Surface termination of an ultrathin V2O3-film on Au(111) studied via ion beam triangulation

The structure of the topmost layer of thin V₂O₃ films on a Au(111) substrate is studied via Ion Beam Triangulation. From electron emission induced by fast H atoms scattered from the film surface under a grazing angle of incidence as function of azimuthal rotation of the target, we find evidence for a reconstructed O₃ termination as proposed from DFT-calculations and recent experimental work using methods based on large angle impact of fast ions. From our studies we derive detailed lateral positions for the topmost O atoms.     

The influence of chemically active gas on the light emission of metallic targets bombarded by positive ions

The introduction of oxygen in the vicinity of a metallic target surface, bombarded with positive argon ions of twenty kiloelectron-volts, increases the number of sputtered atoms in the excited state. This phenomenon of exaltation, very sensitive in the case of nickel and aluminum, is much less marked in the case of molybdenum. Moreover, the emission of excited particles, coming from the beam's ions, is not modified. A quantum-mechanical model of a kinetic emission process, which permits the interpretation of the clean metallic target's emission phenomena, seems insufficient to explain all of the results obtained in the presence of oxygen. In this last case one can therefore use a thermodynamic model in which excited metallic particles can be formed directly by chemical surface reactions of neutralization or reduction.     

Mesoscopic molecular ions in Bose-Einstein condensates

We study the possible formation of large (mesoscopic) molecular ions in an ultracold degenerate bosonic gas doped with charged particles (ions). We show that the polarization potentials produced by the ionic impurities are capable of capturing hundreds of atoms into loosely bound states. We describe the spontaneous formation of these hollow molecular ions via phonon emission and suggest an optical technique for coherent stimulated transitions of condensate atoms into a specific bound state. These results open up new possibilities for manipulating tightly confined ensembles.     

Interaction of a surface glow discharge with a gas flow

A surface glow discharge in a gas flow is of particular interest as a possible tool for controlling the flow past hypersonic aircrafts. Using a hydrodynamic model of glow discharge, two-dimensional calculations for a kilovolt surface discharge in nitrogen at a pressure of 0.5 Torr are carried out in a stationary gas, as well as in a flow with a velocity of 1000 m/s. The discharge structure and plasma parameters are investigated near a charged electrode. It is shown that the electron energy in a cathode layer reaches 250–300 eV. Discharge is sustained by secondary electron emission. The influence of a high-speed gas flow on the discharge is considered. It is shown that the cathode layer configuration is flow-resistant. The distributions of the electric field and electron energy, as well as the ionization rate profile in the cathode layer, do not change qualitatively under the action of the flow. The basic effect of the flow’s influence is a sharp decrease in the region of the quasineutral plasma surrounding the cathode layer due to fast convective transport of ions.     

Energy loss straggling of fast charged particles

Energy loss straggling of fast charged particles colliding with atoms have been considered in the eikonal approximation. The result is represented in the form of the Fano formula with a nonperturbative correction. The known nonperturbative Titeica formula (which is transformed to the Fano formula when perturbation theory is applicable) is obtained only under certain approximations in eikonal calculations. It has been shown that straggling calculated with allowance for nonperturbative effects at large charges of the projectile can be significantly different (by an order of magnitude) from the results obtained by Titeica, Fano, and Bohr. Energy loss straggling of fast highly charged ions on hydrogen and copper atoms have been calculated. The latter results are compared to experimental data.     

Field electron emission and field desorption of cesium from graphene

Field electron emission and field desorption of cesium ions from a monatomic graphene film on Ir and graphene clusters in amorphous carbon are investigated using field electron microscopy and continuous-mode field desorption microscopy. The deposition of cesium on amorphous carbon with graphite clusters leads to inversion of the emission (i.e., emission from the emission centers disappears against the back-ground of uniform emission from the previously nonemitting surface). In both systems, ion current pulses are observed during field desorption in a stationary electric field. During field desorption from the graphene film, current pulses of Cs⁺ ions with a duration shorter than 0.1 s appear from the plane faces of the iridium point. During desorption from graphite clusters, ion current pulses form a pattern of “collapsing rings” on the screen. Possible mechanisms of the observed processes are considered using the model of cesium intercalation by graphite and by the graphene layer and the desorption of Cs atoms under the action of the electric field, as well as the “flip” of the dipole moment during the cesium intercalation.     

Optical studies of atoms and ions in superfluid helium using a ccd-camera system

The spectroscopic study ions and atoms immersed into liquid helium can contribute to the understanding of the structure of pointlike defects in helium and their interaction with the superfluid phase as well. Ions and atoms serve as microprobes in the form of so calledbubble orsnowball type defects in the quantum fluid. The optical emission of these structures is recorded. From the optical spectra of previous experiments the influence of the surrounding helium on the electronic configuration of the impurity atoms or ions was examined. In this experiment the light emitted from the defect atoms is observed by a camera. The pictures obtained yield information about the distribution and the motion of the defect particles in the superfluid. As an example the fluorescence light resulting from the recombination of magnesium, barium and thallium ions with excess electrons in superfluid helium was recorded.     

The scattering of hydrogen from a cesiated tungsten surface

The reflection of protons from a partially cesiated tungsten surface is studied in the energy domain between 100 and 2000 eV and in the angular domain between 75° and 85° with respect to the surface normal. The study is performed by measuring the angular and energy distribution of the scattered negative ions. The reflection can take place along two paths. One path is reflection from the cesium surface layer, the other one is reflection from the tungsten substrate. A dependence of the final charge state on the path is observed. It is inferred that this phenomenon is due to incomplete neutralization of the protons scattered from the cesium layer. The energy loss of the reflected ions cannot be accounted for by using only the binary collision model. Also the electronic stopping of the atoms by the metal electrons is shown to be an important energy loss mechanism. Total conversion measurements of H⁺ to H^- combined with the measurements of the negatively charged fraction of the scattered particles, as reported in the proceeding paper, yield the particle reflection coefficient as a function of the angle of incidence. These reflection coefficients show that for angles of incidence less than 75° already more than 50% of the particles do not reflect from the surface. Total conversion efficiency measurements with H^- ions as primary ions show that the influence of the initial charge state on the total conversion is very small.     

Neutron yield when fast deuterium ions collide with strongly charged tritium-saturated dust particles

The ultrahigh charging of dust particles in a plasma under exposure to an electron beam with an energy up to 25 keV and the formation of a flux of fast ions coming from the plasma and accelerating in the strong field of negatively charged particles are considered. Particles containing tritium or deuterium atoms are considered as targets. The calculated rates of thermonuclear fusion reactions in strongly charged particles under exposure to accelerated plasma ions are presented. The neutron generation rate in reactions with accelerated deuterium and tritium ions has been calculated for these targets. The neutron yield has been calculated when varying the plasma-forming gas pressure, the plasma density, the target diameter, and the beam electron current density. Deuterium and tritium-containing particles are shown to be the most promising plasmaforming gas–target material pair for the creation of a compact gas-discharge neutron source based on the ultrahigh charging of dust particles by beam electrons with an energy up to 25 keV.     

Monte Carlo model for the argon ions and fast argon atoms in aradio-frequency discharge

A three-dimensional Monte Carlo model has been developed for the argon ions and fast argon atoms in the RF sheath of a capacitively coupled RF glow discharge in argon. Our interest in the argon ions and fast argon atoms in the RF sheath arises from the fact that the glow discharge under study is used as sputtering source for analytical chemistry. This source operates at typical working conditions of a few torr pressure and about 10 W incoming power. The argon ion and atom Monte Carlo model has been coupled to a hybrid Monte Carlo-fluid model for electrons and argon ions developed before to obtain fully self-consistent results. Typical results of this model include, among others, the densities, fluxes, collision rates, mean energies, and energy distributions of the argon ions and atoms. Moreover, we have investigated how many RF cycles have to be followed before periodic steady state is reached, and the effects of all previous RF cycles are correctly accounted for. This is found to be the case for about 20-25 RF cycles     

Experimental Measurements on the Current Balance at the Cathode of a Cylindrical Hollow Cathode Glow Discharge

The contribution of positive ions, photons and metastable particles to the secondary emission of electrons from the cathode of an argon glow discharge with cylindrical cathode is measured by sampling the plasma species through a small orifice in the cathode wall and analyzing them in a high vacuum region. Additional experiments are made with a second apparatus which enables the sampling fo plasma species from the negative glow and the dark space. The paper describes the experimental techniques used in both cases and presents estimates on the validity of the results obtained. The positive ions are found to be the dominant contribution to the discharge current at the cathode. The secondary emission of electrons is — in decreasing order of importance — due to impact fo singly charged ions, doubly charged ions and metastable atoms.     

Effects of transverse magnetic field on a laser-produced Zn plasma plume and ZnO films grown by pulsed laser deposition

By adopting a fast photography and time-resolved optical emission spectrometry, we have investigated the effects of transverse magnetic field on the expansion dynamics and enrichment of Zn atoms and Zn⁺ ions in a plume produced by laser ablation of a Zn target in oxygen atmosphere. Plume splitting due to the magnetic field was apparent but the splitting patterns of Zn and Zn⁺ were totally different. The surface morphology and photoluminescence characteristics also changed significantly. In particular, the growth rate increased by as much as 2.4-4.3 times compared to the conventional PLD method.     

Polarization bremsstrahlung at collisions of fast ions with multiatomic targets

The processes of polarization bremsstrahlung at collisions of fast ions with linear chains consisting of isolated atoms have been considered. The intensities and angular distributions of radiation spectra have been obtained for an arbitrary number of atoms in a chain. It has been shown that the interference of the photon emission amplitudes leads to a noticeable change in the spectral angular distributions of polarization bremsstrahlung as compared to the distributions at collisions with an isolated atom. The results allow standard generalization to the cases of polarization bremsstrahlung at channeling of fast ions over surfaces and in solid lattices.     

Effect of the solar wind on the formation of a photoinduced dusty plasma layer near the surface of the Moon

A dusty plasma layer formed near the illuminated part of the surface of the Moon under the action of ultraviolet radiation, as well as fast and slow solar wind, has been numerically simulated. The numerical calculations including the photoemission properties of lunar regolith samples delivered to the Earth have been compared to estimates within known theoretical models. It has been shown that the flux of solar wind particles plays an important role in the formation of the surface photoelectron layer. The conditions of the charging and stable levitation of dust particles in the surface plasma layer of the Moon have been analyzed.     

Surface channeling of swift light ions

A fast computer code is developed to provide information about the trajectories of swift light particles incident on crystalline targets under surface channeling conditions. The approximations used in the model are tested by comparison of trajectory calculations with the MARLOWE simulation program. The simulation of experimental energy distributions allows discussing various inelastic energy loss models for the interaction of 150 keV protons with a nickel surface. The results suggest that plasmon excitations are not sufficient to account for the measured energy losses. It is found that the Oen-Robinson formula, including inelastic energy losses by single electron excitations in dense materials reasonably well applies to the reflection of light ions from metallic surfaces in channeling conditions. The measured light intensity emitted from 200 keV He⁺ reflected ions in various directions close to compact atomic surface rows is compared with the calculated reflection coefficient. The results suggest that most of the particles reflected in ionic state do not penetrate the target surface. Detailed comparison between light emission measurements and calculated reflection intensities, however, requires accurate modelling of the surface topography as well as of the deexcitation mechanisms involved in the surface reflection of light ions.     

Probability of ionization of sputtered particles as a function of their energy: Part I: Negative Si ions

In this study we have investigated how the probability of ionization of sputtered Si atoms to form negative ions depends on the energy of the atoms. We have determined the ionization probability from experimental SIMS energy distributions using a special experimental technique, which included de-convolution of the energy distribution with an instrumental transmission function, found by separate measurements.We found that the ionization probability increases as a power law ∼E^0.677 for particles sputtered with energies of 0-10 eV, then becomes a constant value (within the limits of experimental error) for particles sputtered with energies of 30-100 eV. The energy distributions of Si⁻ ions, measured under argon and cesium ion sputtering, confirmed this radical difference between the yields from low and high-energy ions.To explain these results we have considered ionization mechanisms that are different for the low energy atoms (<10 eV) and for the atoms emitted with higher energy (>30 eV).     

Generation of energetic electrons and ions by interaction of relativistic laser pulses with a supercritical plasma

Generation of hot electrons and fast ions under the action of a relativistic femtosecond laser pulse propagating through a supercritical-density plasma is studied in terms of the kinetic theory by constructing propagators for plasma particle distribution functions. Calculations are carried out for different laser pulse intensities, different sorts of multicharged ions, and different degrees of plasma inhomogeneity. Mechanisms of electron and ion acceleration in the plasma are explored.     

Electron emission features of the derivatives of radiation carbonization of poly(vinylidene fluoride)

It has been studied how photoelectron and CKVV spectra of partially crystalline poly(vinylidene fluoride) (PVDF) are modified during a long-term degradation of its surface under soft X-rays (AlK _α), which is accompanied by a flow of secondary electrons having different energies, and upon exposure to a unfocused beam of 600 eV Ar⁺ ions. In both cases, the surface layer of the sample is enriched with carbon owing to defluorination. The shape of the electron emission spectra of the carbonized layer depends on an external effect; that is, whether soft X-ray photons or ions are used for defluorination. In the case of bombardment with Ar⁺, there is clear evidence for the dominance of the sp2 bonds between carbon atoms, as can be seen from the specific shape of the C KVV band and the C1s spectrum. The most surprising result of this study is that both photons and ions produce the same depth gradient of residual fluorine at an equal fluorine concentration in the carbonized surface layer. The reason for this is not clear and needs further investigation.     

Influence of the Emission of Charged Particles on the Characteristics of Glow Discharges with Oscillating Electrons

The influence of the extraction of charged particles on the conditions of sustaining and the characteristics of a glow discharge with oscillating electrons is considered. It is shown that there is some pressure-dependent optimum level for the extraction of ions at which the energy efficiency of the ion source reaches a maximum. Experimentally, it has been established that the sustaining of a discharge is adversely affected by the run-off of fast ionizing electrons from the discharge, whereas the emission of slow plasma electrons can facilitate the sustaining of the high-current variety of discharge and even lead to a stabilization of an unstable gas-discharge structure. It has been shown that due to the different character of the spatial distributions of fast and slow particles in discharges with electrons oscillating in a magnetic field it is possible to realize highly efficient electron emission without loss of discharge stability by extracting electrons from the near-anode region.     

Analysis of the nitrogen ion beam generated in a low-energy plasmafocus device by a Faraday cup operating in the secondary electronemission mode

The energy distribution and flux of fast nitrogen particles generated in a Mather-type plasma focus device operating at 0.4 Torr of N₂ pressure is reported. A Faraday cup operating in the secondary electron emission mode was employed. To determine the total number of beam particles, multiple scattering of the ions was taken into account. It has been possible to register the ion energy up to a lower kinetic energy threshold of ≈50 keV, which is a value much lower than that obtained with a Thomson spectrometer in a previous work