Extremely low-energy projectiles for SIMS using size-selected gas cluster ions

A new cluster time-of-flight secondary ion mass spectrometry (TOF-SIMS) was developed using a size-selected gas cluster ion as a projectile. Since a large gas cluster ion can generate many low-energy constituent atoms in a collision with the surface, it causes multiple and ultra low-energy sputtering. The mean kinetic energy of constituent atoms is provided by dividing the acceleration energy of the gas cluster ion by the number of constituent atoms. Therefore, the sputtering can be controlled to minimize the decomposition of sample molecules and substrate material by precisely adjusting the number of constituent atoms (the cluster size) and/or acceleration energy of the gas cluster ion. The cluster size was selected on the basis of the time-of-flight method using two ion deflectors attached along the ion-beam line. A high resolution of 11.7 was achieved for the cluster size/size width (M/ΔM) of Ar-cluster ions.     

No enhanced electron emission from high-density atomic collision cascades in metals

Ion-induced secondary electron emission was studied for vanadium and niobium cluster ions at incident energies between 12.5 and 25 keV. The number of electrons ejected per incident atomic particle was found to be solely a function of the velocity, being independent of the number of atoms in the cluster. In spite of the extremely high energy densities created by cluster ion impact, there is no enhanced electron emission from collision spikes. This is discussed in terms of energy coupling between the system of lattice atoms and that of the electrons in the solid.     

Thermonuclear fusion in a strong laser field

Thermonuclear fusion induced by the irradiation of solid deuterated cluster targets and foils with fields of strong femtosecond and picosecond laser pulses is discussed. The thermonuclear-fusion process D(d, n)³He in a collision of two deuterons at an energy of 50 to 100 keV in a deuterium cluster target irradiated with a strong laser pulse is discussed. A theory of thermonuclear fusion proceeding upon the irradiation of clusters formed by deuterium iodide (DI) molecules with the field of a superintense femtosecond laser pulse is developed. This theory is based on an above-barrier process in which the sequential multiple inner ionization of atomic ions within a cluster is accompanied by field-induced outer ionization. The yield of neutrons from thermonuclear fusion in a deuteron-deuteron collision after the completion of a laser pulse is calculated. The yield of neutrons is determined for the thermonuclear-fusion reaction proceeding in the interaction of an intense picosecond laser pulse with thin TiD₂ foils. A multiple ionization of titanium atoms at the front edge of the laser pulse is considered. The heating of free electron occurs in induced inverse bremsstrahlung in the process of electron scattering on multiply charged titanium ions. The yield of alpha particles in the thermonuclear-fusion reaction involving protons and ¹¹B nuclei that is induced in microdrops by a strong laser field is determined. Experimental data on laser-induced thermonuclear fusion are discussed.     

Molecular dynamics simulations of nanopore processing in a graphene sheet by using gas cluster ion beam

The formation of a nanopore in a graphene sheet by collision with an argon cluster is simulated using molecular dynamics method. The number of removed carbon atoms and the size of the nanopore are obtained as a function of the kinetic energy of the cluster. In contrast to nanosculpting with a monomer ion beam, the size of the nanopore that is created by one shot of the cluster varies because of the variety of atom configuration. However, the mean size of the nanopore can be controlled over a wide range only by changing the kinetic energy of the cluster. This implies that the cleaning and processing of the graphene sheet may be realized simultaneously by changing the acceleration energy of the cluster.     

低能原子簇轰击金属簿膜引起的级联碰撞(Ⅱ)——注入簇原子的加速

研究了能量为１ｋｅＶ／ａｔｏｍ的金原子簇和０．２ｋｅＶ／ａｔｏｍ的铝原子簇轰击金薄膜产生的级联碰撞。用分子动力学模拟计算了注入靶后的簇原子能量分布及其随时间的演化。结果表明，在原子簇注入引起的级联碰憧中，簇原子除了将能量传递给靶原子外，尚有可能破加速。簇原子的最高能量可大于它的初始能量；分析了原子簇注入引起的多次碰撞效应，并用经典力学守恒定律计算了一个簇原子发生二次散射后的能量增益，用以解释注入原子的加速机制。 关键词：     

Model for the emission of quasi-thermal atoms during sputtering of metals in the nonlinear collision cascade regime

The ion sputtering of metals and frozen inert gases in the nonlinear collision cascade regime, where the density of the energy released in the bulk of the thermal peak exceeds the critical medium temperature, initiates the emission of quasi-thermal atoms. The energy spectrum of such atoms is substantially shifted toward low energies and is not described by a Maxwellian distribution. A simple emission model is proposed on the assumption of collisional motion of sputtered atoms in their flight from a target, and this model is used to derive an analytical formula for the calculation of the energy spectra of quasi-thermal atoms. A comparison of the calculated energy spectra of indium, krypton, and xenon atoms and the spectra measured during ion sputtering of indium and frozen inert gases in the nonlinear collision cascade regime shows their agreement at reasonable values of fitting parameters.     

Charge Composition of Ions in a Cluster Plasma Formed under the Action of a High-Power Laser Pulse

A model describing the ionization of atoms and ions in a cluster under irradiation by a short laser pulse has been constructed. It is shown that the electron-impact ionization weakly affects the final charge composition, and the main mechanism of ion formation in the cluster is the over-barrier ionization by an electric field. The electric field acting on atoms and ions is the result of combined action of the external laser field and the intrinsic electric field of the cluster. The key parameters of the cluster beam and the laser pulse, which determine the properties of the charge composition of the cluster plasma, have been established.     

Quantum size effect during the electron exchange between a negative hydrogen ion and a cluster of aluminum atoms

This paper deals with the study of the process of electron exchange between a negative hydrogen ion and a cluster of aluminum atoms. The method of wave packet propagation not using perturbation theory is applied to solve the problem. It is shown that the quantum size effect exists during the electron exchange between a negative hydrogen ion and an atomic cluster.     

The Fate of a Cluster Colliding onto a Substrate Dissipation of Translational Kinetic Energy

The fate of clusters emitted onto a substrate falls into several categories including repulsion, soft landing, migration, and explosion, depending on the interaction between the cluster and the substrate. This dynamic behavior of the clusters controls thin-film formation processes from clusters such as cluster ion beam deposition and chemical vapor deposition. Here we describe the collision processes of both Al and Au clusters with a kinetic energy of 0.56 eV/atom onto an amorphous SiO₂ substrate studied by molecular dynamics simulation, focusing on the dissipation of translational kinetic energy during the collision process. The simulation elucidated that the activation of thermal vibrational energy of the substrate promoted the sticking of the colliding clusters on the substrate. This result suggests that the dissipation of the translational kinetic energy of the colliding cluster is one of the factors that determine the structure formed on a substrate from vapor phase.     

Distributions of ions in a cluster plasma created by a laser pulse

Energy and charge distributions of ions are calculated for a cluster beam irradiated by a high-power ultrashort laser pulse. It is shown that the self-consistent field of a cluster ionized by the laser beam strongly affects the characteristics of the ion distributions obtained after the cluster explodes. The mean concentration of atoms bound into clusters in a beam, the cluster size distribution, and the focal-spot diameter are found to have a weak effect on both energy and charge distributions of the ions, whereas the energy spectrum of the produced ions is determined by the mean cluster size.     

On the Theory of Neutral and Charged Cluster Ion Sputtering of Metal

A calculation method for large neutral and/or charged cluster (with number of atoms N > 5) elastic sputtering of a metal during ion bombardment is proposed. The result is presented as a simple assymptotic formula for the probability of cluster ejection and cluster charge state. A conclusion is made on the exponential nature of the dependence of the total cluster yield on the number of atoms the cluster consists of.     

载能团簇离子诱发的非线性二次离子发射

载能团簇离子进入固体时, 由于集体相互作用,在入射路径上产生非常高的能量沉积密度。 实验发现, 载能团簇离子的作用结果并不等于团簇中各原子独立作用的总和, 而是具有非线性效应。 就二次离子发射而言, 这种非线性通常与团簇的能量、 团簇的大小、 离子的电荷态以及靶物质的结构有关。 通过研究二次离子发射有助于理解载能团簇离子与物质相互作用过程中的能量沉积与释放机制。 Using energetic cluster as projectile is a unique way to produce simultaneous impacts of several atoms and deposit extremely high energy density in a very small area. The cluster impingement on solids has exhibited some non linear effects not presented in collisions of individual atoms with those solids. The study of the secondary ion emissions can give insight into the energy deposition and relaxation steps of the cluster solid interaction. The dependence of the yields of secondary ion emission induced by clusters on the energy of clusters,cluster sizes,charge states and material structures of the targets was reviewed in this paper.     

低能CU6团簇在CU(001)表面和AU(001)表面沉积的分子动力学模拟研究

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Collision-induced dissociation of hydrated adenosine monophosphate nucleotide ions: protection of the ion in water nanoclusters

Fragmentation of singly charged anions of adenosine 5'-monophosphate (AMP-) induced by collisions with neutral atoms (Ne, Na) has been studied at a collision energy of 50 keV. The experiments were performed with isolated AMP- as well as with AMP- anions nanosolvated in a cluster with a given number m of water molecules. In the first case, the dominant fragmentation channels concern the loss of adenine, PO3- and H2PO4-. In the latter, loss of water molecules becomes the dominating process, and the AMP- ion is fully protected when m is larger than approximately 13. The observed fragment distributions are well described with the model of an evaporative ensemble.     

High-intensity Si cluster ion emission from a silicon target bombarded with large Ar cluster ions

Secondary ions emitted from Si targets were measured with a quadrupole mass spectrometer under large Ar cluster and monomer ion bombardment. Incident ion beams with energies from 7.5 to 25 keV were used and the mean size of the Ar cluster ion was about 1000 atoms/cluster. Si_n⁺ ions with n values up to n = 8 were detected under Ar cluster ion bombardment, whereas Si cluster ions were scarcely detected under Ar monomer ion bombardment. These cluster ion yields showed the power law dependence on the cluster size.     

A simple calculation of the angular distribution of sputtered atoms

Angular distributions of atoms sputtered with fast ions from smooth amorphous targets are estimated by a straightforward treatment of the collision cascade. When the geometrical situation at oblique ion impact is considered, the angle of maximum emission and the sputtering yield in dependence on the angle of ion incidence are described.     

Charged cores in ionized $\mathsf{{^{4}He}}$ clusters III: A quantum modeling for the collisional relaxation dynamics

When a pure ⁴He droplet is ionized by electron impact, the most abundant fragment detected in mass spectra after ionization is He₂ ⁺ . All the models that have been proposed thus far to explain the experimental evidence therefore involve the formation of the He₂ ⁺ molecular ion. The understanding of the interactions between this ion and the surrounding He atoms in the cluster and of their dynamical behavior during cluster break-up is an important element for the modeling of the cluster evolution after the ionization event. In previous works [1,2] we have computed and described the Potential Energy Surface (PES) of the electronic ground state for the He₃ ⁺ system that provides the required forces between He₂ ⁺ and He. After ionization He₂ ⁺ is presumably formed by association of an He ⁺ and any of the neutral atoms in the cluster via a 3-body collision process. The ensuing vibrational quenching of the hot molecular ion may release the energy necessary to evaporate the entire droplet, or most of it, and give the fragmentation patterns detected by experiments. We present here a model quantum dynamics that generates vibrational deexcitation cross-sections and the corresponding rate coefficients for the collision of He₂ ⁺ with He. A timescale of the cluster evaporation due to vibrational relaxation is estimated and the present findings are compared with earlier studies on the same system.Received: 15 June 2004, Published online: 24 August 2004PACS: 31.15.Qg Molecular dynamics and other numerical methods - 34.50.Ez Rotational and vibrational energy transfer - 36.40.Wa Charged clusters     

Inelastic processes in the collision of fast multiply charged ions with a hydrogen molecule

The collision of a fast (including relativistic) multiply charged ion with the simplest (hydrogen) molecule is considered on the basis of the eikonal approximation. The analytical formula for the reaction cross section, i.e., the total cross section of all inelastic electron processes in the domain where the Born approximation is inapplicable, is derived. The cross section is compared to the corresponding doubled inelastic cross sections for collisions of multiply charged ions with hydrogen atoms calculated within perturbative and nonperturbative approaches.     

A comparison of radiation from sputtered atoms with radiation from projectiles scattered on oxidized magnesium at varying angle of incidence

The intensity of light emitted from sputtered atoms and neutralized, scattered primary ions, excited during 4 keV Ne⁺ and Ar⁺ bombardment of oxidized magnesium has been measured as a function of the incidence angle. It was found that the photon yield of sputtered atoms increases with the angle of incidence much more rapidly than the theoretical sputtering yield and the photon yield of scattered projectiles. In order to explain the experimental results a numerical approach was made based on the following assumptions: (1) the sputtered atom can be excited when it crosses the surface after getting the momentum from the collision cascade; (2) at oblique incidence the sputtered excited atom can be directly emitted after a gentle collision between the incident ion and the surface atom; (3) the neutralized primary ion can only be excited in a violent collision with the surface atom.     

Corona Discharge Ion Mobility Spectrometry of Ten Alcohols

利用电晕放电离子源离子迁移谱装置对十种醇类有机物的迁移谱进行了研究. 在质子化的水合氢离子为反应离子和纯净空气为迁移气体的条件下,各种醇类有机物在其离子迁移谱中都获得了不同的产物离子峰. 利用指数稀释法得到各种样品的检测限在几个pmol/L量级. 利用所得到的离子迁移谱,以硬球碰撞为模型,得到了大气压下离子-分子相互作用的多个化学物理参数,包括离子分子碰撞、扩散系数、碰撞速率常数和离子半径.