Short-laser-pulse-driven emission of energetic ions into a solid target from a surface layer spalled by a laser prepulse

An efficient emission of picosecond bunches of energetic protons and carbon ions from a thin layer spalled from a organic solid by a laser prepulse is demonstrated numerically. We combine the molecular dynamics technique and multi-component collisional particle-in-cell method with plasma ionization to simulate the laser spallation and ejection of a thin (∼20–30 nm) solid layer from an organic target and its further interaction with an intense femtosecond laser pulse. In spite of its small thickness, a layer produced by laser spallation efficiently absorbs ultrashort laser pulses with the generation of hot electrons that convert their energy to ion energy. The efficiency of the conversion of the laser energy to ions can be as high as 20%, and 10% to MeV ions. A transient electrostatic field created between the layer and surface of the target is up to 10 GV/cm. Received: 13 March 2001 / Accepted: 20 March 2001 / Published online: 20 June 2001     

Optimum ion implantation and annealing conditions for stimulating secondary negative ion emission

The type, energy, ion dose, and heating temperature required to ensure a stable minimum work function of a surface in one experimental cycle (at least 2–3 min) are determined. Secondary ion mass spectrograms are recorded using Cs⁺, Ba⁺, and Ar⁺ ions. Cu, Al, and Mo samples are studied. The optimum ion implantation conditions and the activation temperature that provide a stable minimum work function of the sample surfaces are found. The samples implanted by Ba⁺ ions withstand higher temperature and current loads than the samples implanted by Cs⁺ ions. However, the work function in the case of Cs⁺ ions decreases stronger (to 1.9 eV). It is shown that neutral sputtered particles do not leave the surface at eφ ≤ 1.85–1.90 eV.     

Stability of negative ions near the surface of a solid

Stationary states of molecular negative ions (anions) near the surface of a solid are investigated. The lone electron is assumed to interact with a diatomic molecule and the surface of the solid. The energies of electron levels are determined by solving the 2D Schrödinger equation. It is shown that its stable solutions exist at distances from the surface greater than some critical distance, otherwise the electron is detached from the anion. In the case of attraction between the electron and the solid, the interaction potential between the anion and the solid appears to have the Lennard-Jones form and the ion is separated from the surface by some equilibrium distance.     

The simulation of two-beam high-dose ion implantation into solid targets

A physicomathematical model and a BEAM2HD program for the dynamic simulation of one-and two-beam high-dose ion implantation into multilayer and multicomponent targets are developed. The number of target layers is no more than three, and the number of sorts of atoms in each of the layers is no more than seven. The simulation is performed by the Monte Carlo method. Numerical results for the formation of C _x→3N _y→4 superhard layers by two-beam high-dose implantation of nitrogen ions into the Si₃N₄/C/Si₃N₄/Si system are presented.     

Selective ion heating by energetic ion beam injection into a plasma

High energy neutral beam injected into a plasma has been predicted to be susceptible to microinstabilities. If the growth time of these microinstabilities is shorter than the collisional relaxation time, then the energy transfer from the beam to the plasma may take place, in the initial phase, by collisionless wave-particle interaction. We report here an experiment where this effect has indeed been observed. Here ion Bernstein waves excited by an energetic ion beam lead to selective ion heating of the plasma in a collisionless manner.     

二次电子发射和负离子存在时的鞘层结构特性

建立了包括电子、离子、器壁发射二次电子以及负离子多种成分的等离子体无碰撞鞘层的基本模型,讨论了二次电子发射和负离子对1维稳态等离子体鞘层结构的影响,并且分析了多种成分等离子体鞘层内的二次电子和负离子的相互作用。结果表明：二次电子发射系数的增加和负离子含量的增加,都将导致鞘层的厚度有所减小;二次电子发射系数超过临界发射系数之后,鞘层不再是离子鞘。随着器壁材料二次电子发射系数的增加,鞘层中的负离子密度分布也逐渐增加;负离子的增加,导致二次电子临界发射系数有所增加。另外,在等离子体鞘层中二次电子发射和负离子的存在,也影响着鞘层中电子的放电特性与器壁材料的腐蚀。     

二次电子发射和负离子存在时的鞘层结构特性

 建立了包括电子、离子、器壁发射二次电子以及负离子多种成分的等离子体无碰撞鞘层的基本模型,讨论了二次电子发射和负离子对1维稳态等离子体鞘层结构的影响,并且分析了多种成分等离子体鞘层内的二次电子和负离子的相互作用。结果表明：二次电子发射系数的增加和负离子含量的增加,都将导致鞘层的厚度有所减小;二次电子发射系数超过临界发射系数之后,鞘层不再是离子鞘。随着器壁材料二次电子发射系数的增加,鞘层中的负离子密度分布也逐渐增加;负离子的增加,导致二次电子临界发射系数有所增加。另外,在等离子体鞘层中二次电子发射和负离子的存在,也影响着鞘层中电子的放电特性与器壁材料的腐蚀。     

Anomalous enhancement of negative sputtered ion emission by oxygen

Enhancement of negative sputtered ion yields by oxygen (either O⁺₂ bombardment or O₂ gas with Ar⁺ bombardment) is demonstrated for Si^?, As^?, P^?, Ga^?, Cu^? and Au^?, sputtered from a variety of matrices. Because oxygen also enhances positive ion yields of the same species, this effect cannot be simply explained on the basis of existing sputtered ion emission models. To rationalize these phenomena, a surface polarization model is developed which invokes localized electron emissive or electron retentive sites associated with differently oriented surface dipoles in the oxygenated surface. Such sites are considered to dominate the emission of negative and positive ions respectively. The model is shown to correctly predict that Au⁺ and Au^? ion yields are much more strongly enhanced by oxygen in dilute Au-Al alloys than in pure gold.     

Enhancement of the secondary ion emission induced by fast clusters

The emission yields of the secondary ions are measured by using a conventional time of flight (TOF) technique under bombardments of Mg and C₂, Ni and Si₂ with different energies, and C_n, Si_n and Ni_n (n = 1-3) with the different charge states and with energy of 1.5 MeV per atom, respectively. For the bombardments of C_n, Si_n and Ni_n, the enhancements of the secondary ion emissions increase with increasing cluster sizes and charge states. For the bombardments of Mg and C₂, Ni and Si₂, although the mass and the nuclear charges of C₂ and Si₂ are the same as or equivalent to Mg or Ni, respectively, the enhancements of the secondary ion emissions induced by the clusters of C₂ and Si₂ in a wide energy range are also clearly indicated. The instantaneous collective interaction of the cluster constituents plays an important role in the secondary ion emissions.     

Observation of the emission of positive and negative ions in triple and quadruple collisions in a solid under bombardment with keV argon ions

Particles that leave a solid as a result of several consecutive binary collisions are detected in the energy spectra of positive and negative ions emitted upon the irradiation of C, Si, Ge, and In targets with 2-to 5-keV Ar⁺ ions. The appearance of a spectral structure due to the sequence of three and four collisions in the solid can be attributed to the selective role of the surface, which is similar to the channeling effect in crystals.     

Experimental investigations into secondary electron-electron emission from the surface of vacuum chambers

An experiment on measuring the secondary electron yield (SEY) of samples coated with titanium nitride (TiN₂) is in progress at the Recuperator test bench at the Dzhelepov Laboratory of Nuclear Problems, Joint Institute for Nuclear Research. This work is related the problem of electron-cloud formation in the vacuum chambers of accelerators and is of practical importance for the NICA project. The results of the experiment on the SEY measurement will make it possible to choose the most appropriate material for coating the vacuum chamber. In this experiment samples of stainless steel with titanium nitride coating and without any coating are compared.     

Study of nitrogen ion implantation and diffusion phenomena on thin chromium layers followed by the atomic force microscopy and secondary ion mass spectroscopy techniques characterization

This research investigates the effect of ion implantation dosage level and further thermal treatment on the physical characteristics of chromium coatings on Si(1 1 1) substrates. Chromium films had been exposed to nitrogen ion fluencies of 1 × 10¹⁷, 3 × 10¹⁷, 6 × 10¹⁷ and 10 × 10¹⁷ N⁺ cm⁻² with a 15 keV energy level. Obtained samples had been heat treated at 450 °C at a pressure of 2 × 10⁻² Torr in an argon atmosphere for 30 h. Atomic force microscopy (AFM) images showed significant increase in surface roughness as a result of nitrogen ion fluence increase. Secondary ion mass spectroscopy (SIMS) studies revealed a clear increased accumulation of Cr₂N phase near the surface as a result of higher N⁺ fluence. XRD patterns showed preferred growth of [0 0 2] and [1 1 1] planes of Cr₂N phase as a result of higher ion implantation fluence. These results had been explained based on the nucleation-growth of Cr₂N phase and nitrogen atoms diffusion history during the thermal treatment process.     

Determination of the negative ion yield of copper sputtered by cesium ions

Abstract

This paper describes the determination of secondary ion yields for negative ions obtained by bombardment of copper by cesium ions. Stable and reproducible surface conditions are reached by high rate sodium deposition simultaneously with sputtering. An optimum thickness of sodium corresponding to about one monolayer is found. Total negative ion yields K ^? _Σ are measured by a double modulation technique. Individual negative ion yields K ^? _i are then found by mass spectrometrically determining the various negative ion intensities, the sum of which relates linearly to K ^? _Σ. This method is based on the assumption of an equal angular and energy distribution of all sputtered negative ions. Data are given for K^? _Σ and K ^? _Cu and K^? _O. The dependence of K ^? _i on primary ion energy (500 to 2500 eV) is similar to ordinary sputtering which points to the same basic mechanism in both cases.     

Electron emission from a metal surface bombarded by highly charged ions

Electron emission yields and energy spectra for impact of Ar^q+ (1⩽q⩽12) and Ta^q+ (7⩽q⩽21) ions on clean tungsten and gold are presented. The contribution of Auger emission processes have been investigated as function of ion energy, cleanness of surfaces and ion charge state.     

Analysis of the outermost atomic layer of a surface by low-energy ion scattering

Noble gas ion scattering is used to study the surface of solid targets. It is shown that this technique can be used to obtain a mass spectrum of the first atomic layer of the surface. Since the outermost atoms will largely determine the chemical reactivity and physical properties of the surface, this is an important property for an analytic tool to have. In addition to reflected ions, high-energy recoil ions are sometimes observed.Both the reflected and the recoil ions provide information about the atomic structure of the surface. The various possibilities of ion scattering are demonstrated for bromine, oxygen, sodium and potassium adsorbed on a Si(111) surface, for halogens adsorbed on Ni(100) and for GaP (110), (111) ($\text{111}$) surfaces. The influence of thermal vibrations of surface atoms and of electronic excitation on the spectra is discussed.     

Femtosecond electron dynamics on solid surfaces probed by low energy ion scattering and stimulated desorption of secondary ions

The cations emission from condensed matter surfaces has been investigated on the basis of localization and delocalization of valence hole(s) in the femtosecond timescale. The yield of scattered H⁺ (E₀=100 eV), though negligibly small from the Pt(1 1 1) substrate, increases markedly when Ar is adsorbed on it, indicating the localization of a valence (H⁺ 1s) hole on the physisorbed Ar layer. However, the yield of H⁺ scattered from a thick H₂O layer is considerably small relative to that from Ar and CO layers. The delocalized nature of a valence hole in water ice is caused by some covalency in hydrogen bonds. Hydrated protons, H⁺(H₂O)_n, n=1,2,…,10, are emitted efficiently in electron stimulated desorption from water molecules adsorbed on the Ar layer; the ion yields are highest at the initial adsorption stage and decay steeply with increasing coverage. Coulombic repulsion between the hydrated protons confined in physisorbed nanoclusters is responsible for the explosive ion emission.