Path-length distribution of ions reflected from a solid: Theory and computer simulation

Theoretical methods and Monte Carlo procedure are used to study path-length distributions of ions reflected from a solid. The theoretical analysis is based on the solution of the integral Chandrasekhar equation for the Laplace transform of the distribution function. A family of curves is obtained for path-length distributions at several ion energies and mass ratios of ions and target atoms. A computer code for simulation is based on the approximation of pair collisions and a gas model of solid. The simulated results are compared with the theoretical results and published data.     

Charge exchange between low energy alkali ions and cerium oxide surfaces

The fraction of low energy Na⁺ ions neutralized during single scattering from CeO₂(1 0 0) surfaces was measured with time-of-flight spectroscopy. The projectile ionization level is resonant with the surface electronic states, so that the neutralization results from a non-adiabatic charge exchange process that depends on the exit velocity and the local electrostatic potential (LEP) along the exit trajectory at a point close to the surface. Variations of the measured neutral fraction with ion energy and exit angle differ from the results obtained from clean metals due to the inhomogeneity of the LEP on an oxide surface. The results suggest that neutral fraction data collected as a function of emission angle and ion energy could be used to quantitatively map the shape of an inhomogeneous LEP.     

Angular distribution of reflected ions at oblique incidence on a target

The angular distribution of ions reflected from a solid surface has been obtained as a result of solving the Boltzmann equation, written in the diffusion approximation by the method of moments. The angular distribution is dependent on the energy and angle of ion incidence, the ratio of the ion and target atomic masses, and the inelastic energy loss parameter. Good agreement has been shown between the theory and the experimental and computer simulation data.     

Charge state distribution analysis of Al and Pb ions from thelaser ion source at IMP

A prototype laser ion source that could demonstrate the possibility of producing intense pulsed high charge state ion beams has been established with a commercial Nd:YAG laser (E_max=3 J,1064 nm,8-10 ns) to produce laser plasma for the research of Laser Ion Source (LIS). At the laser ion source test bench,high purity (99.998%) aluminum and lead targets have been tested for laser plasma experiment. An Electrostatic Ion Analyzer (EIA) and Electron Multiply Tube (EMT) detector were used to analyze the charge state and energy distribution of the ions produced by the laser ion source. The maximum charge states of Al¹²⁺ and Pb⁷⁺ were achieved. The results will be presented and discussed in this paper.     

Charge distribution of the Kr ions resulting from the X-ray irradiation at 1.3 keV

The model of effective atomic potential and the Monte Carlo method are used to calculate the charge distribution of the Kr ions that are formed due to irradiation of neutral atoms by X-ray photons at an energy of 1.3 keV. The calculated results are in good agreement with the recent experimental data and can be used for development and maintenance of gas detectors that monitor the intensity of X-ray free-electron lasers.     

Energy distribution of reflected ions at normal incidence on the target surface

Summary The energy distribution of reflected ions at normal incidence of the primary beam on the target surface is obtained as a result of approximate solution of the Boltzmann equation by the method of discrete streams. This method gives the opportunity to avoid the problem of negative distribution function, arising in the generally applied method of spherical harmonics, and to present the final result in analytical form. The analysis is performed for a power scattering cross-section and the theoretical energy distribution demonstrates one maximum when the ion mass is equal to the mass of a target atom, and two maxima when the ion mass is much less than the mass of a target atom.     

Revisiting the Poisson-Boltzmann theory: Charge surfaces, multivalent ions and inter-plate forces

The Poisson-Boltzmann (PB) theory is extensively used to gain insight on charged colloids and biological systems as well as to elucidate fundamental properties of intermolecular forces. Many works have been devoted in the past to study PB-related features and to confirm them experimentally. In this work we explore the properties of inter-plate forces in terms of different boundary conditions. We treat the cases of constant surface charge, constant surface potential and mixed boundaries. The interplay between electrostatic interactions, attractive counter-ion release, and repulsive van ’t Hoff contribution is discussed separately for each case. Finally, we discuss how the crossover between attractive and repulsive interactions for constant surface charge case is influenced by the presence of multivalent counter-ions, where it is shown that the range of the attractive interaction grows with the valency.     

Charge state distribution analysis of AI and Pb ions from the laser ion source at IMP

A prototype laser ion source that could demonstrate the possibility of producing intense pulsed high charge state ion beams has been established with a commercial Nd：YAG laser （E =3 J, 1064 rim, ~ 10 ns） to produce laser plasma for the research of Laser Ion Source （LIS）. At the laser ion source test bench, high purity （99.998%） aluminum and lead targets have been tested for laser plasma experiment. An Electrostatic Ion Analyzer （EIA） and Electron Multiply Tube （EMT） detector were used to analyze the charge state and energy distribution of the ions produced by the laser ion source. The maximum charge states of A112＋ and Pb7＋ were achieved. The results will be presented and discussed in this paper.     

Angular distribution of copper atoms sputtered with energetic ions

Measurements of the angular distribution of copper atoms which are sputtered by noble gas ions within the energy range between 0.1 and 1 MeV have been carried out for different angles of ion incidence. The hemisphere over the target surface could be studied with a microphotometer inside the sputtering chamber and the distributions can be plotted in tri-dimensional diagrams. The results are in principle similar to those obtained at lower energies. The angle of maximum emission varies with ion energy and with the angle of incidence and can be related to the sputtering yield.     

Cold emission of negative ions from porous graphite

The electric field strength in the near-wall layers of thermonuclear facilities can be sufficient for the emission of negative ions from the surface of plasma facing materials. The mass spectra of negative ions from MPG-8 porous graphite at electric field strengths up to 4 × 10⁶ V/m and the surface composition of the target have been studied by ion scattering spectroscopy. The dependence of the mass composition of negative ion emission and residual gas on the sample’s temperature has been measured and a correlation between the intensity of negative hydrogen-ion emission and the desorption of water has been established. The temperature dependence of the emission of negative ions and clusters of carbon and hydrocarbon is in qualitative agreement with the chemical erosion of graphite.     

Secondary emission of carbon ions from graphite nanocrystallites

Emission of secondary C⁺ ions from the surface of well-oriented graphite nanocrystallites irradiated with 10 keV Ar⁺ ions has been studied. The energy distribution of emitted secondary ions has been measured. It has been found that the form of energy distribution and the energy corresponding to the maximum of distribution E_max depend on the polar angle θ of C⁺ ion emission. The linear dependence of energy in the maximum of energy spectrum of single-charged secondar y carbon ions on the polar angle of their emission E_max(θ) has been obtained. The mechanism responsible for the observed features of the energy spectra is discussed.     

Emission of radiation by charged particles reflected from solid surfaces

It is predicted that charged particles reflected from solid surfaces could emit radiation due to the change in their direction of motion upon reflection. The theoretical spectrum of this radiation is obtained for specular reflection. The general concept of ‘transition radiation’ is reconsidered to include this new effect which is found to dominate for low-energy, non-penetrating electrons and for high-energy electrons at grazing incidence.     

Charge exchange in Li scattering from Si surfaces

Neutral fractions are measured for 4 keV 7Li(+) scattering from clean, hydrogen-covered, and cesiated Si surfaces. The neutral fraction in scattering from clean Si is approximately 26% and it decreases with hydrogen adsorption. When Cs is adsorbed on Si, the neutral fraction does not distinguish the local potential at the Cs sites from the Si sites, unless hydrogen is coadsorbed. These results demonstrate that resonant charge transfer occurs due to coupling of the Li ionization level with the dangling bond surface states, and that the influence of the dangling bonds extends beyond the local scattering sites.     

Single-parameter expression for ions reflection from surfaces

Based on a detailed analysis of the Boltzmann equation for ion transport in solids, it has been shown that for low-energy ions incident on a heavy element target the distribution function of the ions can be determined by one single parameter, called the scaled transport cross-section, which was defined earlier [1]. This means that the transport quantities of different ion-target-energy combinations should be similar only when their scaled transport cross-section is the same. To test this significant conclusion, we undertook a set of systematic and extensive calculations of reflection coefficients using the improved bipartition model of ion transport. The systematic calculations include 3410 ion-target-energy combinations, namely H, D, T, He, Li, B, C, N, O, Ne ions with energy ranges from 10 eV to 1 MeV normally incident to C, Al, Cu, Mo, Ag, W, Au, Pb, U targets. The only restrictions isM ₁/M ₂<1/6. The calculations verify that particle and energy reflection coefficients present an excellent one-to-one correspondence to the scaled transport cross-section. Furthermore, based on the calculations, universal expressions for both particle reflection coefficients and energy reflection coefficients for normal ion incidence have been obtained by fitting the numerical data. By comparing the results calculated by the universal expressions with experimental and Monte Carlo data, it is shown that the expression can describe reflection coefficients well.     

Energy distribution of metal and noble gas ions traversing single-crystal copper films

A comparative investigation of the energy distribution of ions that traversed single-crystal cooper films reveals that the energy loss of channeled and nonchanneled particles depends not only on the mass but also on the radius of bombarding ions. It is established that the energy spectra of transmitted ions are highly sensitive to a change in the composition and structure of the films. From the change in these spectra, one can estimate the degree of disordering in thin films under various applied forces.     

Angular distributions of electrons and light ions elastically reflected from a solid surface

The angular distributions of elastically reflected electrons are described on the basis of the transfer equation for a radiation particle. The exact solution of the problem is obtained. This solution is compared with experimental spectra of elastically reflected electrons and with the approximate solutions.     

Reaction of cyanide ions with copper on Si surfaces and its use for Si cleaning

A Si cleaning method has been developed by use of potassium cyanide (KCN) dissolved in methanol. When silicon dioxide (SiO₂)/Si(1 0 0) specimens with 10¹⁴ atom/cm² order copper (Cu) contaminants are immersed in 0.1 M KCN solutions of methanol at 25 °C, the Cu concentration is reduced to below the detection limit of total X-ray fluorescence spectrometer of ∼3 × 10⁹ atoms/cm². X-ray photoelectron spectra show that the thickness of the SiO₂ layers is unchanged after cleaning with the KCN solutions. 10¹⁴ cm⁻² order Cu contaminants on the Si surface can also be removed below ∼3 × 10⁹ atoms/cm², without causing contamination by potassium ions. UV spectra show that Cu-cyano complex ions are formed in the KCN solutions after the cleaning. The main Cu species in the KCN solutions is ions with the concentration of []:[Cu⁺] = 1:1.6 × 10²³. Even when the KCN solutions are contaminated with 64 ppm Cu²⁺ ions in the solutions, which form ions, the cleaning ability does not decrease, showing that ions are not re-adsorbed. The KCN solutions can also passivate defect states such as Si/SiO₂ interface states, leading to the improvement of characteristics of Si devices.     

Charge distribution in graphene from quantum calculation

The local charge distributions of different shape graphene sheets are investigated by using the quantum calculations.It is found that the charge distribution on carbon atom is not uniform, strongly depending on its position in the graphene and its local atomic environment condition. The symmetrical characteristic and geometrical structures of graphene also have an important influence on the charge distribution. The charges of atom at the graphene edge are strongly related to their surrounding bonds. It is found that the charges of double-bonded atom at the zigzag edge are closely related to the bond angle, but the charges of double-bonded atom at the armchair edge are mainly influenced by the area of triangle. The charges of triple-bonded atom at the edge are mainly affected by the standard deviation of the length of the associated triple bonds.