Showing papers by "Bulgarian Academy of Sciences published in 1982"

On the relation between nucleation work, nucleus size, and nucleation rate

Abstract: A general thermodynamic relation is proved to exist between the nucleation work and the nucleus size regardless of the model for the excess free energy of the nucleus. When this energy is supersaturation independent, the relation reads as follows: the number of atoms (or molecules) in the nucleus equals the derivative with minus sign of the nucleation work with respect to the supersaturation Δμ. It is shown that, experimentally, a reliable determination of the nucleus size is possible when data for the stationary nucleation rate J are plotted in coordinates kT ln J vs. Δμ (k is the Boltzmann constant and T is the absolute temperature). The slope of such an experimental curve gives information about the nucleus size independently of the kind of nucleation: classical or atomistic, homogeneous or heterogeneous, three dimensional or two dimensional, etc.

Localized Excitons in CdS1−xSex Solid Solutions

Abstract: The emission spectra of CdS1–xSex solid solutions (x ≦ 0.3) at helium temperatures are studied under monochromatic excitation in different regions of the absorption spectrum. In all samples very intense luminescence due to excitons localized by composition fluctuations is observed. High intensity of luminescence and complete absence of free exciton emission indicate a high probability of exciton localization. A model for exciton localization is proposed. It is assumed that only the hole component of the exciton is localized by the potential fluctuations. The electron is bound to this hole by Coulomb attraction. The energy migration between the localized states is discussed. Three different regions can be distinguished in the excitation spectrum of localized excitons. The first region corresponds to the excitation of free excitons which are trapped to localization sites in the process of energy migration. There is no energy migration in the third spectral region and all the emission is caused by directly photo-excited states. In the intermediate region II a limited energy transfer between the localized states takes place via phonon assisted tunneling. Die Emissionsspektren von CdS1–xSex-Mischkristallen (x ≦ 0,3) werden bei Heliumtemperaturen unter monochromatischer Anregung in verschiedenen Bereichen des Absorptionsspektrums unter- sucht. In allen Proben wird sehr intensive Lumineszenz von Exzitonen beobachtet, die an Zusam- mensetzungsfluktuationen lokalisiert sind. Die hohe Intensitat der Lumineszenz und das vollstandige Fehlen der Emission freier Exzitonen weist auf eine hohe Wahrscheinlichkeit von Exzitonlokalisierung hin. Ein Modell fur die Exzitonlokalisierung wird vorgeschlagen. Es wird angenommen, ds nur der Lochanteil des Exzitons durch die Potentialfluktuationen lokalisiert ist. Das Elektron ist an diesem Loch durch Coulombanziehung gebunden. Die Energieubertragung zwischen den lokalisierten Zustanden wird diskutiert. Drei unterschiedliche Bereiche lassen sich im Anregungsspektrum der lokalisierten Exzitonen unterscheiden. Der erste Bereich entspricht der Anregung freier Exzitonen, die an Lokalisierungsplatzen wahrend der Energieubertragung getrapt werden. Im dritten Spektralbereich existiert keine Energieubertragung und die gesamte Emission wird durch direkt photo-angeregte Zustande verursacht. Im Zwischenbereich II findet ein begrenzter Energieubergang zwischen den lokalisierten Zustanden uber phononassistierte Tunnelung statt.

Parallel arrays of microtubles formed in electric and magnetic fields

Abstract: The influence of electric and magnetic fields on microtubule assembly in vitro was studied. Both types of field caused alignment of microtubules in parallel arrays, as demonstrated by electron micrographs. These Iindings suggest a possible role of microtubules in the biological effects of exogenous as well as endogenous

THE RECENT (MIOCENE‐QUATERNARY) REGMATIC SYSTEM OF THE WESTERN MEDITERRANEAN REGION: A New Model of Ensialic Geodynamic Evolution, in a Context of Plastic/Rigid Deformation

Abstract: By comparing the features observed on Landsat images with the known geological data, a regmatic system that controlled the entire Neogene to Recent structural evolution of the Western Mediterranean area has been recognized.

Determination of parameters of excited states of DNA and RNA bases by laser UV photolysis

Abstract: — The mechanism of photodecomposition of nucleic acid bases in a neutral aqueous solution upon two-step excitation of high-lying electronic states by a powerful laser UV radiation is discussed. Experimental dependences of photodecomposition efficiency versus UV radiation intensity are measured both under picosecond and nanosecond laser UV irradiations. By comparison of experimental dependences with a theoretical model, we obtain some characteristics of excited states, such as lifetime t1 of the first electronic excited state S1 intersystem crossing yield φ, photosensitivity from an intermediate excited state and others for all five nucleic acid bases.

Spectral functions and hole nuclear states

Abstract: An explicit expression for the spectral function of hole nuclear states is given in the framework of the coherent fluctuation model. It is shown that the spectral function and single-particle width, centroid energy, quasiparticle effective mass, etc., are functionals of the ground-state nucleon density distribution. Calculation of the spectral functions for58Ni,40Ca,28Si is carried out and the results are compared with the experimental data.

Dynamic Analysis of Multirigid-Body System Based on the Gauss Principle

Abstract: This paper on the basis of the Gauss principle an efficient computer oriented algorithm for solving the equations of motion of multibody mechanical system. No constraints on the type of the kinematic chain of the system are imposed, i.e. the system may contain an arbitrary number of “closed loops”. In the context of the investigation of the dynamics of the industrial manipulators, robots and similar mechanisms, the mechanical systems considered are linked rigid bodies with joints permitting relative rotation or translation. The successive computation is due to a suitable expression of the Gauss “leas constraint” and using of the powerful recently developed iterative programming methods for constraint and unconstrain minimization of a functional.

Instability of thin liquid films between membranes

Abstract: The conditions for instability of the thin liquid film between two plane-parallel membranes were derived taking into account the influence of the membrane tension, the membrane bending elasticity, the film viscosity and the disjoining pressure. It was shown that the liquid film could be unstable if the negative (attractive) disjoining pressure is predominant. The characteristic timeτ m of growth of perturbation due to thermal or other fluctuations of the membrane shape increases with increasing the film viscosity, the membrane tension and the membrane bending elasticity, and decreasing the film thickness and the negative disjoining pressure. It is of the order of 10−2÷103 sec. When the membranes approach each other at certain value of the average film thicknessh cr called critical, the fastest growing perturbations lead to formation of a liquid film with smaller (or zero) thickness. It was found that the critical thickness increases with increasing the negative disjoining pressure and the membrane area and decreasing the membrane tension and the bending elasticity having typical values of the order of 10−6÷10−5 cm. The case of a membrane approaching a solid plane was also considered. Excluding the small differences in numerical coefficients the results are similar to the case of two identical membranes.

Multipole Model of the Molecular Asymmetry in Thermotropic and Lyotropic Liquid Crystals. Volume and Surface Effects

Abstract: The molecular asymmetry of mesogenic molecules is defined as a deviation of the corresponding molecular property from spherical and axial symmetry. Three types of asymmetry-electrical, steric and biphilic, are considered using a multipole representation. Two basic molecular shapes are included-rod-like and plate-like. The first three multipoles-monopole, dipole, and quadrupole, are involved. In the steric case a monopole does not exist. The effective shape of a molecule includes not only steric, but also electrical intermolecular repulsive interactions. The biphilic asymmetry is determined by the distribution of the hydrophilic and hydrophobic groups along the molecule. Three types of generalized fields—electrical, deformational, and biphilic, are also defined and their volume and surface sources are discussed. The effects of the molecular asymmetry on the intermolecular interactions in thermotropic (rod-like and disc-like) and lyotropic liquid crystals are qualitatively discussed in an analogy t...

Adiabatic polaron theory of electron hopping in crystals: A reaction-rate approach

Abstract: The paper presents an application of reaction-rate theory to nonradiative electron transitions in polar crystals based on a "two-site" model. For this purpose localized electronic states are described by using the adiabatic approximation. Both "large" polarons in ionic crystals and "small" polarons in molecular crystals are considered from a unified point of view. By assuming a single frequency of the lattice vibrations (Einstein model) one can calculate the transition probability for both the limits of adiabatic and nonadiabatic electron transfer as well as for the whole intermediate range. The relevant expressions, earlier derived in treating electron transfer in solution, are reviewed and discussed in order to give a justification of their application to polaron hopping in crystals. In a similar way, a review is made of the reaction-rate approach to electron transfer to obtain the appropriate rate equations for the low- and high-temperature limits and the intermediate temperature range as well. New expressions for the polaron-hopping rate are also derived. The conditions of its validity are discussed. This paper provides an essentially new approach to nonradiative electron transfer in polar crystals. It permits one to overcome the limitations of the usual methods, such as the multiphonon approach, entirely based on time-dependent perturbation theory and/or the Franck-Condon approximation which restrict their applicability only to nonadiabatic polaron hopping. The reaction-rate treatment exactly reproduces the results of the more rigorous multiphonon theory of nonadiabatic transitions. Moreover, it yields correct rate expressions for adiabatic transitions, in particular, in the high-temperature range, in which the classical occurrence-probability approach greatly overestimates the activation energy. Some problems concerning the Einstein one-frequency oscillator model assumed and the irreversibility of the electron transfer are discussed with some details from standpoint of both reaction-rate theory and multiphonon theory. The advantages of the reaction-rate approach are emphasized.