetics of quenching of donor luminescence in the presence of acceptors with allowance for migration of energy over the donor subsystem is derived. An extended criterion for the applicability of the hopping migration model is obtained.     

Luminescence, intrinsic photoeffect, and color-center conversion in anisotropic crystals under femtosecond laser excitation

The spatial-temporal dynamics of the photochemical conversion of color centers with varying multipolarity and orientation of quantum transitions in irradiated anisotropic crystals exposed to pulsed polarized femtosecond optical radiation and the resulting spatial distribution of the residual and newly-formed color centers are studied. The properties of the concurrent luminscence and photoconductive processes closely related to the photochemical transformation of light-absorbing quantum systems are also examined. A femtosecond laser technique is proposed, which allows extended spatial-periodic structures or narrow profiles of color-center concentrations, including single thin layers or dips, to be formed at a specified depth from the crystal surface through photochemical transformations. Institute of Applied Physics at the Irkutsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 31–42, March, 2000.     

Mechanisms of femtosecond energy transfer upon strong excitation in crystals

It is shown that the fast temperature-independent shift of the long-wavelength edge of fundamental absorption observed upon electron bombardment of alkali halide, alkaline-earth, and oxygen-containing crystals is caused by the impact mechanism. In the course of the impact shift of lattice ions, short-lived perturbation (mainly, of their valence shells) occurs. The perturbation fields in the vicinity of impact-shifted anions act for no more than 100 fs. As a result, the distribution of the density of states in the valence band of a crystal is distorted, and the long-wavelength shift of the fundamental-absorption edge may increase for less than 100 fs.     

Photoluminescence quantum yield and excitation energy migration in low viscosity solutions

An expression for the relative quantum yield is derived for the general case of energy transfer accompanied by diffusion. The migration of excitation energy among donors has been taken into account. Detailed discussion of the case of moderate diffusion is given.