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.     

Luminescence and electronic excitation energy migration in stilbene crystals

Absorption and luminescence spectra are reported for various conditions. A new band occurs at the short-wave edge of the luminescence spectrum above 90 ° K, which probably represents the short-wave component of the exciton radiation. Reabsorption has a marked effect on the energy distribution in the luminescence spectrum.     

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.     

The diffusion and non-radiative transfer of excitation energy in molecular crystals containing foreign molecules

A study is made of the non-radiative transfer and diffusion of excitation energy in molecular crystals containing foreign molecules and its influence on the luminescent properties of these crystals.

---

,

, .

     

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.     

Wave propagation in a one-dimensional randomly perturbed periodic medium

We study the variance of the solution of a periodic randomly perturbed one-dimensional Schrödinger operator after propagation through N periods. It is shown that if the frequency of propagation lies inside the band, then the total variance is proportional to Nσ², where σ is the intensity of the white noise. However, if the wave frequency is close to the band edge (where the transfer matrix has a Jordan block structure), the resulting variance is proportional to Nσ^2/3. Thus, propagation becomes highly sensitive to random perturbations.

Numerical simulations reveal that even low noise in a periodic potential can suppress transmission near the band edges and make it strongly irregular inside the band. Further increase of the noise amplitude leads to intermittent behaviour of the transmission coefficient, and makes transmission possible only for a few random frequencies in the band.     

Excitons perturbed by self-trapped excitons in alkali iodine crystals

A strong transient optical absorption band on the exciton shoulder has been observed in pure alkali iodine crystals following pulsed electron irradiation. The ultraviolet transition is ascribed to creation of a second exciton in the vicinity of a self-trapped exciton. The perturbation caused by the proximity of the self-trapped exciton lowers the energy needed to form the second exciton by 0.14 eV for KI and 0.12 eV for RbI.     

Curvature-dependent excitation propagation in cultured cardiac tissue

The geometry of excitation wave front may play an important role on the propagation block and spiral wave formation. The wave front which is bent over the critical value due to interaction with the obstacles may partially cease to propagate and appearing wave breaks evolve into rotating waves or reentry. This scenario may explain how reentry spontaneously originates in a heart. We studied highly curved excitation wave fronts in the cardiac tissue culture and found that in the conditions of normal, non-inhibited excitability the curvature effects do not play essential role in the propagation. Neither narrow isthmuses nor sharp corners of the obstacles, being classical objects for production of extremely curved wave front, affect non-inhibited wave propagation. The curvature-related phenomena of the propagation block and wave detachment from the obstacle boundary were observed only after partial suppression of the sodium channels with Lidocaine. Computer simulations confirmed the experimental observations. The explanation of the observed phenomena refers to the fact that the heart tissue is made of finite size cells so that curvature radii smaller than the cardiomyocyte size loses sense, and in non-inhibited tissue the single cell is capable to transmit excitation to its neighbors.     

Ultraviolet excitation of rare-earth-doped YSZ crystals

Abstract

The optical properties of nominally pure and Er³⁺- or Pr³⁺ -doped yttria-stabilized zirconia single crystals were investigated under UV light excitation. In the excitation spectra of both types of doped crystals, a broad UV band is observed. Under excitation with light of different wavelengths inside this band, the luminescence features of the doped crystals are different. YSZ: Pr³⁺ samples exhibit the characteristic 4f → 4f emission of the Pr³⁺ ions. In YSZ: Er³⁺ crystals, both the Er³⁺ ion and the intrinsic luminescence are observed. Host to Er³⁺ ion radiative energy-transfer is also demonstrated. No dependence of the transfer process with the excitation wavelength was found. These results suggest that the UV band in Er³⁺ -doped crystals is associated with the lattice-dopant ion interaction rather than with the 4f5d interconfigurational band of the Er^3? ions.     

Resonant excitation of intense acoustic waves in crystals

The resonant excitation of an intense elastic wave through nonspecular reflection of a special pump wave in a crystal is described. The choice of the plane and angle of incidence is dictated by the requirement that the excited reflected wave be close to the bulk eigenmode with its energy flow along a free boundary. The resonance parameters have been found for a medium with an arbitrary anisotropy. General relations are concretized for monoclinic, rhombic, and hexagonal systems. A criterion is formulated for an optimal selection of crystals in which the resonant reflection is close to the conversion one, when almost all of the energy from the incident beam of the pump wave falls into the near-surface narrow high-intensity reflected beam. Estimates and illustrations are given for such crystals as an example. The intensity of the reflected beam increases with its narrowing, but its diffraction divergence also increases with this narrowing. Nevertheless, the intensity of the beam can be increased by a factor of 5–10 at sufficiently high frequencies while keeping its divergence at an acceptable level. Amplification by two orders of magnitude can be achieved by compressing the beam in two dimensions through its double reflection.     

The influence of propagation on off-resonant excitation of a model molecule

We show that the proposal of Makarov, Cantrell, and Louisell regarding stimulated inelastic resonance fluorescence (SIRF) does not accurately represent the effects of propagation on off-resonant excitation of two-level model molecular absorbers.     

掺质KCl晶体F_A色心电子能级

在计算KCl晶体中的F_A心电子能级时,本文认为是基质和掺质对色心电子云束缚的松紧程度之差确定了电子云分布,据此对三种典型的掺质:Li:KCl:F_A(II);Cu:KCl:F_A(III);Ti:KCl:F_A(III),采用不同的出发点进行计算,结果和实验值比较,误差在5%以内.     

Nerve propagation and wall in liquid crystals

The relevant equations for describing the Brochard-Leger wall in liquid crystals are found and analyzed. The analogy between the motion of the wall in liquid crystals and nerve propagation is traced. The theoretical results in this work are in excellent agreement with experiments. The basic consideration for the wall in this letter applies to describing other walls in liquid crystals.     

Mechanism of self-trapping of electron excitation in alkali-halide crystals

A dynamic method is used to calculate the equilibrium positions of ions near heavy and light homologous impurities for KCl:J and KJ:Cl crystals. The most probable sites of self-traps are shown.Lattice ralaxation near an excited anion is modeled. The localization of electron excitations (EE) in alkali-halide crystals is examined from the viewpointof the formation of a covalent bond between anions as a relust of their approach toward each other. The EE localization time is determined by the lifetime of local vibrations in the crystal near the excited anions (5-10^–13–10^–11 sec).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 35–38, January, 1986.     

Microscopic theory of sound propagation in dielectric crystals

The time evolution of the atomic displacement field in a dielectric crystal subjected to an external force is studied in the domain of linear response by means of imaginary time Green's functions. For slowly varying disturbances two coupled equations have to be solved: a differential equation for the amplitude of an acoustic wave and a linearized Boltzmann equation. The latter results from the integral equation for the vertex part and includes an additional integral operator. The collision equation is solved for different relative magnitudes of the sound frequency and the frequencies for normal and Umklapp processes using the method developed by Weiss. Some of the expressions showing up in the velocity and damping of the sound wave are estimated numerically for rare gases with two-body forces in the form of the Morse potential.     

Absorption and propagation of light in quaterthiophene crystals

We examine absorption of light in quaterthiophene crystals for different directions of the ray and wave-normal vector after refraction on the vacuum/crystal interface. Experimental non-normal incidence spectra are reported where different absorption bands show different behaviour as a function of the angle of incidence. This allows to identify those originating from the b_u Frenkel exciton state.