Simulation of the Processes of Isomerization of Complex Molecules and of Their Spectra with Time Resolution

Based on the method developed earlier to calculate the dynamic electronic-vibrational spectra of complex molecules with allowance for isomeric transformations, a more general method has been developed making it possible to model the spectra of several isomeric forms in any number depending on the problem posed. Population dynamics analysis of the energy levels of isomers is possible. Computational algorithms have been developed and optimized, and a specialized software has been developed for a PC and a supercomputer (of the type MVS 1000). Computer experiments were carried out, and the possibility of simulating dynamic spectra and kinetic intramolecular processes in real time for complex compounds with allowance for their intermolecular transformations is shown.     

Calculation of the Probabilities of Molecular Transitions in Modeling Interisomeric Structural Transformations

The problems of a computational procedure in modeling structural isomer–isomer transitions in multiatomic molecules are considered. A number of approximations that allow one to considerably simplify calculation of the probabilities of these transformations are suggested. In comparison with traditional calculations of potential surfaces and determination of their saddle points, in the proposed approach the relationship between the electronic and vibrational components of a general problem is virtually changed. A particular emphasis is put on the vibrational part, which seems to be more physical. It is shown that the problem of structural isomer–isomer conversions can be solved without resorting at all to explicit calculation of the contribution of an electronic matrix element to the probability of transition. There is no need to study in detail the shapes of potential surfaces; it suffices only to know the positions of their minima. The calculation of multidimensional vibrational integrals is replaced by calculation of one-dimensional integrals. The results obtained form a basis for constructing methods for modeling the dynamic spectra of complex molecules considering their structural transformations.     

Modeling of the vibrational structure of the vibronic spectra of diphenylpolyenes by the parametric method

The structures of the electronic-vibrational spectra and of the excited states of a number of diphenylpolyene molecules are determined within the framework of the second approximation of the parametric method. The system of parameters of the structural fragments of molecules is improved and good agreement with spectral experiment is obtained. It is shown that there is a high degree of transferability of the polyene and acene parameters of the method and that the models obtained are adequate to the real structure of molecules. It is also shown that the method proposed makes it possible to perform predictive qualitative and quantitative calculations of the spectra of these molecules, as well as of the spectral characteristics necessary for modeling photochemical molecular transformations. In the series of diphenylpolyene molecules, an interpretation of the vibrational structure of the spectra is proposed and the specific features of variation of the geometry upon excitation of molecules are considered.     

Modeling of the time-resolved vibronic spectra of polyatomic molecules: The formulation of the problem and analysis of kinetic equations

A semiempirical parametric method is proposed for modeling three-dimensional (time-resolved) vibronic spectra of polyatomic molecules. The method is based on the use of the fragment approach in the formation of molecular models for excited electronic states and parametrization of these molecular fragments by modeling conventional (one-dimensional) absorption and fluorescence spectra of polyatomic molecules. All matrix elements that are required for calculations of the spectra can be found by the methods developed. The time dependences of the populations of a great number (>10³) of vibronic levels can be most conveniently found by using the iterative numerical method of integration of kinetic equations. Convenient numerical algorithms and specialized software for PC are developed. Computer experiments showed the possibility of the real-time modeling three-dimensional spectra of polyatomic molecules containing several tens of atoms.     

Polarization and spectral characteristics of anisotropic ring laser cavities with nonideal mirrors

A mathematical model for calculating the polarization and spectral characteristics of eigenmodes of nonplanar ring optical cavities with allowance for the phase anisotropy of oblique-incidence mirrors is described. The results of the numerical modeling and experimental measurements of the characteristics of laser mirrors and mode spectra of practically important four-mirror cavities with a nonplanar symmetric axial contour are compared. It is shown that the proposed mathematical model can be applied to optimal selection of lasers for laser cavities with allowance for their real parameters.     

Quantum-mechanical description of the intensity distribution in two-photon absorption spectra of dihalogenated benzenes

Using molecules of dihalogenated benzenes as an example, the verification of an earlier proposed method of direct quantum-mechanical calculation of the relative intensities of lines in the two-photon absorption spectra of polyatomic molecules is continued. The method of calculation is based on the adiabatic model in the context of the Herzberg-Teller approximation. A comparative analysis of the intensity distribution in the two-photon absorption spectra of dihalogenated and monosubstituted benzenes is carried out, and the vibrational characteristics of these molecules in the lowest singlet excited electronic states are determined. A satisfactory agreement with experimental values, along with the analysis of the possibilities of the method, confirms the expediency of its use for gaining information about the intensity distribution in two-photon absorption spectra of polyatomic molecules.     

Mössbauer total external reflection: Composition of surface layers

An ambiguity of Mössbauer spectra measured under total external reflection conditions due to the increase of the number of parameters which should be determined can be compensated by measuring a series of experimental spectra. The interpretation of all experimental data must be done by means of numerical modeling (or fitting). The method enables us to study phase transformations within a layer of about 10 nm thickness.     

Modeling of the vibrational structure of the vibronic spectrum and an excited state of the dinaphthylethylene molecule

The vibrational structure of the fluorescence spectrum and the structure of a dinaphthylethylene molecule in an excited state are calculated in the first and second approximations of the parametric method employed in the theory of vibronic spectra. The calculated spectra are in quantitative agreement with experimental data. The role of the angular parameters of the parametric method in the quantitative prediction of the vibrational structure of the fluorescence spectrum and changes observed in the geometry of the dinaphthylethylene molecule under excitation is determined. It is demonstrated that the polyene and acene parameters of the parametric method possess a high degree of transferability and that the models obtained are quite adequate to the real structures of the molecules under investigation. The proposed approach permits qualitative predictions and quantitative predictive calculations of the spectra of the studied molecules, as well as the spectral characteristics necessary for simulation of photochemical molecular transformations.     

New methodologies for computer simulation of paramagnetic resonance spectra

A new software package, SOPHE, for computer simulation of randomly oriented EPR spectra is described. A central feature of SOPHE is that all interactions associated with the electronic spins and major nuclear spins are treated equally through full matrix diagonalisation. In order to make this approach “workable”, several new methodologies have been developed, which include a novel partition scheme for partitioning the unit sphere, an interpolation scheme involving a combination of cubic spline and linear interpolations, and a segmentation method for handling multiple resonances between a given pair of energy levels. As a result, these new developments have led to a significant reduction in computational times. The SOPHE package can be used to simulate a variety of EPR spectra arising from isotropic organic radicals to complex coupled spin systems. It also incorporates the major linewidth models developed previously for the simulation of randomly oriented EPR spectra from magnetically isolated spin systems.     

The numerical determination of dipole hyperpolarizabilities

The paper is devoted to calculations of the contour shapes in the infra-red and Raman spectra of the 1 : 1 and 1 : 2 type complexes of ordinary and heavy water with two organic bases, dioxane and dimethyl sulphoxide, by means of a computer. The calculations have been made using the mathematical formalism developed in the previous papers of this series which takes into account the coupling between two stretching vibrations of the water molecule and the overtone of its bending vibration for a set of the liquid molecules variously perturbed by non-equal H-bonds. The basic parameters of the model are obtained from the spectra of HOD molecules involved in the same complex as the one considered, and from empirical correlations discussed in the previous papers. Starting from a set of parameters determined in this manner we calculate three spectra simultaneously for each of the above-mentioned complexes, namely the infra-red spectrum and the isotropic and anisotropic components of the Raman spectrum. Every spectrum is composed of three contours belonging to the overtone of the bending vibrations, and also to the in-phase and out-of-phase stretching vibrations respectively. These contours are asymmetrical and have different half-widths, maximum frequencies and intensities in all three types of spectra. The model demonstrates the inadequacy of the widespread interpretation of such spectra in terms of the symmetric and antisymmetric vibrations of undistorted water molecules characterized by the symmetry C _2v . Good agreement between calculated and experimental spectra is considered to provide strong evidence of the fluctuation model.     

Analysis of electronic-vibrational spectra of uracil,thymine, and cytosine

A theoretical analysis of absorption spectra of uracil, thymine, and cytosine—nucleic acid bases— is carried out. Structural dynamic models of these molecules in their electronically excited states are constructed. On the basis of the calculated vibrational structure of the electronic spectra, different tautomeric forms of these molecules are determined. The possibility of modeling the influence of hydrogen bonds on the electronic-vibrational spectra is shown.     

Calculation and interpretation of the vibronic spectra of pyridine and <Emphasis Type="Italic">trans</Emphasis>-1,2-di(2′-pyridyl)ethylene in the second approximation of the parametric method

The structures of pyridine and dipyridylethylene molecules in an excited state and their vibronic spectra are calculated within the second approximation of the parametric method. The system of parameters obtained, including parameters of the σ and π types, ensures a quantitative agreement between the theoretical and experimental spectra of the pyridine and dipyridylethylene molecules. This agreement indicates that the proposed model is adequate to the real structure of the molecules. The parameterization is sufficiently complete and provides a means for quantitatively modeling the vibrational structure of the spectra of complex molecules containing fragments similar to those characterized by the σπ* and ππ* transitions. It is demonstrated that, after such essential substitutions of atoms in the molecules, the system of parameters of the acene and polyene fragments retains the high stability. The vibrational structure of the electronic spectra of the pyridine and dipyridylethylene molecules is interpreted, and the changes observed in the geometry of these molecules under electronic excitation are analyzed.     

Measurement of mass flow rate of particulate solids in gravity chute conveyor based on laser sensing array

This paper describes the design, computer simulation and experimental evaluation of a novel measurement system that has been recently developed for the measurement of mass flow rate of particles in a gravity chute convey pipeline. Comprehensive computer simulation and numerical calculations show that the proposed method with laser-sensing array is theoretically correct and the programmed algorithm is effective. This method has been characterized by dynamic testing, real time and continuous measurements. The results of simulation and experiment have demonstrated that the multi-laser source fan-shaped geometry can be an effective approach to interrogate the mass flow rate of particles over the pipe cross-section. The mass flow rate of particles derived from extinction principle has shown a good agreement with the projection sum of light beams and the real mass flow rate.     

CdS在室温热光非线性状态下的动态光谱研究

提供了一种研究半导体在光学非性状态下的动态光谱方法.将此方法应用到半导体CdS上,观察到在热光非线性状态下,其透过光谱(在吸收带边缘附近)随时间的变化(在ms数量级上).计算机模拟结果与实验相符,并得出了在CdS上获得室温光学双稳态的条件.     

Mössbauer line sharpening

A novel analytical method of Mössbauer spectra resolution improvement has been developed. This method gives an opportunity to narrow the effective line of a source and an absorber by simple integral transformations of an experimental spectrum, that is, to transform the Lorentzian linewidth to the Lorentzian power one. The resulting spectrum may be considered as a new experimental one since the statistical error in each point of the spectrum is evaluated exactly. The procedure of sharpening gives many advantages for Mössbauer spectra analysis. To illustrate this, a number of applications of the proposed method is demonstrated.     

Dynamic stiffness method for exact inplane free vibration analysis of plates and plate assemblies

The inplane free vibration behaviour of plates is investigated using the dynamic stiffness method. Some distinctive modes which went unnoticed in earlier investigations using the dynamic stiffness method have been addressed by revisiting the problem and focusing on the special set of missing solutions. Results are validated extensively both by published results as well as by numerical studies using NASTRAN and ABAQUS. The accuracy of the finite element method for inplane free vibration analysis is assessed and critically examined through the provision of benchmark solutions. Some representative modes that are missed by well-established dynamic-stiffness-based computer programs are presented. The inplane dynamic stiffness matrix presented is of great importance when combined with the out of plane matrix in order to obtain the closed-form solution for free vibration analysis of structures with complex geometries.     

Numerical simulation of electromagnetic acoustic transducers using distributed point source method

In spite of many advances in analytical and numerical modeling techniques for solving different engineering problems, an efficient solution technique for wave propagation modeling of an electromagnetic acoustic transducer (EMAT) system is still missing. Distributed point source method (DPSM) is a newly developed semi-analytical technique developed since 2000 by Placko and Kundu (2007) [12] that is very powerful and straightforward for solving various engineering problems, including acoustic and electromagnetic modeling problems. In this study DPSM has been employed to model the Lorentz type EMAT with a meander line and flat spiral type coil. The problem of wave propagation has been solved and eddy currents and Lorentz forces have been calculated. The displacement field has been obtained as well. While modeling the Lorentz force the effect of dynamic magnetic field has been considered that most current analyses ignore. Results from this analysis have been compared with the finite element method (FEM) based predictions. It should be noted that with the current state of knowledge this problem can be solved only by FEM.     

Vibrational spectra of water in solutions

On the basis of results obtained by the method of theoretical modeling, the spectra of water in solutions are interpreted as the sums of the spectra of water molecules associated with one and two molecules of the solvent. The concentration, isotopic, and temperature variations of the spectra investigated fully confirm the proposed interpretation.     

The fluorescence and electronic structure of phenyl-substituted tetraazachlorin molecules

The effect of the addition of phenyl groups to pyrrole rings of tetraazachlorins, a new class of tetrapyrroles, on the photophysical properties and electronic structure of the molecules has been investigated by a complex of experimental and theoretical methods. Characteristics of fluorescence at 293 and 77 K have been determined for phenyl-substituted tetraazachlorins. The objects of this study include unsubstituted tetraazaporphine. The introduction of phenyl groups affords a marked increase in the fluorescence quantum yield. For tetraazaporphine and phenyl-substituted tetraazachlorins, fluorescence buildup occurs as the temperature is decreased from 293 to 77 K, but to a lesser extent than for tetraazachlorins having no phenyl groups, which were earlier studied by the authors. The fluorescence buildup mechanism is discussed. The singlet oxygen generation quantum yield has been determined for the tetrapyrroles examined. This characteristic increases upon tetrapyrrole is phenylation. The electronic structure and absorption spectra of unsubstituted porphine and chlorin, tetraazaporphine, tetraazachlorin, octaphenyltetraazaporphine, and tetramethylhexaphenyltetraazachlorin have been calculated by the INDO/Sm method (original modification of the INDO/S method) with molecular geometry optimization using DFT. The results of the quantum-chemical calculation of the absorption spectra are in good agreement with experimental data for transitions to the lowest excited electronic states Q _x (S ₁) and Q _y (S ₂).     

Methods of measuring characteristics of oscillations and waves in micromechanics (A review)

Methods of recording the electric signals generated during wave oscillations and wave propagation and the results of experiments on determination of dynamic characteristics of thin fibers and films are reviewed. New experimental setups have been developed. A possibility of studying the spectra of fiber oscillations is demonstrated, and a method for determining nonlinear stress-strain diagrams based on variations in the frequencies of transverse oscillations of this fibers as strings is proposed. The effect of sharp deceleration of dispersal of a charge being coated on the specimen surface in case of a damaged glass fiber is discovered. A complex method for measuring wave and mass velocities of elastic waves and instant elasticity moduli of a fiber during simultaneous high-speed photography and recording of electromagnetic radiation is developed. The dependences of these quantities on the wave intensity are given, and the scale effect is revealed. Application of this method for studying wave processes in thin polymer films is demonstrated.