Numerical simulation of the intramolecular dynamics of photoisomerization

The results of mixed quantum-classical and quantum-mechanical numerical calculations of the intramolecular dynamics of photoisomerization under conditions similar to ordinary natural conditions, i.e., for irradiation of the molecule by a light pulse not shorter than the lifetime of the resonant excited electronic state of the molecule and with an intensity comparable to that of solar light at the Earth’s surface, are presented. It was concluded that the dynamics of such photoisomerization should be modeled using quantum-mechanical methods. The simplest approach to modeling the photoisomerization of a molecule with two isomeric forms can be based on the density matrix formalism for describing the interaction of a light pulse with a three-level system of Λ configuration.     

Numerical simulation of non-Abelian particle-field dynamics

Numerical 1D-3V solutions of the Wong-Yang-Mills equations with anisotropic particle momentum distributions are presented. They confirm the existence of plasma instabilities for weak initial fields and of their saturation at a level where the particle motion is affected, similar to Abelian plasmas. The isotropization of the particle momenta by strong random fields is shown explicitly, as well as their nearly exponential distribution up to a typical hard scale, which arises from scattering off field fluctuations. By variation of the lattice spacing we show that the effects described here are independent of the UV field modes near the end of the Brioullin zone.     

Numerical simulation of process dynamics during laser beam drilling with short pulses

In the last years, laser beam drilling became increasingly important for many technical applications as it allows the contactless production of high quality drill holes. So far, mainly short laser pulses are of industrial relevance, as they offer a good compromise between precision and efficiency and combine high ablation efficiency with low thermal damage of the workpiece. Laser beam drilling in this pulse length range is still a highly thermal process. There are two ablation mechanisms: evaporation and melt expulsion. In order to achieve high quality processing results, a basic process understanding is absolutely necessary. Yet, process observations in laser beam drilling suffer from both the short time scales and the restricted accessibility of the interaction zone. Numerical simulations offer the possibility to acquire additional knowledge of the process as they allow a direct look into the drill hole during the ablation process. In this contribution, a numerical finite volume multi-phase simulation model for laser beam drilling with short laser pulses shall be presented. The model is applied for a basic study of the ablation process with μs and ns laser pulses. The obtained results show good qualitative correspondence with experimental data.     

具有原子分辨率的x射线荧光全息术的数值模拟研究

根据x射线荧光全息术的成像原理，对体心立方晶系的Fe单晶进行了数值模拟（包括荧光全 息图及其重构像），在各个晶面（001，010，100）上得到Fe原子的像，与Fe的晶格模型的 原子位置一致，表明运用这种x射线荧光全息术，能够在原子水平上得到单晶或准晶体的内 部结构图像. 关键词： x射线荧光全息术 同步辐射 晶体结构 傅里叶变换     

Numerical simulation of continuum models for fluid-fluid interface dynamics

This paper is concerned with numerical methods for two-phase incompressible flows assuming a sharp interface model for interfacial stresses. Standard continuum models for the fluid dynamics in the bulk phases, for mass transport of a solute between the phases and for surfactant transport on the interface are given. We review some recently developed finite element methods for the appropriate discretization of such models, e.?g., a pressure extended finite element (XFE) space which is suitable to represent the pressure jump, a space-time extended finite element discretization for the mass transport equation of a solute and a surface finite element method (SurFEM) for surfactant transport. Numerical experiments based on level set interface capturing and adaptive multilevel finite element discretization are presented for rising droplets with a clean interface model and a spherical droplet in a Poisseuille flow with a Boussinesq-Scriven interface model.     

Numerical simulation of space-charge dynamics in a gyrotron trap

The particle-in-cell (PIC) method is used to simulate the self-consistent accumulation and bunching of space charge in the trap of a gyrotron electron-optical system. It is shown that it is possible to generate charge bunches that oscillate along the direction of the magnetic field. The dependence of the characteristics of these oscillations on the magnitude of the electron current into the trap is determined, along with the effect of the accumulated charge on the velocity distribution of electrons in the current passing through the magnetic mirror. Satisfactory agreement with the experimental data is obtained. Zh. Tekh. Fiz. 67, 98–101 (September 1997)     

Numerical investigation of the effect of dual frequency sonication on stable bubble dynamics

A computational study aiming to simulate an oxygen single acoustic bubble oscillation under a dual-frequency sonication was presented in this paper. The non-linear response of the bubble to the superposition of two fields of ultrasonic waves was investigated through dynamics parameters, collapse ratios and average velocities. The main goal of this analyze is to link the properties of the wave resulting from the dual-frequency excitation to the dynamics behavior of the bubble. The obtained results prove that, in contrast with the mono-frequency, coupling a wave to lower frequencies enhances the collapse duration and raises the compression ratio in the case of 35 kHz, while associating any of the studied waves to a higher frequency elevates the number of bubble oscillations during a time interval as compared to mono-frequency. The total sonochemical production has been investigated in accordance with the dynamics results, as well as the proportions of the three predominant free radicals, that show a dependency on the value of the basic frequency.     

Spectro-streak picosecond studies of intramolecular charge-transfer fluorescence

A spectro-streak photometer, an instrument for simultaneously measuring fluorescence intensity, time, and wavelength,I(t, ), with a single picosecond excitation pulse, has been constructed. Two typical and currently highly topical examples of mesurements are discussed. (1) the temporal development of the fluorescence form the intramolecular charge-transfer (ICT) state of the rigid aromatic compound 4,5-(1-methylindolino)3,4-naphthanthracene is studied in the protic solvent hexanol. (2) Propyl chain-linked pyrene/N,N-dimethylaniline is used as the model compound to study conformational changes associated with the transition from a contact ion pair to a sandwich exciplex.     

Numerical simulation of the vortex dynamics in the two-band Ginzburg-Landau model

The dynamics of Abrikosov vortices in the intermediate state of a two-band superconductor is analyzed by numerical simulation of the nonstationary two-band Ginzburg-Landau equations.     

Numerical simulation of momentumless turbulent wake dynamics in linearly stratified medium

This paper presents comparison of two numerical models of the momentumless turbulent wake dynamics behind a body of revolution in a linearly stratified medium, namely, the model based on direct (DNS) numerical integration of Navier–Stokes equations in the Oberbeck–Boussinesq approximation and the mathematical model with application of a semi-empirical turbulence model of the third order. The results of calculations by these two models agree with the known experimental data.     

A mathematical simulation of intramolecular proton phototransfer

A system of equations for the matrix elements of the density operator of a seven-level model molecule interacting with a light pulse was solved numerically to determine the time dependences of the populations of molecule states at various radiation pulse parameters and parameters characterizing radiative and nonradiative spontaneous molecule transitions and reversible transitions between some of its states. The results were used to characterize the photoisomerization of molecules between states with different positions of the proton of the intramolecular H-bond (the keto and enol forms). Examples of oscillating molecular state population modulation in isomer-isomer tunnel proton shifts are given. Changes in the development of photoionization in time as molecular parameters and radiation pulse width and intensity changed were considered. An analysis of the results obtained is an example of the use of mathematical simulation of intramolecular dynamics for increasing the effectiveness of using spectral-time data in the determination of the mechanism of proton phototransfer in molecules with intramolecular H-bonds     

Molecular dynamics simulation of iron nanoparticle sintering during flame synthesis

The sintering process of iron nanoparticles produced in a flame environment is investigated by molecular dynamic (MD) simulations. The thermodynamic characteristics and restructuring pathways are studied for two-body and three-body sintering processes. The melting point, energy, and structures are computed for nanoparticles before and after sintering. A simplified model is proposed to predict the equilibrium temperature of nanoparticles upon sintering. The MD results are used to explain the formation mechanisms of two size ranges of nanoparticles during the flame synthesis. The role of sintering during nanoparticle growth is analyzed.     

Numerical simulation of the one-dimensional population dynamics with nonlocal competitive losses and convection

Numerical solutions of the generalized one-dimensional Fisher–Kolmogorov–Petrovskii–Piskunov equation with nonlocal competitive losses and convection are constructed. The influence function for nonlocal losses is chosen in the form of a Gaussian distribution. The effect of convection on the dynamics of the spatially inhomogeneous distribution of the population density is investigated.     

Numerical simulation of particle dynamics in a storage ring by using BETACOOL program

The BETACOOL program, developed by electron cooling group of the Joint Institute for Nuclear Research (JINR), is a set of algorithms based on common format of input and output files. The program is oriented toward simulation of the ion beam dynamics in a storage ring in the presence of cooling and heating effects. The version presented in this report includes three basic algorithms: simulation of root-mean-square (rms) parameters of the particle distribution function evolving in time, simulation of the distribution function evolution using the Monte Carlo method, and a tracking algorithm based on a molecular dynamics technique. The general processes investigated with the program are intrabeam scattering in the ion beam, electron cooling, and interaction with residual gas and an internal target.     

电离层钡云释放早期动力学行为的数值模拟

在中性钡云扩散动力学模型的基础上,考虑钡原子的氧化和电离损耗, 探讨了钡云释放早期(t≤ 100 s)的演化基本特征、钡云形态、亮度以及电子密度分布等问题. 得到了不同释放量(1, 10 kg)、不同释放高度(250, 300 km)和不同初始形状因子(1, 10) 条件下钡云释放早期动力学行为的数值模拟结果.     

Influence of intramolecular hydrogen bond formation sites on fluorescence mechanism

The fluorescence mechanism of HBT-HBZ is investigated in this work. A fluorescent probe is used to detect HClO content in living cells and tap water, and its structure after oxidation by HClO (HBT-ClO) is discussed based on the density functional theory (DFT) and time-dependent density functional theory (TDDFT). At the same time, the influence of the probe conformation and the proton transfer site within the excited state molecule on the fluorescence mechanism are revealed. Combined with infrared vibrational spectra and atoms-in-molecules theory, the strength of intramolecular hydrogen bonds in HBT-HBZ and HBT-ClO and their isomers are demonstrated qualitatively. The relationship between the strength of intramolecular hydrogen bonds and dipole moments is discussed. The potential energy curves demonstrate the feasibility of intramolecular proton transfer. The weak fluorescence phenomenon of HBT-HBZ in solution is quantitatively explained by analyzing the frontier molecular orbital and hole electron caused by charge separation. Moreover, when strong cyan fluorescence occurs in solution, the corresponding molecular structure should be HBT-ClO(T). The influence of the intramolecular hydrogen bond formation site on the molecule as a whole is also investigated by electrostatic potential analysis.     

Dynamic quenching of the dual fluorescence of molecules

Mathematical relations describing the properties of spontaneous steady-state dual fluorescence under conditions of dynamic quenching of excited states by foreign impurities are derived. It is shown that, in the case of a kinetic character of the reaction, the initial form of the dye and its photoproduct are quenched, the intensity ratio of the fluorescence bands of the initial form and the product linearly increasing with the quencher concentration. Analysis performed is applicable to a wide range of photoreactions accompanied by the dual fluorescence (charge transfer, proton transfer, complexation, etc.). The properties of the fluorescence, absorption, and dual fluorescence excitation for 3-hydroxyflavone in acetonitrile under conditions of dynamic quenching by the TEMPO spin quencher with a concentration below 1.25 × 10^?2 M are studied. 3-Hydroxyflavone is characterized by the excited-state intramolecular proton transfer and by the fluorescence spectrum consisting of two well-spaced bands. The observed dependences of the intensity of both fluorescence bands on the quencher concentration correspond to the theoretical conclusions. The Stern-Volmer constants calculated from the experimental data on the assumption of diffusion quenching of the excited states are 858 and 1141 M^?1 for the normal and tautomeric fluorescence bands, respectively. The experimental results reveal the kinetic character of the excited-state proton transfer in 3-hydroxyflavone in acetonitrile.     

Size effect during two-particle agglomeration: Results of molecular dynamics simulation

The microscopic mechanisms controlling the atomic rearrangements during agglomeration of Ni particles 3 to 7 nm in size at temperatures T = 0.6–0.95 T _m have been studied by the molecular dynamics method. Pentagonally twinned crystals were obtained as a result of coalescence for the disorientations corresponding to special Σ11 and Σ27 large-angle boundaries.