Adsorption kinetics and dynamics of small organic molecules on a silica wafer: Butane, pentane, nonane, thiophene, and methanol adsorption on SiO2/Si(1 1 1)

The adsorption kinetics (by thermal desorption spectroscopy) and adsorption dynamics (by molecular beam scattering) have been determined for a number of alkanes, methanol, thiophene, and water on a silica wafer—SiO₂/Si(1 1 1). No indications for bond activation were present, i.e., all probe molecules adsorb molecularly obeying 1st order kinetics. The coverage-dependent heat of adsorption has been determined accordingly. The adsorption dynamics are precursor-mediated with Kisliuk-like shapes of the adsorption probabilities at low impact energies and adsorbate-assisted adsorption at large impact energies.     

X-ray intensity fluctuation spectroscopy studies on phase-ordering systems

The order-disorder phase transition in Cu3Au has been studied by x-ray intensity fluctuation spectroscopy. Following a quench from the high-temperature, disordered phase, the ordering kinetics is well described by a universal scaling form that can be measured by time-resolved (incoherent) x-ray scattering. By using coherent scattering, we have measured the fluctuations about this universal scaling form. In the late stages of the ordering process, these fluctuations give a two-time correlation function C(q,t1,t2) which has a scaling form with natural variables deltat=/t1-t2/ and t =(t1+t2) / 2. The scaling form crosses over from linear in t to t1/2. These present the first such results for a nonconserved system.     

Chaos,fractional kinetics,and anomalous transport

Chaotic dynamics can be considered as a physical phenomenon that bridges the regular evolution of systems with the random one. These two alternative states of physical processes are, typically, described by the corresponding alternative methods: quasiperiodic or other regular functions in the first case, and kinetic or other probabilistic equations in the second case. What kind of kinetics should be for chaotic dynamics that is intermediate between completely regular (integrable) and completely random (noisy) cases? What features of the dynamics and in what way should they be represented in the kinetics of chaos? These are the subjects of this paper, where the new concept of fractional kinetics is reviewed for systems with Hamiltonian chaos. Particularly, we show how the notions of dynamical quasi-traps, Poincaré recurrences, Lévy flights, exit time distributions, phase space topology prove to be important in the construction of kinetics. The concept of fractional kinetics enters a different area of applications, such as particle dynamics in different potentials, particle advection in fluids, plasma physics and fusion devices, quantum optics, and many others. New characteristics of the kinetics are involved to fractional kinetics and the most important are anomalous transport, superdiffusion, weak mixing, and others. The fractional kinetics does not look as the usual one since some moments of the distribution function are infinite and fluctuations from the equilibrium state do not have any finite time of relaxation. Different important physical phenomena: cooling of particles and signals, particle and wave traps, Maxwell's Demon, etc. represent some domains where fractional kinetics proves to be valuable.     

Kinetic theory of gas-surface interaction

The kinetics of particle and energy exchange between gas and surface is treated in analogy to the transport of particles and energy in solutions using Onsager's theory of nonequilibrium thermodynamics. In addition to the coefficients of sticking and thermal accommodation a third kinetic coefficient is introduced describing the coupling of particle and energy transport. This coupling turns out to be particularly important at low temperatures reducing the accommodation coefficient from 2 to 1.The Onsager coefficients are then expressed in terms of the scattering properties of the surface. A simple model is treated to illustrate the general results and to provide estimates for the kinetic coefficients.Extract from doctoral thesis of H.M. submitted to Fachbereich Physik, Techn. Universität München, 1978     

Relativistic particle scattering

In the presence of competing relativistic formalisms for interacting particle dynamics, a model-independent axiomatic approach is proposed for the study of the following asymptotic aspects of relativistic classical particle dynamics: the definition of the scattering operator, scattering angle and time-delay, and the specification of a general functional interdependence between the objects so defined.     

Quantum simulation of the Klein paradox with trapped ions

We report on quantum simulations of relativistic scattering dynamics using trapped ions. The simulated state of a scattering particle is encoded in both the electronic and vibrational state of an ion, representing the discrete and continuous components of relativistic wave functions. Multiple laser fields and an auxiliary ion simulate the dynamics generated by the Dirac equation in the presence of a scattering potential. Measurement and reconstruction of the particle wave packet enables a frame-by-frame visualization of the scattering processes. By precisely engineering a range of external potentials we are able to simulate text book relativistic scattering experiments and study Klein tunneling in an analogue quantum simulator. We describe extensions to solve problems that are beyond current classical computing capabilities.     

Polymerization-induced Phase Separation: Phase Behavior Developments and Hydrodynamic Interaction

The process of polymer synthesis based on polymerization-induced phase separation (PIPS) is revisited from the theoretical point of view. Cahn–Hilliard–Cook theories for spinodal decomposition are adapted to describe the kinetics of phase separation and deduce the time-resolved scattering function, while the double reaction model is used to describe the kinetics of polymerization. Coupling of these two kinetics is provided by the Carother's equation relating the fraction of reacted monomers to the degree of polymerization at time t, denoted N(t). It is argued that the approach to criticality is governed by a critical parameter, χ_c, that is different from the usual parameter for spinodal decomposition, χ_s, deduced from the second derivative of the free energy. While the latter parameter depends on the reciprocal degree of polymerization N^?1(t), the former one depends on its time integral. This leads to significant consequences on the phase behavior developments during the PIPS process. Hydrodynamic interactions are found to speed up the emergence of instability modes. Although the qualitative trends remain similar to those of the Rouse dynamics, important quantitative changes are found due to the long-range viscous flow effects.     

Nonlinear conductance fluctuations in a mesoscopic ring

We study the conductance of a single particle on a ring subject to an arbitrary dc electric field, which is generated by a linearly in time increasing magnetic flux. The full quantum mechanical time development is calculated numerically by splitting the dynamics into independent consecutive Zener tunneling transitions and free motion on the ring. The Zener transitions occur near the avoided crossings of the bandstructure which arises from the adiabatic eigenstates as a function of flux in the presence of a static scattering potential. To account for the necessary dissipation the particle is coupled to an appropriate oscillator bath which is adjusted to give a strictly linear current-voltage characteristic for arbitrary voltage and temperature in the absence of scattering. Taking a single δ-function scatterer we find that the dissipative coupling eliminates the localization in energy space found previously and leads to a well defined resistive steady state. The scattering introduces reproducible fluctuations around the average Ohmic behavior which are caused by coherent backscattering. Their magnitude depends on the strength of the scattering potential and decays slowly for large voltages. The associated correlation energy is determined by the uncertainty of the eigenstates due to the dissipative bath coupling. Thermal averaging leads to a decrease of the conductance fluctuations proportional to T^?1.     

气体分子与纳米粒子碰撞的分子动力学仿真分析

当气体分子与纳米粒子碰撞的时候,纳米粒子传输理论预测到当纳米粒子的直径由小变大时,碰撞会由镜面反射转化为漫反射.文章利用分子动力学仿真研究了气体分子与纳米粒子碰撞的过程.在验证了这种转化存在同时,又探讨了碰撞转化的机理,即漫反射的起因.仿真结果揭示了漫反射的起因是由于纳米粒子对气体分子的吸附作用.这种吸附作用是由于纳米粒子对能量的容纳特性而产生的.     

The application of quasi-elastic neutron scattering techniques (QENS) in surface diffusion studies

Neutron scattering techniques such as quasi-elastic neutron scattering, QENS, have proven to be well-suited tools for studying structure and dynamics of surface adsorbed molecules. In contrast to many more widely used surface science techniques neutron scattering allows the microscopic characterization of samples under a wide range of thermodynamic conditions, as the samples are not constrained to ultra high vacuum environment. Moreover, neutron scattering allows the separation of coherent and incoherent scattering, giving access to different diffusive mechanisms such as single particle diffusion, mass transport, rotations, or vibrations. In this paper we will review recent progress and the state-of-the-art in neutron scattering experiments on surface adsorbed molecules in the sub-monolayer coverage range with a specific emphasis on studies of carbon and other high surface density substrates. We will also cover recent progress in theoretical modeling, since the usefulness of neutron scattering data on surface dynamics can be strongly enhanced by computational modeling, such as molecular dynamics (MD) simulations and the development of analytical models.     

Quantum kinetics and thermalization in a particle bath model

We study the dynamics of relaxation and thermalization in an exactly solvable model of a particle interacting with a harmonic oscillator bath. Our goal is to understand the effects of non-Markovian processes on the relaxational dynamics and to compare the exact evolution of the distribution function with approximate Markovian and non-Markovian quantum kinetics. There are two different cases that are studied in detail: (i) a quasiparticle (resonance) when the renormalized frequency of the particle is above the frequency threshold of the bath and (ii) a stable renormalized "particle" state below this threshold. The time evolution of the occupation number for the particle is evaluated exactly using different approaches that yield to complementary insights. The exact solution allows us to investigate the concept of the formation time of a quasiparticle and to study the difference between the relaxation of the distribution of bare particles and that of quasiparticles. For the case of quasiparticles, the exact occupation number asymptotically tends to a statistical equilibrium distribution that differs from a simple Bose-Einstein form as a result of off-shell processes whereas in the stable particle case, the distribution of particles does not thermalize with the bath. We derive a non-Markovian quantum kinetic equation which resums the perturbative series and includes off-shell effects. A Markovian approximation that includes off-shell contributions and the usual Boltzmann equation (energy conserving) are obtained from the quantum kinetic equation in the limit of wide separation of time scales upon different coarse-graining assumptions. The relaxational dynamics predicted by the non-Markovian, Markovian, and Boltzmann approximations are compared to the exact result. The Boltzmann approach is seen to fail in the case of wide resonances and when threshold and renormalization effects are important.     

Path-length-resolved diffusive particle dynamics in spectral-domain optical coherence tomography

We describe a new method to measure the decorrelation rate of the optical coherence tomography (OCT) magnitude simultaneously in space and time. We measure the decorrelation rate of the OCT magnitude in a Fourier-domain OCT system for a large range of translational diffusion coefficients by varying the sphere diameter. The described method uses the sensitivity advantage of Fourier-domain OCT over time-domain OCT to increase the particle diffusion imaging speed by a factor of 200. By coherent gating, we reduce the contribution of multiple scattering to the detected signal, allowing a quantitative study of diffusive particle dynamics in high concentration samples. We demonstrate that this technique is well suited to image diffusive particle dynamics in samples with a complex geometry as we measure the morphology and diffusive particle dynamics simultaneously with both high spatial and high temporal resolution.     

A schematic model discussion of the conductor to insulator transition for particle motion in a random potential

A schematic model for the dynamics of a particle moving in a random environment is discussed where the latter is assumed to cause normal scattering and also scattering with time inversion symmetry breaking. The second mechanism changes the exponent unity to one half for the vanishing of the conductivity near the percolation threshold. It alters also the exponent of the critical conductivity spectrum from 1/2 to 1/3. The dynamics near the transition including all cross over phenomena to the usual effective medium theory results are described by a two parameter scaling law.Dedicated to Professor W. Brenig on the occasion of his 60th birthday     

Exact kinetic transport equation solutions in the particle propagation theory in the scattering medium

Charged particle kinetics in an inhomogeneous medium (stochastic magnetic field) is investigated. Exact analytic expressions for Green function of kinetic equation in relaxation-time approximation are derived in one and three dimensions with arbitrary particle absorption. We separately consider the case of isotropic particle injection as well as the case of unidirectional instantaneous particle injection. The new way of solution makes it possible to get off any Cauchy-Principal Value integrals in some solutions which arise in the inverse Fourier-Laplace transform. Weak scattering regime and diffusion approximation is considered, and particle density is derived in three dimensions and arbitrary particle source.     

孤立波在一维复合颗粒链中传播特性的模拟研究

采用分子动力学方法模拟研究了孤立波在重轻颗粒相间排列的一维复合颗粒链中的传播特性.结果发现,在轻重颗粒的质量比较大或较小时,散射作用较弱,颗粒的速度和孤立波的速度衰减较慢.在轻重颗粒的质量比为中等时,散射作用较强,颗粒的速度和孤立波的速度衰减较快.孤立波在通过重-轻颗粒界面时,存在有增速效应,可以提高孤立波的传播速度.并且,轻重颗粒的质量比越小增速效应越强.在散射作用和增速效应的共同作用下,改变轻重颗粒的质量比可以调控孤立波在重-轻颗粒链中的传播时间.     

Trapping of projectiles in fixed scatterer calculations

Zeitschrift für Physik A Hadrons and nuclei - We study multiple scattering off nuclei in the closure approximation. Instead of reducing the dynamics to one particle potential scattering, the...     

Model for single-particle dynamics in supercooled water

We analyze a set of 10 M-step molecular dynamics (MD) data of low-temperature SPC/E model water with a phenomenological analytical model. The motivation is twofold: to extract various k-dependent physical parameters associated with the single-particle or the self-intermediate scattering functions (SISFs) of water at a deeply supercooled temperature and to apply this analytical model to analyses of new high resolution quasielastic neutron scattering data presented elsewhere. The SISF of the center of mass computed from the MD data show clearly time-separated two-step relaxations with a well defined plateau in between. We model the short time relaxation of the test particle as a particle trapped in a harmonical potential well with the vibrational frequency distribution function having a two-peak structure known from previous inelastic neutron scattering experiments. For the long time part of the relaxation, we take the alpha relaxation suggested by mode-coupling theory. The model fits the low-temperature SISF over the entire time range from 1 fs to 10 ns, allowing us to extract peak positions of the vibrational density of states, the structural relaxation rate 1/tau of the cage (the potential well) and the stretch exponent beta. The structural relaxation rate has a power law dependence on the magnitude of the wave vector transfer k and the stretch exponent varies from 0.55 at large k to unity at small k.     

Inclusive processes at large transverse momenta

A general model for processes at large transverse momentum is constructed on the assumption that events with large transverse momentum particles posses short range correlations. It is shown that particle ratios and scaling properties are not sensitive to the detailed dynamics of large angle scattering, and also that the character of the inclusive spectrum can be understood in terms of (quasi-) elastic scattering.     

Nonlinear dynamics of charged particles in an oblique electromagnetic wave

We study dynamics of a charged particle under action of an electromagnetic wave that propagates obliquely to a background uniform magnetic field. The dynamics is described by a slow-fast Hamiltonian system. We show that long-term dynamics is dominated by phenomena of capture of particle into resonance with the wave and escape from this resonance, as well as of scattering on resonance. We find that the variation of the particle?s kinetic energy on the time interval between capture and escape is bounded and accumulated in the motion along the background field. We discuss possible applications of the obtained results.     

烟尘簇团粒子光学截面和散射矩阵的数值计算

用离散偶极子近似方法，计算了单个烟尘簇团粒子的光学特性，得到了簇团粒子的散射截面、吸收截面及不对称因子随入射角的变化关系，为研究波在烟尘粒子中的传输特性提供了有效的计算方法；给出了不同入射角情况下烟尘簇团粒子散射矩阵元素的角分布，为研究散射体的散射特性、极化特性以及散射体结构特性提供了一种理论方法. 关键词： 烟尘簇团粒子 散射矩阵 光学截面 DDA