The dynamics of desorption induced by atomic hydrogen: HD/Cu(111)

Recent observations of a direct reaction between adsorbates and hydrogen atoms incident from the gas phase are interpreted in terms of an Eley-Rideal reaction. A detailed comparison of the experimental data for the HD/Cu(111) system with quantum mechanical model calculations corroborates such an interpretation. The peculiar isotope effect observed can be understood from the different dynamical implications of appropriately rescaled potential energy surfaces. The width of the measured time-of-flight spectrum is explained from the overlapping contributions of the populated vibrational levels. The angular distributions are rationalized by contributions both from ‘indirect’ events, where the incident atoms make several bounces in the surface well prior to reaction, and ‘direct’ reactive events.     

The dynamics of desorption induced by atomic hydrogen: HD/Cu(111)

Recent observations of a direct reaction between adsorbates and hydrogen atoms incident from the gas phase are interpreted in terms of an Eley-Rideal reaction. A detailed comparison of the experimental data for the HD/Cu(111) system with quantum mechanical model calculations corroborates such an interpretation. The peculiar isotope effect observed can be understood from the different dynamical implications of appropriately rescaled potential energy surfaces. The width of the measured time-of-flight spectrum is explained from the overlapping contributions of the populated vibrational levels. The angular distributions are rationalized by contributions both from ‘indirect’ events, where the incident atoms make several bounces in the surface well prior to reaction, and ‘direct’ reactive events.     

Dynamics of adsorption,exchange reactions,and defect formation at solid surfaces

In real time computer simulations of reactive atoms incident on solid surfaces, several interesting fundamental processes have been observed. These include (1) a cascade of inelastic collisions between incident atom and substrate atoms that eventually results in equilibration and adsorption; (2) for sufficiently reactive atoms, and sufficiently high substrate temperatures, an exchange reaction in which the adsorbed atom replaces a substrate atom; and (3) the formation of defects as a result of adsorption or exchange events.     

Interactions of incident H atoms with metal surfaces

Atomic hydrogen is a highly reactive species of interest because of its role in a wide range of applications and technologies. Knowledge about the interactions of incident H atoms on metal surfaces is important for our understanding of many processes such as those occurring in plasma-enhanced catalysis and nuclear fusion in tokamak reactors. Herein we review some of the numerous experimental surface science studies that have focused on the interactions of H atoms that are incident on low-Miller index metal single-crystal surfaces. We briefly summarize the different incident H atom reaction mechanisms and several of the available methods to create H atoms in UHV environments before addressing the key thermodynamic and kinetic data available on metal and modified metal surfaces. Generally, H atoms are very reactive and exhibit high sticking coefficients even on metals where H₂ molecules do not dissociate under UHV conditions. This reactivity is often reduced by adsorbates on the surface, which also create new reaction pathways. Abstraction of surface-bound D(H) adatoms by incident H(D) atoms often occurs by an Eley-Rideal mechanism, while a hot atom mechanism produces structural effects in the abstraction rates and forms homonuclear products. Additionally, incident H atoms can often induce surface reconstructions and populate subsurface and bulk absorption sites. The absorbed H atoms recombine to desorb H₂ at lower temperature and can also exhibit higher subsequent reactivity with adsorbates than surface-bound H adatoms. Incident H atoms, either directly or via sorbed hydrogen species, hydrogenate adsorbed hydrocarbons, sulfur, alkali metals, oxygen, halogens, and other adatoms and small molecules. Thus, H atoms from the gas phase incident on surfaces and adsorbed layers create new reaction channels and products beyond those found from interactions of H₂ molecules. Detailed aspects of the dynamics and energy transfer associated with these interactions and the important applications of hydrogen in plasma processing of semiconductors are beyond the scope of this review.     

Computer simulation of single-crystal and polycrystal sputtering I

Sputtering of Cu single-crystal and polycrystal targets by 27 keV Ar ions has been simulated using the new binary collision cascade computer program OKSANA. The sputtering yield, the sputtering and reflection efficiencies, and the absolute and relative contributions to sputtering from various components have been calculated in a broad range of incidence angles. The obtained angular dependences of the sputtering yield have proved to agree with experimental data. Some features of sputtering due to semichannel focusing of incident particles have been found. The contributions to sputtering from several types of linear collision chains and from the primary knock-on atoms are considered in most detail. It has been shown, in particular, that the pure focused, pure defocused, and mixed focused-defocused collision chains contribute noticeably to sputtering. The contribution from the primary knock-on atoms is angle-dependent and reaches its maximum in the range of glancing angles for both single-crystal and polycrystal targets.     

CH基团与金刚石(111)面的碰撞反应及其对碳膜生长的影响

等离子体增强化学气相沉积技术中的碳膜选择性自组装机理是高性能碳膜制备过程中的挑战性基础课题.采用经典分子动力学方法,模拟了不同能量(1.625-65 eV)的CH基团在清洁金刚石和吸氢金刚石(111)面上的轰击行为,获得了吸附、反弹、反应等各类事件的发生概率,并据此探讨了含氢碳膜制备过程中CH基团的贡献.结果表明,随着入射能量的增加,CH基团对薄膜生长的贡献由单纯的吸附、反弹机理向反应、吸附混合机理转变,其中最主要的反应过程是释放一个或两个氢原子的反应,而释放氢分子的反应则很少发生.这些反应不仅使薄膜生长过程更均匀、薄膜表面更平整,还降低了薄膜的氢含量.生长机理的转变导致低能量条件下所成薄膜中的多数碳原子都包含一个氢原子作为配位原子,而高能量条件下的薄膜中的碳原子则很少有氢原子作为配位原子.另外,通过分析sp~3-C和sp~2-C数目的变化,研究了CH基团对金刚石基底的破坏作用.     

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The scattering behaviour of H atoms on Be surface by molecular dynamics simulations was reported in this paper. When the incident energy increases from 1 to 9eV, the outcome shows the H atoms scattering rate decrease with incident energy increasing, and increase with incident angle increasing. When incident energy is 1eV, all of incident H atoms scatter above the Be surface. When incident energy are 5 and 9eV, 14.7% and 35.8% of H atoms inject into the Be sample and then scatter, respectively, and incident depths increase with energy increasing. Scattering density and energy as function of scattering angle were also discussed.     

Etching of SiC by energetic F2: Molecular dynamics simulation

Molecular dynamics (MD) simulations were performed to investigate F₂ continuously bombarding silicon carbide (SiC) surfaces with energies in the range of 50-200 eV at normal incidence and room temperature. The Tersoff-Brenner form potential was used. The simulation results show that the uptake of F atoms, the etch yields of C and Si from the initial substrate, and the surface structure profile are sensitive to the incident energy. Like occurrence in Si etching, steady-state etching is observed and an F-containing reaction layer is formed through which Si and C atoms are removed. A carbon-rich surface layer after bombarding by F₂ is observed which is in good agreement with experiments. In the reaction layer, SiF in SiF₂ species are dominant; with increasing incident energy, the total fraction of SiF and SiF₂ increases, while the amount of SiF₃ and SiF₄ decreases. Finally, etching mechanisms are discussed.     

Modeling the efficiency of neutron detection scintillators in the energy range of 5–70 MeV

The instrumental functions for neutron godoscope detectors used in studying the nd-breakup reaction are obtained. The effect on light output of contributions from different processes that occur in a scintillator under the effect of neutrons is examined at different energies of the incident neutrons. The relationship between the detector??s efficiency and neutron energies ranging from 5 to 70 MeV is obtained for different thresholds.     

F原子与SiC(100)表面相互作用的分子动力学模拟

本文采用分子动力学模拟方法研究了F原子(能量在0.5—15 eV之间)与表面温度为300 K的SiC(100)表面的相互作用过程. 考察了不同能量下稳定含F反应层的形成过程和沉积、刻蚀过程的关系以及稳定含F反应层对刻蚀的影响. 揭示了低能F原子刻蚀SiC的微观动力学过程. 模拟结果表明伴随着入射F原子在表面的沉积量达到饱和,SiC表面将形成一个稳定的含F反应层. 在入射能量小于6 eV时,反应层主要成分为SiF₃,最表层为Si-F层. 入射能量大于6 eV时,反应层主要成分为SiF. 关键词： 分子动力学 刻蚀 能量 SiC     

Experimental realization of an optical one-way barrier for neutral atoms

We demonstrate an asymmetric optical potential barrier for ultracold 87Rb atoms using laser light tuned near the D2 optical transition. Such a one-way barrier, where atoms incident on one side are transmitted but reflected from the other, is a realization of Maxwell's demon and has important implications for cooling atoms and molecules not amenable to standard laser-cooling techniques. In our experiment, atoms are confined to a far-detuned dipole trap consisting of a single focused Gaussian beam, which is divided near the focus by the barrier. The one-way barrier consists of two focused laser beams oriented almost normal to the dipole-trap axis. The first beam is tuned to present either a potential well or barrier, depending on the state of the incident atoms. On the reflecting side of the barrier, the second beam optically pumps the atoms to the reflecting (barrier) state, thus producing the asymmetry.     

Quasiclassical studies of Eley-Rideal and hot-atom reactions on a surface at 94 K: H(D) → D(H) + Cu(1 1 1)

Reactions and reaction dynamics of gas-phase H(or D) atoms with D(or H) atoms adsorbed onto a Cu(1 1 1) surface have been investigated by the quasi-classical molecular dynamics method. To simulate the H(D) → D(H) + Cu(1 1 1) system at a 94 K surface temperature, D(or H) adsorbates were disseminated arbitrarily on the surface of Cu(1 1 1) to form 0.50, 0.28 and 0.18 ML of coverages. The interaction of hydrogen atoms and the surface system is worked out by an LEPS function. LEPS parameters have been determined by using the total energy values which were calculated by a density functional theory (DFT) method and the generalized gradient approximation (GGA) for the exchange-correlation energy for various configurations of one and two hydrogen atoms on the Cu(1 1 1) surface. The Cu(1 1 1) surface, imitated by an embedded-atom method which is a many-body potential parameterized by Voter-Chen, is formed as a multilayer slab. The slab atoms are permitted to move. Various processes, trapping onto the surface, inelastic reflection of the incident projectile and penetration of the adsorbate or projectile atom into the slab, are examined. The dependence of these mechanisms on isotopic replacement has also been analyzed. Considerable contributions of the hot-atom pathways for the product formations are consequently observed. The rate of subsurface penetrations is obtained to be larger than the sticking rate onto the surface.     

Quantum reflection

Recent experimental and theoretical results concerning the sticking coefficient at ultralow energy are described. The need for an accurate treatment of long range forces, including retardation, is emphasized. The system involving H atoms incident on liquid helium provides the first clear evidence of quantum reflection. New results are reported for the sticking of D atoms incident on helium. The energy upper bound for the regime of quantum reflection for alkali atoms is found to be extremely low, but ultimately achievable.     

Few nucleon transfer versus fragmentation in the production of projectile-like fragments in the40Ar+68Zn reaction at 27.6 MeV/nucleon

The isotope distributions, momentum width distributions, and velocities of the projectile-like fragments in the⁴⁰Ar on⁶⁸Zn reaction have been measured at 27.6 MeV/nucleon incident energy. The results show the existence of a fragmentation process well described in the framework of the high energy fragmentation model. However, important contributions from direct nucleon transfer and damped collisions are present.     

Molecular dynamics simulations of reactive etching of SiC by energetic fluorine

Molecular dynamics simulations were performed to investigate F continuously bombarding Si-terminated 3C-SiC(001) surfaces with incident energies of 10, 100 and 200 eV at normal incidence and room temperature. For an energy of 10 eV, deposition only occurs on the surface. For energies larger than 10 eV, accompanying the saturation of F uptake, a balance between F deposition from the incident atoms and F removal from the fluorinated substrate is established, while the steady-state etching is reached. The simulated results demonstrate that Si atoms in SiC are preferentially etched, which is in good agreement with experiments. The preferential etching of Si results in formation of a C-rich interfacial layer whose thickness increases with increasing incident energy. The analysis shows that Si-containing etch products are dominant. PACS 52.65.Yy; 81.65.Cf; 52.77.Dq     

Chemical sputtering,a discussion of mechanisms

Sputtering can be defined as the process whereby particles leave the surface as a direct consequence of the presence of incident radiation. When particles leave the surface as a result of receiving momentum from the collision cascade induced by the incident radiation, the process is called “physical sputtering”. If the incoming radiation (ions, electrons, or photons) induces a chemical reaction which leads to the subsequent desorp-tion of particles, the process could be classified as “chemical sputtering”. There are a number of molecules such as CH₄, CF₄, CF₃H, CF₃CI, etc., whose binding energy to a large variety of surfaces is believed to be only a few kcal/mole. Therefore, these molecules will not remain absorbed at room temperature. Consequently, if they are generated from surface atoms by radiation-induced processes, they will almost immediately desorb into the gas phase. This process is one type of chemical sputtering. Recent data obtained in plasma environments suggest that this type of reaction is a widely occurring phenomena: however, few systematic quantitative investigations of the subject have been completed. In this paper we will review the evidence for chemical sputtering and discuss mechanisms based on experimental information obtained for the chemical sputtering of silicon and SiO₂ under argon ion bombardment in the presence of a molecular beam of XeF2. Under these conditions, 25 or more silicon atoms can leave the surface per incident argon ion. About 75% of the silicon is emitted as SiF₄ (gas) and the rest leaves as silicon atoms or SiFx radicals. The total yield (silicon plus fluorine) is greater than 100 atoms/ion. The measured yields are a strong function of XeF₂ flux and a much weaker function of ion energy in the range 500-5000 eV. The chemical-sputtering yield for SiO₂ is smaller than that of silicon by about an order of magnitude, but it is still larger than the physical-sputtering yield. Moreover, SiO₂ is also sputtered by electrons. These results indicate that the incident radiation induces a chemical reaction between silicon and adsorbed fluorine which produces SiF₄, and the SiF4 is subsequently desorbed into the gas phase. We define this process as chemical sputtering. The large yields are probably a consequence of weak binding between the surface and the SiF₄ molecule.     

Pt(111)表面低能溅射现象的分子动力学模拟

利用嵌入原子方法的原子间相互作用势，通过分子动力学模拟，详细研究了贵金属原子在Pt (111)表面的低能溅射现象.模拟结果显示：对于垂直入射情况，入射原子的质量对Pt (11 1)表面的溅射阈值影响不大.当入射原子的能量小于溅射阈值时，入射原子基本以沉积为主 ；当入射原子的能量大于溅射阈值时，溅射产额随入射原子能量的增加而线性增大；当入射 原子能量达到200 eV时，各种入射原子的溅射产额都达到或接近1，此时入射原子主要起溅 射作用.溅射原子发射的角分布概率和溅射花样与高能溅射相类似.研究表明：与基于二体碰 撞近似的线性级联溅射理论不同，当入射原子能量大于溅射阈值时，低能入射原子的溅射产 额正比于入射原子的约化能量和入射原子与基体原子的质量比.通过对低能入射原子的钉扎 能力分析，提出了支配低能溅射的入射原子反射物理机理. 关键词： 分子动力学模拟、低能溅射     

入射能量对H2+与SiC表面相互作用影响的分子动力学模拟

用分子动力学方法研究了入射能量对 H2+与 SiC 样品表面相互作用的影响.模拟结果表明,在 H2+轰击 SiC 样品表面的初始阶段,样品中 H 原子的滞留量增加较快,其后,增加的速率减慢,并逐渐趋于饱和.入射能量越大,样品中 H 原子的滞留量也就越大.样品在 H2+的轰击下,样品 Si、C 原子会发生刻蚀.入射能量越...     

分子动力学模拟H原子与Si的表面相互作用

通过分子动力学模拟了入射能量对H原子与晶Si表面相互作用的影响. 通过模拟数据与实验数据的比较, 得到H原子吸附率随入射量的增加 呈先增加后趋于平衡的趋势. 沉积的H原子在Si表面形成一层氢化非晶硅薄膜, 刻蚀产物(H₂, SiH₂, SiH₃和SiH₄)对H原子吸附率趋于平衡有重要影响, 并且也决定了样品的表面粗糙度. 当入射能量为1 eV时, 样品表面粗糙度最小. 随着入射能量的增加, 氢化非晶硅薄膜中各成分(SiH, SiH₂, SiH₃)的量以及分布均有所变化. 关键词： 分子动力学 吸附率 表面粗糙度 氢化非晶硅薄膜     

Scattering of CO and N(2) molecules by a graphite surface

Measurements of angular distributions for the scattering of well-defined incident beams of CO and N(2) molecules from a graphite surface are presented. The measurements were carried out over a range of graphite surface temperatures from 150 to 400?K and a range of incident translational energies from 275 to over 600?meV. The behavior of the widths, positions and relative intensities of the angular distributions for both CO and N(2) were found to be quite similar. The experimental measurements are discussed in comparison with calculations using a classical mechanical model that describes single collisions with a surface. Based on the behavior of the angular distributions as functions of temperature and incident translational energy, and the agreement between measured data and calculations of the single-collision model, it is concluded that the scattering process is predominantly a single collision with a collective surface for which the effective mass is significantly larger than that of a single carbon atom. This conclusion is consistent with that of earlier experiments for molecular beams of O(2) molecules and Xe atoms scattering from graphite. Further calculations are carried out with the theoretical molecular scattering model in order to predict translational and rotational energy transfers to and from the molecule during scattering events under similar initial conditions.