Understanding gold-thiolate cluster emission from self-assembled monolayers upon kiloelectronvolt ion bombardment

This article focuses on the emission of organometallic clusters upon kiloelectronvolt ion bombardment of self-assembled monolayers. It is particularly relevant for the elucidation of the physical processes underlying secondary ion mass spectrometry (SIMS). The experimental system, an overlayer of octanethiols on gold, was modeled by classical molecular dynamics, using a hydrocarbon potential involving bonding and nonbonding interactions (AIREBO). To validate the model, the calculated mass and energy distributions of sputtered atoms and molecules were compared to experimental data. Our key finding concerns the emission mechanism of large clusters of the form MxAuy up to M6Au5 (where M is the thiolate molecule), which were not observed under sub-kiloelectronvolt projectile bombardment. Statistically, they are predominantly formed in high-yield events, where many atoms, fragments, and (supra)molecular species are desorbed from the surface. From the microscopic viewpoint, these high-yield events mostly stem from the confinement of the projectile and recoil atom energies in a finite microvolume of the sample surface. As a result of the high local energy density, molecular aggregates desorb from an overheated liquidlike region surrounding the impact point of the projectile.     

Electric polarizability of small metal spheres

The electric polarizability of small sodium spheres is calculated in a semiclassical approximation based on the spherical jellium model and on the Vlasov equation. Static and dynamic polarizabilities are compared for two kinds of mean field with a smooth and a sharp surface. It is deduced that the position of the centroid of the surface-plasmon resonance is not sensitive to the surface diffuseness of the mean field. Comparison with analogous quantum calculations suggests that an unrealistic feature of the zero-range exchange-correlation force employed in the local-density approximation might play an important role in increasing the static polarizability and decreasing the frequency of the surface-plasmon resonance.     

Bi-Directional Ion Emission from Massive Gold Cluster Impacts on Nanometric Carbon Foils

Carbon cluster emission from thin carbon foils (5-40 nm) impacted by individual Au(n) (+q) cluster projectiles (95-125 qkeV, n/q = 3-200) reveals features regarding the energy deposition, projectile range, and projectile fate in matter as a function of the projectile characteristics. For the first time, the secondary ion emission from thin foils has been monitored simultaneously in both forward and backward emission directions. The projectile range and depth of emission were examined as a function of projectile size, energy, and target thickness. A key finding is that the massive cluster impact develops very differently from that of a small polyatomic projectile. The range of the 125 qkeV Au(100q) (+q) (q ≈ 4) projectile is estimated to be 20 nm (well beyond the range of an equal velocity Au(+)) and projectile disintegration occurs at the exit of even a 5 nm thick foil.     

Ballistic Impact Behaviour of Glass/Epoxy Composite Laminates Embedded with Shape Memory Alloy (SMA) Wires

This paper aims to estimate the enhancement in the energy absorption characteristics of the glass fiber reinforced composites (GFRP) by embedding prestrained pseudo-elastic shape memory alloy (SMA) that was used as a secondary reinforcement. The pseudo-elastic SMA (PE-SMA) embedded were in the form of wires and have an equiatomic composition (i.e., 50%–50%) of nickel (Ni) and titanium (Ti). These specimens are fabricated using a vacuum-assisted resin infusion process. The estimation is done for the GFRP and SMA/GFRP specimens at four different impact velocities (65, 75, 85, and 103 m/s) using a gas-gun impact set-up. At all different impact velocities, the failure modes change as we switch from GFRP to SMA/GFRP specimen. In the SMA/GFRP specimen, the failure mode changed from delamination in the primary region to SMA-pull out and SMA deformation. This leads to an increase in the ballistic limit. It is observed that energy absorbed by SMA/GFRP specimens is higher than the GFRP specimens subjected to the same levels of impact energy. To understand the damping capabilities of SMA embedment, vibration signals are captured, and the damping ratio is calculated. SMA dampens the vibrations imparted by the projectile to the specimen. The damping ratio of the SMA/GFRP specimens is higher than the GFRP specimens. The damping effect is more prominent below the ballistic limit when the projectile got rebounded (65 m/s).     

Second-order theories for electron loss in fast collisions with He atoms

The momentum distribution of projectile electrons ejected in collisions with light targets is calculated within the second-order Born approximation for direct ionisation and within the electron impact approximation and the impulse approximation for electron capture to the target continuum. From comparison with available experimental data it is found that for forward emission angles the electron is well described by a projectile eigenstate, while at backward angles a target final state is more appropriate. At all angles the inclusion of simultaneous target excitation is very important.     

H(D) → D(H) + Cu(111) collision system: molecular dynamics study of surface temperature effects

All the channels of the reaction dynamics of gas-phase H (or D) atoms with D (or H) atoms adsorbed onto a Cu(111) surface have been studied by quasiclassical constant energy molecular dynamics simulations. The surface is flexible and is prepared at different temperature values, such as 30 K, 94 K, and 160 K. The adsorbates were distributed randomly on the surface to create 0.18 ML, 0.28 ML, and 0.50 ML of coverages. The multi-layer slab is mimicked by a many-body embedded-atom potential energy function. The slab atoms can move according to the exerted external forces. Treating the slab atoms non-rigid has an important effect on the dynamics of the projectile atom and adsorbates. Significant energy transfer from the projectile atom to the surface lattice atoms takes place especially during the first impact that modifies significantly the details of the dynamics of the collisions. Effects of the different temperatures of the slab are investigated in this study. Interaction between the surface atoms and the adsorbates is modeled by a modified London-Eyring-Polanyi-Sato (LEPS) function. The LEPS parameters are determined by using the total energy values which were calculated by a density functional theory and a generalized gradient approximation for an exchange-correlation energy for many different orientations, and locations of one- and two-hydrogen atoms on the Cu(111) surface. The rms value of the fitting procedure is about 0.16 eV. Many different channels of the processes on the surface have been examined, such as inelastic reflection of the incident hydrogen, subsurface penetration of the incident projectile and adsorbates, sticking of the incident atom on the surface. In addition, hot-atom and Eley-Rideal direct processes are investigated. The hot-atom process is found to be more significant than the Eley-Rideal process. Furthermore, the rate of subsurface penetration is larger than the sticking rate on the surface. In addition, these results are compared and analyzed as a function of the surface temperatures.     

Collision Dynamics of an Energetic Carbon Ion Impinging on the Stone-Wales Defect in a Single-walled Carbon Nanotube

By employing atomistic simulations based on an empirical potential model and a self-consistent-charge density-functional tight-binding method, the collision dynamics process of an energetic carbon ion impinging on the Stone-Wales defect in a single-walled carbon nanotube was investigated. The outwardly and inwardly displacement threshold energies for the primary knock-on atom in the Stone-Wales defect were calculated to be 24.0 and 25.0 eV, respectively. The final defect configuration for each case was a 5-1DB-T(DB=dangling bond) defect formed in the front surface of the nanotube. Moreover, the minimum incident energy of the projectile prompting the primary knock-on atom displacement was predicted to be 71.0 eV, and the time evolutions of the kinetic and potential energies of the projectile and the primary knock-on atom were both plotted to analyze the energy transfer process.     

Charge asymmetry effects in inner-shell alignment induced by ion-atom collisions

Charge asymmetry effects in an alignment of inner-shell vacancies induced by ion-atom collisions are studied theoretically. The corrections to the first order calculations of the alignment which depend on the sign of the projectile charge are discussed. The effect of these corrections on the differential alignment is large and can be studied in coincidence experiments with proton and antiproton beams of low energies. The calculations show that with increasing projectile energy the charge asymmetry effects decrease and become practically negligible when the projectile velocity is comparable with the velocity of the electron in the shell under consideration. The total alignment depends only weakly on the charge of the projectile. The calculated differences of the integralA g (2p ₃ ^2/?1 ) alignment produced by protons and antiprotons are not greater than 20% in the whole energy range under study.     

Collisions of slow polyatomic ions with surfaces: The scattering method and results

Surface-induced dissociation (SID) and reactions following impact of well-defined ion beams of polyatomic cations C₂H₅OH⁺, CH₄⁺, and CH₅⁺ (and its deuterated variants) at several incident angles and energies with self-assembled monolayers (SAM), carbon surfaces, and hydrocarbon covered stainless steel were investigated by the scattering method. Energy transfer and partitioning of the incident projectile energy into internal excitation of the projectile, translational energy of products, and energy transferred into the surface were deduced from the mass spectra and the translational energy and angular distributions of the product ions. Conversion of ion impact energy into internal energy of the recoiling ions peaked at about 17% of the incident energy for the perfluoro-hydrocarbon SAM, and at about 6% for the other surfaces investigated. Ion survival probability is about 30–50 times higher for closed-shell ions than for open-shell radical cations (e.g., 12% for CD₅⁺ versus 0.3% for CD₄⁺, at the incident angle of 60° with respect to the surface normal). Contour velocity plots for inelastic scattering of CD₅⁺ from hydrocarbon-coated and hydrocarbon-free highly oriented pyrolytic graphite (HOPG) surfaces gave effective masses of the surface involved in the scattering event, approximately matching that of an ethyl group (or two methyl groups) and four to five carbon atoms, respectively. Internal energy effects in impacting ions on SID were investigated by comparing collision energy resolved mass spectra (CERMS) of methane ions generated in a low pressure Nier-type electron impact source versus those generated in a Colutron source in which ions undergo many collisions prior to extraction and are essentially vibrationally relaxed. This comparison supports the hypothesis that internal energy of incident projectile ions is fully available to drive their dissociation following surface impact.     

CDW and CDW-EIS investigations in an independent electron approximation for the resonant double electron capture by swift He2+ in helium

The CDW and CDW-EIS approximations are used to study the resonant double electron capture by He²⁺ in helium at intermediate and high impact velocities. Calculations of the total cross sections as a function of the impact energy are performed in an Independent Electron Approximation. A very good agreement with recent experimental data for capture into all states of the projectile is found although neither angular nor dynamical electron correlations are included in the calculations.     

Sensitivity of ion induced X-ray analysis for surface layers on thick silicon substrates

Surface contaminations on silicon slices and contaminations in surface layers deposited on thick silicon substrates were investigated by means of ion induced X-ray analysis. Sensitivity limits of foreign atoms are given for proton and nitrogen impact in dependence of projectile energies. They were derived using calculated K-shell ionization cross-sections as well as measured X-ray and γ-ray background spectra. The validity of the model used for characteristic cross-section calculations was proved by comparing the theoretical with experimental results. Some examples are given for the analysis of the basic material for the production of semiconductor electronic devices.     

Classical trajectory study of orientation and alignment effects for charge exchange processes in H+- Na*(3p m) collisions

The orientation and alignment effects for charge exchange in H⁺ + Na*(3p) collisions are studied using the classical trajectory Monte-Carlo method in the energy range from 1 to 8 keV. For Na*(3p _-1) → H*(2s, 2p _±1) transitions a large orientation effect is predicted by the probability functions, in very good agreement with semiclassical calculations. Angular differential cross sections are also calculated and interpreted using the impact parameter dependence of the proton deflection angle. They predict left-right asymmetry in agreement with semiclassical calculations or experimental results, but slightly smaller. Another geometry, not experimentally realized, is considered, where the proton velocity is parallel to the quantization axis of the p _±1 oriented states. Charge exchange from different aligned states with respect to the direction of the projectile velocity is also investigated, but the alignment effects are not as well described as the orientation effects. Total cross sections from oriented or aligned states with cylindrical symmetry around the projectile velocity direction are calculated and allow the hypothesis of velocity matching to be tested.     

Enthalpy of solution of phenoxyalcohols solubilized in aqueous sodium dodecylsulfate and hexadecyltrimethylammonium bromide solutions

The enthalpy of solution of phenoxy 2-ethanol, 1 phenoxy,-3 propanol and benzylalcohol was determined at 25 °C in aqueous sodium dodecylsulfate and hexadecyltrimethylammonium bromide solution, up to 0.2 mol/kg surfactant concentration. Using the pseudo-phase model, the standard enthalpy of transfer and the partition coefficient of the alcohols between micelle and water are calculated. The latter quantity is found to be systematically larger when derived from enthalpy than from free energy measurements. Using the so-called compensation plot, the solution thermodynamics of aromatic and aliphatic alcohols in aqueous sodium dodecylsulfate and in the octane+water systems are compared. Aromatic alcohols display an anomalous behavior in the octane+ water system but not in the micellar one.The standard enthalpy of solution of various alcohols presents, when plotted against hexadecyltrimethylammonium bromide concentration, a shoulder in the region around 0.05 mol/kg; a discussion is presented on the evidence for alleged micellar structural changes in aqueous micellar systems.     

Molecular sputter depth profiling using carbon cluster beams

Sputter depth profiling of organic films while maintaining the molecular integrity of the sample has long been deemed impossible because of the accumulation of ion bombardment-induced chemical damage. Only recently, it was found that this problem can be greatly reduced if cluster ion beams are used for sputter erosion. For organic samples, carbon cluster ions appear to be particularly well suited for such a task. Analysis of available data reveals that a projectile appears to be more effective as the number of carbon atoms in the cluster is increased, leaving fullerene ions as the most promising candidates to date. Using a commercially available, highly focused C₆₀^q+ cluster ion beam, we demonstrate the versatility of the technique for depth profiling various organic films deposited on a silicon substrate and elucidate the dependence of the results on properties such as projectile ion impact energy and angle, and sample temperature. Moreover, examples are shown where the technique is applied to organic multilayer structures in order to investigate the depth resolution across film-film interfaces. These model experiments allow collection of valuable information on how cluster impact molecular depth profiling works and how to understand and optimize the depth resolution achieved using this technique.     

Resonant double electron capture by fast He2+ from helium: the first-order Born approximation with correct boundary condition

The first-order Born approximation with correct boundary condition is applied in a study of the resonant double electron capture by fast He²⁺ from helium. A configuration-interaction wave function is employed to describe the ground state of helium. Total cross section as a function of the impact energy is calculated and compared with experimental and theoretical values.     

添加CaO的准东煤中温水蒸气气化特性的研究

以内在碳捕集气化为背景,利用加压热重分析仪开展CaO对准东煤中温(700-750℃)水蒸气气化反应动力学特性的影响研究,采用氮气吸附仪对准东煤焦的比表面积进行测定,并对煤中不同赋存形态碱金属含量采用电感耦合等离子体发射光谱仪(ICP-OES)进行测定。结果表明,准东煤中的可溶性碱金属元素能有效催化气化反应,引入的二氧化碳吸收剂CaO与碱金属间表现出协同催化作用。水洗后的准东煤焦活性最高,添加CaO后的气化活性最好。Ca/C物质的量比对准东煤气化特性的影响研究表明,CaO的添加存在饱和量,Ca/C物质的量比为1.0较为合适。利用均相模型(HM)、缩核模型(SCM)以及修正体积模型(MVM)对反应动力学实验数据进行拟合,结果表明,修正体积模型可以较好地体现添加CaO的准东煤中温水蒸气气化反应动力学特性,由此获得反应活化能为160.90 kJ/mol。     

Case for projectile kinetic energy gain in stopping power studies

Under certain collision conditions, a swift ion projectile colliding with a target will gain rather than lose kinetic energy, contrary to the standard conception of stopping power. In this work, we consider the conditions for such a collision such that the energy loss is negative, that is, that there will be projectile kinetic energy gain. In particular, for a target initially in the ground state we find that the projectile gains kinetic energy only when charge exchange and de‐excitation processes are involved. This occurs when the electron affinity of the projectile is larger than the ionization potential of the target. Consequences of this effect are analyzed. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 215–221, 2003