离子束引起界面反应和原子混合的唯象模型

离子束引起固体原子反应和混合是近年来国际上研究极为活跃的一个课题,利用这种原理在硅化物形成以及原子微合金方面都取得了很多有益结果。但是,对其物理机理的研究还很不夠,还没有一个完整的理论可以描述“离子混合”的过程。本文从固体中原子碰撞、运动、原子反应和合成的观点出发,讨论了快速离子轰击二元素系统所发生的二类基本过程:原子间的弹性碰撞、原子重排、反应以及形成稳定或亚稳态的过程。提出一个描述离子束引起界面相互作用的原子过程的唯象模型,得到了描述界面反应和扩散的动力学方程,提出了别区受反应控制和受扩散控制过程的判据并与实验结果作了比较。     

异质原子在Cu（001）表面扩散的分子动力学模拟

采用分子动力学方法模拟单个增原子Ag,Pd和Cu在Cu（001）表面上的扩散过程.通过对自扩散和异质扩散过程中扩散机制的观察,统计三种不同的增原子在不同温度下的扩散频率,拟合给出扩散势垒和扩散频率的指前因子,并与扩散势垒的静力学计算结果进行比较.结果表明：在800 K以下时,三种增原子均以简单跳跃机制为主扩散,与衬底不互溶的Ag增原子的跳跃频率最大,与衬底互溶的Pd增原子的跳跃频率最小.同质增原子与异质增原子的扩散频率和温度的关系均较好地符合Arrhenius公式,由Arrhenius公式拟合给出的三种不同增原子的扩散势垒与表面结构和增原子表面结合能有关.Pd和Cu增原子从跳跃机制为主向交换机制为主的转换温度分别在825和937 K左右. 关键词： 表面扩散 分子动力学模拟     

表面物理第六讲 气体在固体表面的吸附

吸附表面和吸附物质多种多样,通常把吸附分为物理吸附和化学吸附两类.物理吸附主要是由于范德瓦耳斯力的作用,其吸附热远小于1eV,典型的实例是各种惰性气体的吸附.化学吸附是由于化学力的作用,即固体表面原子与吸附分子间形成化学键,其吸附热约为1eV,例如各种分子解离吸附.物理     

电介质或导体表面吸附原子的共振光现象

本文借助于线性响应理论方法求得固体表面附近原子的光学布洛赫(Bloch)方程。借助于光学布洛赫方程和量子回归定理,求得了吸附原子的共振荧光谱。通过对该谱的数值分析可得出下述结论:固体表面的存在将对光谱中峰的高度以及两个边带的位置产生明显的影响。当被吸附的原子非常接近固体表面时,两个边带会向两侧移动,其中一个几乎消失,同时各个峰的高度也会大幅度降低。     

Li掺杂对MgH2(001)表面H2分子扩散释放影响的第一性原理研究

本文利用基于密度泛函理论的第一性原理分别研究了MgH₂(001)表面H原子扩散形成H₂分子释放出去的可能路径及金属Li原子掺杂对其影响. 研究结果表明: 干净MgH₂(001)表面第一层释放H原子形成H₂分子有两种可能路径, 其释放能垒分别为2.29和2.50 eV; 当将Li原子替代Mg原子时, 两种H原子扩散释放路径的能垒分别降到了0.31和0.22 eV, 由此表明Li原子掺杂使MgH₂(001)表面H原子扩散形成H₂释放更加容易.     

惰性气体原子和离子在强激光场中的隧道电离

在隧道电离区域利用ADK模型并考虑了激光的体积效应,对惰性气体原子及各价离子在强激光场中的电离产额进行了计算,并与BSI模型及实验结果进行了比较,结果发现ADK模型的结果会随着原子序数的增加而变好,尤其对Xe离子,不需要进行强度转换就能给出很好的结果．由此可以得出结论：对于重原子（或离子）,用ADK模型得到的离子产额与实验结果符合得更好．最后通过分析ADK模型的适用范围,给出了和计算结果相一致的结论．     

电子在固体中的运动

固体(本文指结晶体)的主要特性是其原子或分子的规则排列.不同晶体各种物理性质上的千差万别(例如有金属、半导体和绝缘体之分),都深刻地与电子在晶体中的能量状态及运动规律相联系着. 在晶体中,一个电子的运动是与数目众多的格点上的原子(或分子)以及数目更多的其它电子的运动相关联.物理学家们分析了各种实验和数据,颇费心思地将这种多体问题简化为单电子问题,建立了可信赖的单电子模型──能带论. 一、自由电子气模型1.电子公有化-自由电子气 以金属钠为例来说明这个模型.金属钠为体心立方结构.每个原子在闭壳层外有一个价电子.这个电子…     

分子碰撞振动激发的一种计算方法

研究原子和双原子分子碰撞的振动激发过程，需要求争一组振动耦合微分方程组，给出了此方程组的一种数值求解方法，推导了ＩＯＳ近似下的振动激发截面公式，将方程化简为无量纲形式，然后给出了解方程的方法，最后给出应用这种方法计算的一个例子，并与实验结果进行了比较。     

多组分液滴汽化的一个模型

本文导出在对流运动介质中多组分液滴汽化的通量公式,并给出描述液滴汽化历史的微分方程组.同时作了各种多组分液滴汽化历史的计算,计算结果与实验结果较好地吻合. 一、通量积分式和有效扩散系数 研究一个多组分理想溶液液滴在隋性气体(液滴溶液不吸收也不释放的气体),I中的汽化.设溶液由n种组分构成,则在液滴表面形成n+1种组分的多元气体混合物轴对称扩散膜层,第n+1种组分指的是隋性气体I.用假想膜层理论将轴对称扩散膜层折合为等效的球对称假想膜层,则可按stewart通量公式写出在该膜层中任意半径r处的稳态扩散方程,即:     

固体表面结构的同步辐射研究X射线吸收近边结构（XANES）

表面x-射线吸收近边结构是与团体表面原子的内壳层的电子向未占据轨道跃迁密切相关的。吸收边阈值之上的大约50eV的低能范围的潜图结构含有与吸收原子近邻的局域结构和局城电子态相关的信息。因此它已经成为研究固体表面局域特性，诸如同城几何结构，键长和键角的有用工具．为了和实验的表面XANES谱一致和对实测XANES谱进行解释，已经发展起来多种XANES的理论计算方法，比较成功的有能带结构近似理论和多原子体系的多重散射理论近似方法．在本文中，还简要介绍了利用同步辐射进行表面XANES实验的技术要求和相关方法的比较，并给出了若干实例说明表面XANES在固体表面结构研究方而的应用．     

Positronium in molecular substances

The progress of research into the behavior of the positronium in molecular substances in the last several years is reviewed. The Ps atom can be treated as a solute in a liquid. The repulsive exchange force is the dominant force which reacts against the van der Waals attractive force. One of the most useful general relationships found is the one between the pick-off quenching rate and the surface tension of the liquid. The pick-off quenching rate is found, in general, to be an additive property of the functional groups of a molecule. The quenching of theo-Ps atom by an active quenching agent in an inert solvent has been found to be primarily diffusion controlled. Some deviations from the simple Arrhenius equation have been found. For gases, more theoretical work on the collision problems of Ps with atoms or molecules is needed. For amorphous molecular solids, it is likely that the property of pick-off quenching ofo-Ps is similar to that in liquids. However, in crystalline molecular solids, the situation is quite different. A few definite cases have been found where a Ps atom does not form or exist. Further work in this area should be very interesting. The foregoing leads us to believe that positronium or positron annihilation can be useful in the study of fundamental properties of molecular liquids and solids.     

Equation of state for inert gas solids

The equation of state is a fundamental relation to analyse the thermophysical properties of different class of solids and it plays a key role in basic and applied condensed matter physics research. A lot of work has been done in the field of ionic solids, minerals and metals but a very little work is done in the field of inert gas solids. Most of the equations of state failed to explain the properties of inert gas solid because of their abnormal behavior in the low temperature range. In the present paper, Singh-Gupta equation of state has been used to study the properties of these solids. The results obtained using these equations have shown a good agreement with available experimental results. Thus it is shown that these equations of states successfully explain the behavior of inert gas solids.      

Numerical modelling of methanol liquid pool fires

The focus of this paper is on numerical modelling of methanol liquid pool fires. A mathematical model is first developed to describe the evaporation and burning of a two-dimensional or axisymmetric pool containing pure liquid methanol. Then, the complete set of unsteady, compressible Navier-Stokes equations for reactive flows are solved in the gas phase to describe the convection of the fuel gases away from the pool surface, diffusion of the gases into the surrounding air and the oxidation of the fuel into product species. Heat transfer into the liquid pool and the metal container through conduction, convection and radiation are modelled by solving a modified form of the energy equation. Clausius–Clapeyron relationships are invoked to model the evaporation rate of a two-dimensional pool of pure liquid methanol. The governing equations along with appropriate boundary and interface conditions are solved using the flux-corrected transport algorithm. Numerical results exhibit a flame structure that compares well with experimental observations. Temperature profiles and burning rates were found to compare favourably with experimental data from single- and three-compartment laboratory burners. The model predicts a puffing frequency of approximately 12 Hz for a 1 cm diameter methanol pool in the absence of any air co-flow. It is also observed that increasing the air co-flow velocity helps in stabilizing the diffusion flame, by pushing the vortical structures away from the flame region.     

Theory of inert gas-condensing vapor thermoacoustics: propagation equation

The theory of acoustic propagation in an inert gas-condensing vapor mixture contained in a cylindrical pore with wet walls and an imposed temperature gradient is developed. It is shown that the vapor diffusion effects in the mixture are analogous to the heat diffusion effects in the thermoacoustics of inert gases, and that these effects occur in parallel with the heat diffusion effects in the wet system. The vapor diffusion effects can be expressed in terms of the thermoviscous function F(lambda) used in the theory of sound propagation of constant cross-section tubes. As such, these results can be extended to any shape parallel-walled tube. The propagation equations predict that the temperature gradient required for onset of sound amplification in a wet-walled prime mover is much lower than the corresponding temperature gradient for an inert gas prime mover. The results of a measurement of the onset temperature of a simple demonstration prime mover in air with a dry stack and with a stack wetted with water provide a qualitative verification of the theory.     

自悬浮定向流纳米金属粉末制备的理论模拟

叙述了自悬浮定向流纳米金属粉末的制备原理，建立了在惰性气体介质中金属液滴表面蒸发 形成悬浮微粒过程的数学模型.描述了蒸发金属液滴表面层的热流动、物质迁移、热扩散、 凝聚相初始核的形成、金属蒸气在粒子表面的凝聚、粒子相互凝聚等过程，考虑了物质迁移 系数等动力学参数对温度的依赖关系.预测了指定粉末尺寸分布下的最佳工艺条件. 关键词： 纳米金属粉末 凝聚理论模拟 自悬浮定向流     

Theoretical description of adatom migration in two-dimensional highly-ordered states

Analytical expressions for chemical, jump, and tracer diffusion coefficients are obtained for interacting lattice gases on a square lattice. Strongly repulsive nearest neighbor interactions cause the formation of a highly-ordered c() state in the vicinity of half coverage. It is shown that only strongly correlated successive adatom jumps contribute to the particle flow. This allows to describe the adatom kinetics by considering an almost ideal lattice gas of defects. Two types of defects are considered, adatoms in the empty sublattice and vacancies in the filled sublattice of the c() ordered state. The diffusion equations for these defects are developed considering the generation and recombination of defects. In addition we have considered adatom transport caused by the motion of defect pairs (dimers). Dimer transport mechanism prevails in the high coverage region. The characteristic features of the various diffusion coefficients near half coverage are analyzed and discussed. The theory is compared with the results of sophisticated Monte-Carlo simulations which have been executed with the use of a fully parallelized algorithm on a Cray T3E (LC784-128). The agreement between theoretical and MC results is excellent if the motion of dimers at is taken into account. Received 24 June 1998     

Processes in condensed inert gases involving excess electrons

Drift of an excess electron in dense and condensed inert gases in external electric field and excitation of atoms by electron impact in these systems are analyzed. The effective potential energy surface for an excess electron at a given electric field strength consists of wells and hills, and the actions of neighboring atoms are therefore separated by saddles of the potential energy. At such atomic densities that the difference of interaction potentials for an excess electron between neighboring wells and hills of the potential energy surface becomes small, the electron mobility is large. This is realized for heavy inert gases (Ar, Kr, Xe) with a negative scattering length of an electron on individual atoms. In these cases, the average potential energy of the electron interaction with atoms corresponds to attraction at low atomic densities and to repulsion at high densities. The transition from attraction to repulsion at moderate atomic densities leads to a maximum of the electron mobility. A gas model for electron drift in condensed inert gases is constructed on the basis of this character of interaction. Due to high electron mobility, condensed inert gases provide high efficiency of transformation of the electric field energy into the energy of emitting photons through drifting electrons. It is shown that, although the role of formation of autodetaching states in the course of electron drift is more important for condensed inert gases than for rare gases, this effect acts weakly on exciton production at optimal atomic densities. The parameters of a self-maintained electric discharge in condensed inert gases as a source of ultraviolet radiation are discussed from the standpoint of electron drift processes.     

Diffusion coefficients of metals in inert gases: Comparison of calculations with experimental data

In the present work some simple methods of calculation of the diffusion coefficient by using the model and experimental interaction potentials are discussed. An applicability of such calculations for diffusion of metals in inert gases is analyzed.     

Correlation and trust mechanism-based rumor propagation model in complex social networks

In real life, the rumor propagation is influenced by many factors. The complexity and uncertainty of human psychology make the diffusion model more challenging to depict. In order to establish a comprehensive propagation model, in this paper, we take some psychological factors into consideration to mirror rumor propagation. Firstly, we use the Ridenour model to combine the trust mechanism with the correlation mechanism and propose a modified rumor propagation model. Secondly, the mean-field equations which describe the dynamics of the modified SIR model on homogenous and heterogeneous networks are derived. Thirdly, a steady-state analysis is conducted for the spreading threshold and the final rumor size. Fourthly, we investigate rumor immunization strategies and obtain immunization thresholds. Next, simulations on different networks are carried out to verify the theoretical results and the effectiveness of the immunization strategies. The results indicate that the utilization of trust and correlation mechanisms leads to a larger final rumor size and a smaller terminal time. Moreover, different immunization strategies have disparate effectiveness in rumor propagation.