Cluster emission under ion bombardment of metallic targets

Mass spectroscopic studies of the neutral particles sputtered by Ar⁺ ions at 8 keV from polycrystaline samples have been performed, using non-resonant laser ionization and subsequent time-of-flight mass spectroscopy. Besides sputtered atoms, also dimer and trimer contributions in the order of 10^–1 to 10^–2 and 10^–3 to 10^–4, respectively, are found in the sputtered flux. The data obtained here together with previously published data by other groups for different bombarding energies provide strong support for the validity of the recombination model.     

Structural changes of metallic surfaces induced by ultrasound

The effect of ultrasounds on the surface of metallic plates with cubic faced centered (Al, Ag, Cu) or hexagonal (Zn) structure was studied. Treated surfaces were strongly attacked: a micrometric roughness and a superficial oxidation (characterized by roughness measurements and scanning electron microscopy) were induced. X-ray diffraction analyses showed a preferential reorientation of the maximum density planes for both crystallographic structures.     

Photon emission from rare gas ion bombardment of metal surfaces

Metal surfaces (Mg, Cu, Zr, Mo) are bombarded with He⁺, Ne⁺ and Ar⁺ in the energy range of 400 eV to 8 keV. Radiation from scattered projectiles and sputtered target particles is observed between 200 and 700 nm. It is shown that most of the radiating particles originate from surface collisions. Auger neutralization, resonance tunneling and direct electron transitions are the important electronic processes involved.     

Structure sensitive factors in molecular cluster formation by ion bombardment of single crystal surfaces

The dynamics of molecular cluster formation from a solid bombarded by a 600 eV Ar⁺ ion have been studied classically by computer simulation. The dimers and trimers are found to establish their identity as clusters within interaction range of the solid, but not by a direct ejection of a bound molecule. The Cu₂/Cu and Cu₃/Cu ratios are found to be strongly dependent on crystal orientation. The (111) face is 2–3 times more likely to produce multimers than the (100) face. We find 9 trimers from (111) but none from (110). The relationship between cluster composition and the original arrangement of those atoms on the surface is presented in detail. We find that each multimer forms from atoms that originate within a roughly circular region of area ～70 Ų or less. This region is not necessarily centered on the ion impact point. A consequence of this observation is that dimers can consist of atoms that were several Ångströms apart on the surface but that most trimers contain at least one nearest neighbor pair of atoms. The calculated energy distribution for the dimers matches well with similar experimental studies.     

Calculation of positronium emission fraction from metallic surfaces

Abstract

Theoretical calculations of positronium emission fraction are compared with experiments. The results show that positronium emission fraction is quite sensitive to screening effects. Thus, positronium formation will be a very good example for testing dynamical screening effects. An experimental angle-resolved energy spectrum of positronium is required for more detailed discussion.     

Electron bombardment induced desorption of oxygen from LiNbO3

Low energy (1 keV) electron bombardment of LiNbO₃ single crystals causes the emission of O⁺ ions with a kinetic energy of 20 eV. The emission process can be identified as a Knotek-Feibelman mechanism, whereby the creation of Nb 4p core-holes is followed by an interatomic Auger process which leaves oxygen in a highly repulsive state. This oxygen subsequently desorbs.     

Electron and ion emission from liquid metal surfaces

The application of positive or negative electric fields to small-area liquid metal surfaces leads to very high brightness DC ion or pulsed electron emission. The stabilization of a cone-shaped structure by the balance of electrostatic and surface tensions forces is described. Electron and ion emission occurs by field emission and field evaporation mechanisms     

Terahertz emission from a metallic surface induced by a femtosecond optic pulse

Results of experimental and theoretical investigations on generation of terahertz radiation at the interaction of femtosecond laser pulses with a metal surface are presented. Investigations are performed with the laser pulse intensities higher compared with that used in papers [Opt. Lett.29, 2674 (2004); Opt. Lett.30, 1402 (2005)]. The most effective generation is observed for p-polarized optical pulses with incidence angles in the range 5°-10° (from the surface), depending on the kind of metal. For the copper, the exponential growth of terahertz pulse energy with the increase of optical pulse energy was registered. Theoretical interpretation for some of the experimental results is proposed based on the model of free electrons in metal.     

Autoelectronic emission of metallic films induced by picosecond pulses of infrared laser radiation

The well-known defects of the surface of a solid, microscopic projections and spikes, play a decisive role in electron emission induced by an electric field. If there are mobile electrons of holes in the solid, then the electric field is enhanced by a factor of 10–100 at the tip of a microscopic projection. This effect was discovered in electrostatics more than a century ago. In turn, the probability of tunnel emission of an electron from a metal into a vacuum is an exponential function of the electric field strength. Correspondingly the electron emission current density at the tip of a microscopic spike can be larger than that on a smooth surface by an astronomical factor. This effect is particularly strikingly manifested when picosecond pulses of infrared laser radiation of moderate power are used to initiate autoelectronic emission. Relative to a smooth surface, the emission current density is enhanced by hundreds of orders of magnitude. These experimental conditions can be used to scan the surface of a conducting material with a laser beam and to detect all the microscopic projections, in order to male detailed observations with subpicosecond time resolution of the phase transition from autoelectronic emission to explosive emission. Polytechnic University, Tomsk. Institute of Electrophysics, Ural Branch of the Russian Academy of Sciences. French National Scientific Center, Saclay, France. Translated from Izvestiya Vysshikh Uchenbnykh Zavedenii, Fizika, No. 11, pp. 42–44, November, 1997.     

Analysis of cardiac tissue by gold cluster ion bombardment

Specific molecules in cardiac tissue of spontaneously hypertensive rats are studied by using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The investigation determines phospholipids, cholesterol, fatty acids and their fragments in the cardiac tissue, with special focus on cardiolipin. Cardiolipin is a unique phospholipid typical for cardiomyocyte mitochondrial membrane and its decrease is involved in pathologic conditions. In the positive polarity, the fragments of phosphatydilcholine are observed in the mass region of 700-850 u. Peaks over mass 1400 u correspond to intact and cationized molecules of cardiolipin. In animal tissue, cardiolipin contains of almost exclusively 18 carbon fatty acids, mostly linoleic acid. Linoleic acid at 279 u, other fatty acids, and phosphatidylglycerol fragments, as precursors of cardiolipin synthesis, are identified in the negative polarity. These data demonstrate that SIMS technique along with Au₃⁺ cluster primary ion beam is a good tool for detection of higher mass biomolecules providing approximately 10 times higher yield in comparison with Au⁺.     

Electron emission from metal surfaces by ultrashort pulses: determination of the carrier-envelope phase

The phase varphi of the field oscillations with respect to the peak of a laser pulse influences the light field evolution as the pulse length becomes comparable to the wave cycle and, hence, affects the interaction of intense few-cycle pulses with matter. We theoretically investigate photoelectron emission induced by an intense, few-cycle laser pulse from a metal surface (jellium) within the framework of time-dependent density functional theory and find a pronounced varphi dependence of the photocurrent. Our results reveal a promising route to measuring varphi of few-cycle light pulses (tau<6 fs at lambda=0.8 microm) at moderate intensity levels (I(p) approximately 10(12) W/cm(2)) using a solid-state device.     

Electron emission induced by keV protons from tungsten surface at different temperatures

The electron emission yield is measured from the tungsten surface bombarded by the protons in an energy range of 50 keV-250 keV at different temperatures. In our experimental results, the total electron emission yield, which contains mainly the kinetic electron emission yield, has a very similar change trend to the electronic stopping power. At the same time, it is found that the ratio of total electron emission yield to electronic stopping power becomes smaller as the incident ion energy increases. The experimental result is explained by the ionization competition mechanism between electrons in different shells of the target atom. The explanation is verified by the opposite trends to the incident energy between the ionization cross section of M and outer shells.     

Auger electron emission by ion bombardment of light metals

The energy distribution of secondary electrons ejected from a metallic target upon ion bombardment (Ar⁺, 10 keV) is studied with a 180° magnetic analyser, particularly in the vicinity of the Auger peaks that appear in the electron spectra of the light metals (Li, Be; Na, Mg, Al, Si). A qualitative explanation based on the correlation diagrams which described the evolution of the electronic levels during the collision of two atoms inside the target is proposed to interpret the variation of the Auger peak intensity with the atomic number of the target.     

Ion bombardment of Fe-based amorphous metallic alloys

Fe₇₄Cu₁Nb₃Si₁₆B₆ amorphous metallic alloy is investigated after ion irradiation by 110 keV N^?+? and 593 MeV Au ions. The depth-profiles of the radiation damage were calculated by the SRIM2008 code. Applicability of transmission and conversion electron Mössbauer effect measurements to distinguish between the bulk and surface radiation damage is demonstrated by using different irradiation conditions. The investigated alloy is characterized by ferromagnetic interactions. The implantation does not depict appreciable changes of the samples’ surfaces. Changes in chemical short-range order (SRO) are revealed in N^?+? irradiated alloys. Heavy Au ions caused pronounced effects in the position of the net magnetization though no impact on SRO was observed. After annealing, structural relaxation and annealing-out of the irradiation-induced stresses caused the rotation of the net magnetization back to its original position.     

Scattering of low-energy atoms by metallic surfaces

The problem of a neutral low-energy atom impinging on a well-defined metallic surface is approached from first principles. The solid and its potential energy of interaction with the incident atom is treated in the most general way, but under the following assumptions: (a) the conduction electrons interact adiabatically with the lattice ions and the gas atom; (b) no chemical reactions occur; (c) the one-phonon approximation is valid. The scattering amplitudes for surface and bulk mode excitations are obtained in terms of the dynamical properties of the metallic surface. Direct collisions of the incident atoms with the lattice ions are shown to give a negligible contribution to the scattering. The most important contribution comes from the interaction of the gas atom with the surface conduction electrons; the excitation of lattice vibrational modes occurs through the electron-phonon term of the Hamiltonian. The general expressions for the scattering amplitudes obtained show that the scattering is incoherent. With further assumptions one obtains a separation of the scattering amplitude into a coherent and incoherent part.     

Friction model to describe cluster bombardment

Short time molecular dynamics simulations were performed to model C₆₀ and Au₃ bombardment of an amorphous water sample in the projectile energy range of 5-120 keV. A previously proposed friction model has been applied to describe the fundamental motion of a projectile during cluster bombardment of a solid. This simple analytical model uses a definition of friction on a single particle to describe the cluster movement through a medium. Although the mathematics of the friction model vary among systems, the projectile motion and energy deposition of a single particle into the sample as well as the reactive environment created is close to that of C₆₀ bombardment.