Collisional dynamics of Ag19 on Pd(100): a molecular dynamics study

By means of molecular dynamics simulations based on realistic n-body potentials we investigate structural and dynamical features inherent to the energetic collision of a silver cluster (Ag₁₉) on the Pd(100) substrate. Both the system and the impact energy (E_i = 95 eV) adopted have been chosen to parallel an experimental study of size selected Ag cluster deposition on Pd(100). Our results indicate that the experimental cross section obtained via thermal energy atom scattering at the same collision energy is well reproduced by the simulations.The modeling allows to rationalize the collision outcome in terms of defect production and cluster atoms implantation. The adsorbed structures have an heterogenous nature and are mostly two-dimensional.     

碱金属团簇碰撞动力学研究

With in the framework of distance dependent tight-binding molecular dynamics (DDTBMD), the collision dynamics of sodium cluster Na n has been studied systematically. Some phenomena have been observed at different impact parameters b , such as fusion reaction, deep inelastic collision (DIC) and quasi-elasticcollision, which are similar to the nuclear heavy-ion collisions (HIC). For the system of Na6 (3D) + Na8, the reaction mechanism at b=9a0 is DIC, but when b is equal to 13a0, it corresponds to quasi-elastic collision. Further more the rotation processes during the collisions, are related to the collision energy and parameter. The larger collision energy is, the earlier relative rotation will occur, and the relaxation time becomes shorter and the relative rotation energy is much smaller. There exists maximal relative rotation energy which corresponding to b = 7a0. When b is smaller than 7a0, the rotation energy increases with b increasing, otherwise the energyd ecreases. And the maximal relative rotation energy is corresponding to DIC process. The maximal rotation energy can reach on etenth of total energy, which is much less than that in HIC.     

On the statistical nature of collision and surface-induced dissociation: a theoretical investigation of aluminum clusters

The unimolecular dissociation dynamics of aluminum clusters following collision with either a rare gas atom or a surface is investigated by classical trajectory simulations with model potentials. Two conformers of Al(6) with very distinct shapes, i.e., the spherical O(h) and planar C(2)(h) clusters, are considered in this work. The initial vibrational energy and angular momentum distributions resulting from collision, as well as the energy and angular momentum resolved lifetime distributions, of excited clusters were determined for both collision-induced dissociation (CID) and surface-induced dissociation (SID) processes. The partitioning of excitation energy acquired upon collision was found to depend on the excitation mechanism (CID or SID), as well as on the cluster molecular shape, especially in the case of CID. For both types of processes, the energy and angular momentum resolved excited cluster lifetime distributions were found to decay exponentially, in agreement with statistical theories of chemical reactions, suggesting intrinsic Rice-Ramsperger-Kassel-Marcus (RRKM) behavior. Moreover, the simulated microcanonical rate constants determined from the cluster lifetime distributions are in good agreement with the predictions of the orbiting transition state model of phase space theory (OTS/PST), which further supports the statistical character of cluster CID and SID. Thus, in the CID and SID of highly fluxional systems such as aluminum clusters, the rate of intramolecular vibrational energy redistribution (IVR) is much faster than the dissociation rate, which validates one of the key assumptions, i.e., post-collision statistical behavior, underlying the models that are routinely used to determine cluster binding energies from experimental CID/SID cross sections.     

Postcollision relaxation of small atomic clusters

Molecular-dynamics simulations are performed to investigate the effects caused by the lack of internal equilibration on the dynamics and properties of atomic clusters. The studied systems consist of Lennard-Jones clusters of five to ten atoms and a colliding vapor monomer. Cluster radius and potential energy are shown to reach a time-independent value within 30 ps after a collision with a vapor monomer. The relaxation in terms of rotational energy takes at least 200 ps. During the first couple of picoseconds after the collision time-dependent cluster decay rates are observed. The unrelaxed cluster states are expected to have minimal effect on gas-liquid nucleation rates.     

钯团簇碰撞沉积钯/银基板的分子动力学模拟

运用分子动力学模拟方法研究了常温下较大的钯团簇以不同初始速度撞击不同硬度基板的微观过程,着重分析了沉积形貌的变化、团簇的嵌入深度和原子的扩散程度、基板碰撞接触区域的温度演变以及碰撞过程中团簇与基板间的能量转化,获得了沉积过程中变形形貌、结构特征及能量转化随团簇尺寸、初始速度及基板材质的变化规律.并进一步探究了第二颗团簇以不同时刻沉积时前一团簇的变形形貌及基板接触区域温度变化的特点,发现短时间间隔下第二颗团簇的沉积更有利于团簇与基板的结合.     

Reactions of high translational energy hydrogen atoms with propane

The reactions of translationally excited deuterium atoms with propane have been investigated by use of photochemically produced atoms of 157kJmol(-1) initial energy. The reaction mechanism was tested by comparing the results for the DBr scavenged system to those where Br(2) was used as radical scavenger. Values were obtained for the fraction of atoms undergoing "hot" reaction on collision with propane, and the relative cross sections for the interaction of deuterium atoms with propane and DBr. The effect of the initial energy of the "hot" atoms is discussed.     

Experimental and theoretical studies of charge transfer and deuterium ion transfer between D2O+ and C2H4

The charge transfer and deuterium ion transfer reactions between D(2)O(+) and C(2)H(4) have been studied using the crossed beam technique at relative collision energies below one electron volt and by density functional theory (DFT) calculations. Both direct and rearrangement charge transfer processes are observed, forming C(2)H(4) (+) and C(2)H(3)D(+), respectively. Independent of collision energy, deuterium ion transfer accounts for approximately 20% of the reactive collisions. Between 22 and 36 % of charge transfer collisions occur with rearrangement. In both charge transfer processes, comparison of the internal energy distributions of products with the photoelectron spectrum of C(2)H(4) shows that Franck-Condon factors determine energy disposal in these channels. DFT calculations provide evidence for transient intermediates that undergo H/D migration with rearrangement, but with minimal modification of the product energy distributions determined by long range electron transfer. The cross section for charge transfer with rearrangement is approximately 10(3) larger than predicted from the Rice-Ramsperger-Kassel-Marcus isomerization rate in transient complexes, suggesting a nonstatistical mechanism for H/D exchange. DFT calculations suggest that reactive trajectories for deuterium ion transfer follow a pathway in which a deuterium atom from D(2)O(+) approaches the pi-cloud of ethylene along the perpendicular bisector of the C-C bond. The product kinetic energy distributions exhibit structure consistent with vibrational motion of the D-atom in the bridged C(2)H(4)D(+) product perpendicular to the C-C bond. The reaction quantitatively transforms the reaction exothermicity into internal excitation of the products, consistent with mixed energy release in which the deuterium ion is transferred in a configuration in which both the breaking and the forming bonds are extended.     

Attachment cross sections of protonated water clusters

The attachment of water molecules onto size selected protonated water clusters has been experimentally investigated. Absolute attachment cross sections are measured as a function of cluster size, collision energy, and initial cluster temperature. Although thermal evaporation is ruled out in our experiment, attachment cross sections become significantly smaller than hard sphere cross sections as the collision energy increases. This feature is attributed to a transition from adiabatic to nonadiabatic regime. It is shown to be due to a dynamical effect: as the collision duration becomes shorter than the typical time required for collision energy redistribution into clusters internal energy, the attachment probability is reduced. We relate this typical time to the period of the main surface vibrational mode excited by the collisions. This hypothesis is further supported by results obtained with deuterated water clusters.     

Collision energy transfer in collision of NH(4) (+)(NH(3))(n-1) (n=3-9) with ND(3)

An incorporation of ND(3) into protonated ammonia cluster ions NH(4)(+)(NH(3))(n-1) (n=3-9), together with a dissociation of the cluster ions, was observed in the collision of the cluster with ND(3) at collision energies ranging from 0.04 to 1.4 eV in the center-of-mass frame. The branching fractions of the cluster ion species produced in the reactions were obtained as a function of the collision energy. The branching fractions of the incorporation products were successfully explained in terms of the Rice-Ramsperger-Kassel (RRK) theory at collision energies lower than the binding energy of the cluster ion. In addition, the internal energy distributions of the parent cluster ions were determined, and found to be in good agreement with those predicted using the evaporative ensemble model. In incorporations at collision energies lower than the binding energy of the cluster ion, all of the collision energy was transferred to the internal energy of the cluster ions; subsequently, an evaporation of ammonia molecules occurred in an equilibrium process after a complete energy redistribution in the clusters. In contrast, at collision energies higher than the binding energy of the cluster ion, a release of an ammonia molecule from the incorporation products occurred in a nonequilibrium process. The transition from the complex mode to the direct mode in the incorporation was observed at collision energies approximately equal to the binding energy. On the other hand, the collision energy dependence of the cross sections for the dissociation and for a nonreactive collision were estimated by a RRK simulation in which the collision energy transfer was interpreted by using the classical hard-sphere collision model. A relationship between reactivity and reaction modes in the collision of NH(4)(+)(NH(3))(4) with ND(3) is discussed via a comparison of the experimental results with the RRK simulation.     

Mechanisms of silicon sputtering and cluster formation explained by atomic level simulations

In low‐energy secondary ion MS, collision cascades result in rare sputter events or unfavourably low sputter yields. To better identify the origin of emission products generated by low‐energy ion impacts, we carried out molecular dynamics simulations of the underlying collision cascades, using a reactive force field that accounts for the dynamic breaking and forming of bonds. A detailed explanation of the cluster formation and ejection processes for atomic oxygen and also atomic silicon bombardment of Si (100) is given for comparison. Copyright © 2014 John Wiley & Sons, Ltd.     

Evanescent high pressure during hypersonic cluster-surface impact characterized by the virial theorem

Matter under extreme conditions can be generated by a collision of a hypersonic cluster with a surface. The ultra-high-pressure interlude lasts only briefly from the impact until the cluster shatters. We discuss the theoretical characterization of the pressure using the virial theorem and develop a constrained molecular-dynamics procedure to compute it. The simulations show that for rare-gas clusters the pressures reach the megabar range. The contribution to the pressure from momentum transfer is comparable in magnitude and is of the same sign as that ("the internal pressure") due to repulsive interatomic forces. The scaling of the pressure with the reduced mechanical variables is derived and validated with reference to the simulations.     

准经典轨线方法对Ca+CD3Ⅰ→CaⅠ+CD3同位素效应的动力学研究

在扩展Lond-Eyring-Polanyi-Sato(LEPS)势能面上,采用准经典轨线方法对反应Ca+CD3I→CaI+CD3进行了动力学计算,并讨论了该反应的同位素效应.在同位素效应作用下,产物CaI的振动态分布向低振动态转移,反应体系的散射截面在低碰撞能和高碰撞能处有较小的变化.同时,受到反应物的质量因子变化的影响,产物转动取向值减少,产物转动取向增强.仅有产物的角分布受同位素效应的影响不明显.     

Co(+)-assisted decomposition of h6-acetone and d6-acetone: acquisition of reaction rate constants and dynamics of the dissociative mechanism

Reaction rate constants have been acquired for the gaseous unimolecular decomposition reaction of the Co(+)(OC(CH(3))(2)) cluster ion and its deuterium labeled analog. Each rate constant is measured at a well resolved cluster internal energy within the range 12,300-16,100 cm(-1). The weighted, averaged kinetic isotope effect (KIE), k(H)/k(D) = 1.54 ± 0.05, is about three times smaller than the KIE measured for the rate-determining rate constants in the similar Ni(+)(OC(CH(3))(2)) decomposition reaction. These reactions likely follow the same oxidative addition-reductive elimination mechanism. Thus, this unexpected change in the KIE magnitudes is not due to differences in the dissociative reaction coordinates. Rather, we propose that the unique dissociation dynamics of these two similar systems is due to differences in the low-lying electronic structure of each transition metal ion.     

Water-induced fluorescence quenching of aniline and its derivatives in aqueous solution

Nonradiative deactivation processes of excited aniline and its derivatives in aqueous solution were investigated by steady-state and time-resolved fluorescence measurements to reveal characteristic solvent effects of water on the relaxation processes of excited organic molecules. The magnitude of nonradiative rate (k_nr) of excited aniline derivatives increased significantly in water compared to that in organic solvents (cyclohexane, ethanol, and acetonitrile). The fluorescence lifetime measurements in organic solvent/H₂O mixed solvents suggested that the fluorescence quenching in water was not due to exciplex formation but due to interactions with a water cluster. From temperature effect experiments on the fluorescence lifetime and quantum yield of aniline, N-methylaniline, and N,N-dimethylaniline, the apparent activation energies for the nonradiative deactivation rate in water were determined as 21, 30, and 41 kJ mol^-1, respectively. Upon substitution of hydrogen atoms in the aromatic ring of aniline derivatives for deuterium atoms resulted in normal deuterium isotope effect in cyclohexane, i.e. k_nr decreased by deuterium substitution, while in water the same deuterium substitution led to an increase in k_nr (the inverse isotope effect). The inverse isotope effects implied that a direct internal conversion to vibrationally higher excited states in the electronically ground state is not a dominant mechanism but the transition to a close-lying energy level, e.g. the relaxation to charge transfer to solvent (ctts) state, would be associated with the quenching mechanism in water.     

Energy dependent decay rates of Lennard-Jones clusters for use in nucleation theory

Decay rates of small clusters (containing between 10 and 40 Lennard-Jones atoms) are determined by molecular dynamics simulations. The cluster is defined by the condition that the atoms must lie within a specified distance of their center of mass, and initial isothermal states are generated using a Metropolis Monte Carlo method. Plots of the logarithm of the survival fraction against time are found to be nonlinear, indicating that the decay of constant temperature clusters is non-Markovian and depends on the collision rate with a thermalizing gas. However, when the clusters are banded according to their energies, exponential decay is seen. The energy dependent decay rates from simulations agree to within a factor of 2 with those estimated from equilibrium considerations (using free energies from thermodynamic integration and assuming a Gaussian energy distribution), indicating that clusters defined in this way can be used in Markovian rate equations. During nucleation, the cluster energy distribution is shifted from its equilibrium value, leading to a reduction in the nucleation rate by a temperature dependent factor of 100 or more, in the absence of a thermalizing carrier gas.     

Molecular dynamics of 300 keV (D2O)100 clusters on Ti-D

We use the molecular dynamics code DAMSEL to predict the velocity distributions for beam and lattice atoms after bombardment of Ti-D “foils” of thickness 20.86 Å by 300 keV (D₂O)₁₀₀ cluster ions. From these distributions we estimate the D-D nuclear fusion yield. We find that cluster bombardment reduces the overall energy deposition of the beam in the lattice compared to that of the individual beam atoms of the same velocity. However, a small portion of lattice atoms (<1%), and a larger percentage of beam atoms (～30%), have energies above the maximum present in the case of bombardment by individual D or O atoms. The folding of the standard D-D fusion cross sections over the relative velocity distributions produced by beam and lattice deuterons produce a fusion yield estimate of ～1×10^?21 fusions per cluster, with the high-energy distributions of beam deuterons playing the most important role. This is nine orders of magnitude lower that the data of Beuhler et al. While transient (～10 fs) atom densities 50% higher than that of the initial lattice are recorded in the course of the simulation, the average energy transferred per lattice atom — 23 eV — is insufficient to support any “collision spike” explanation of the observed fusion yield.     

Exploring the importance of quantum effects in nucleation: the archetypical Ne(n) case

The effect of quantum mechanics (QM) on the details of the nucleation process is explored employing Ne clusters as test cases due to their semi-quantal nature. In particular, we investigate the impact of quantum mechanics on both condensation and dissociation rates in the framework of the microcanonical ensemble. Using both classical trajectories and two semi-quantal approaches (zero point averaged dynamics, ZPAD, and Gaussian-based time dependent Hartree, G-TDH) to model cluster and collision dynamics, we simulate the dissociation and monomer capture for Ne(8) as a function of the cluster internal energy, impact parameter and collision speed. The results for the capture probability P(s)(b) as a function of the impact parameter suggest that classical trajectories always underestimate capture probabilities with respect to ZPAD, albeit at most by 15%-20% in the cases we studied. They also do so in some important situations when using G-TDH. More interestingly, dissociation rates k(diss) are grossly overestimated by classical mechanics, at least by one order of magnitude. We interpret both behaviours as mainly due to the reduced amount of kinetic energy available to a quantum cluster for a chosen total internal energy. We also find that the decrease in monomer dissociation energy due to zero point energy effects plays a key role in defining dissociation rates. In fact, semi-quantal and classical results for k(diss) seem to follow a common "corresponding states" behaviour when the proper definition of internal and dissociation energies are used in a transition state model estimation of the evaporation rate constants.     

Fragmentation cross sections of protonated water clusters

We have measured fragmentation cross sections of protonated water cluster cations (H(2)O)(n=30-50)H(+) by collision with water molecules. The clusters have well-defined sizes and internal energies. The collision energy has been varied from 0.5 to 300 eV. We also performed the same measurements on deuterated water clusters (D(2)O)(n=5-45)D(+) colliding with deuterated water molecules. The main fragmentation channel is shown to be a sequential thermal evaporation of single molecules following an initial transfer of relative kinetic energy into internal energy of the cluster. Unexpectedly, that initial transfer is very low on average, of the order of 1% of collision energy. We evaluate that for direct collisions (i.e., within the hard sphere radius), the probability for observing no fragmentation at all is more than 35%, independently of cluster size and collision energy, over our range of study. Such an effect is well known at higher energies, where it is attributed to electronic effects, but has been reported only in a theoretical study of the collision of helium atoms with sodium clusters in that energy range, where only vibrational excitation occurs.     

碳离子碰撞单壁碳纳米管的动力学

采用分子动力学方法研究了碳离子碰撞碳纳米管中顶位、键中心和六元环中心的动力学过程。通过分析低、中、高3种入射能分别对碰撞过程的影响,探索了典型缺陷形成的微观演化过程。研究结果表明,碰撞碳纳米管中不同空间位置,其碰撞结果差异较大,其中顶位碰撞阈能最低,约为20 eV;碰撞六元环中心时碳管会发生严重变形,损伤最为严重。通过分析入射离子动能,碳纳米管热动能、质心动能以及势能随时间的演化规律,阐述了碰撞过程中的能量转移机制。     

碳离子碰撞单壁碳纳米管的动力学

采用分子动力学方法研究了碳离子碰撞碳纳米管中顶位、键中心和六元环中心的动力学过程。通过分析低、中、高3种入射能分别对碰撞过程的影响,探索了典型缺陷形成的微观演化过程。研究结果表明,碰撞碳纳米管中不同空间位置,其碰撞结果差异较大,其中顶位碰撞阈能最低,约为20 e V;碰撞六元环中心时碳管会发生严重变形,损伤最为严重。通过分析入射离子动能,碳纳米管热动能、质心动能以及势能随时间的演化规律,阐述了碰撞过程中的能量转移机制。