Dynamic Potential of Interaction between Nitrogen Atoms and Aluminum Crystal Surface

Specific features of the angular distributions of accelerated neutral nitrogen atoms at the grazing angles of incidence on the Al(001) crystal surface have been investigated by the computer simulation method. The N–Al pair interaction potential is approximated by the three-parameter Morse potential with the energydependent coefficients. The angular distributions of scattered atoms have been simulated taking into account the interaction between atoms and several atomic layers in the lattice and the atomic displacement during thermal oscillations. The parameters of the pair potential of accelerated neutral nitrogen atoms in the energy range from 10 to 70 keV have been determined according to the best agreement between the calculated dependence of the rainbow scattering angle on the energy of particles incident on the crystal surface and the available experimental data.

     

Low-energy ion scattering by monatomic films

The angular and energy distributions of Cs⁺ and Xe⁺ ions scattered by monatomic crystalline films have been calculated via the molecular dynamics method in the scope of the multiparticle interaction mechanism. The calculated dependences of scattered heavy ions with a low initial energy E ₀ = 40 eV on the atomic mass, crystal lattice type, interatomic distance, and binding energy of the film atoms are discussed and compared with the experiment. The presented data can be used to predict the physical properties of a surface covered with a monatomic layer of foreign atoms during experimental surface studies based on the backscattering of ions with low initial energies.     

Polar exit angle distributions of slow H<Superscript>+</Superscript> ions,related to the presence of a regular domain structure,calculated within the diffuse scattering approximation upon interaction of ions with the sample surface

The angular distributions of slow H ions scattered from the surface of a magnetic film with a stripe domain structure have been calculated. The calculations were performed within the models of specular and diffuse scattering upon collisions of ions with surface. It is shown that the magnetic fields of the stripe domain structure block the ions scattered at small exit angles. It is established that, within the more adequate model of diffuse scattering, the anisotropy of interaction of ions with the magnetic field of the stripe structure manifests itself in a wider angular range.     

The ejection of atomic particles from ion bombarded solids

Considerable interest has developed in the last decade in the study of atomic collisions in solids, particularly single crystal solids. This has been heightened by the observation of channelling and the interest in industrial aspects of ion implantation. In the following article, we present an up-to-date account of the work being done on the particles ejected from a single crystal, following ion bombardment. These particles include sputtered atoms and ions, scattered atoms and ions and secondary electrons. An attempt has been made to indicate the similarities and differences, particularly between the energy and angular distributions of these ejected particles. The major link involving the dependence of these distributions of all particles on crystallography is stressed.     

Scattering of excited atoms due to ion bombardment of metals

The spectrum and the spatial distribution of the emission of scattered particles upon bombardment of Be, Ti, Ta, and W with H and He ions were studied. The quantum yield and the velocity and angular distributions of the scattered excited particles were measured. Computer simulation of the scattering of H ions and He ions from the metals under study was performed with the use of the program TRIM-95. Agreement is observed between the calculated and experimentally measured angular distributions of the scattered particles.     

Energy and charge distribution of helium atoms reflected from a Cu surface under grazing incidence

The energy and charge distributions of helium ions and atoms reflected from a Cu surface at grazing incidence have been measured at the energies 250 and 300 keV. The dependence of the relative number of helium atoms in the scattered beam on the scattering angle at incidence angles less than 2° and scattered particle energies below 150 keV has been discovered. It is shown that, for a theoretical estimation of the charge fractions of particles reflected from the target surface, charge-exchange cross sections used in calculations must be corrected when solids are considered instead of gases.     

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.     

Scattering of Li+ from LEED characterized W(110) and Si (111) surfaces at energies between 2 and 20 eV

Energy and angular distributions of Li⁺ ions scattered from W(110) and Si(111) surfaces have been measured for a wide range of scattering angles and for beam energies between 2 and 20 eV. The collision process can be explained in terms of binary collisions with single surface atoms, if the influence of the attractive part of the interaction potential is taken into account. A square well approximation for the potential makes it possible to predict the form of the energy spectrum as well as the behaviour of the angular distribution of the scattered particles for both systems. The influence of adsorbed atoms and molecules on the scattering behaviour has been investigated. Exposure of the W surface to H₂, O₂ and CO shows that surface coverages exceeding 10% of a monolayer very drastically influence the scattering. The results from the clean surfaces indicate that sufficiently precise measurements of the energy spectra of the scattered particles can yield very detailed information about the form of the interaction potential. The form of the energy spectra also contains information as to the extent of vibrational excitation of the surface atoms.     

The effect of inelastic loss on the development of interatomic collision cascades

A theory of interatomic collision cascades in an infinite medium subject to inelastic energy loss (ionization slowdown) of particles is developed. Emphasis is on the angular and energy distributions of primary ions and cascade atoms upon slowdown. Analysis is performed under the assumption that single scattering of the particles follows the hard ball law, and the electronic stopping power of the medium is determined by the Lindhard formula. It is shown that the inclusion of slowdown directly in solving the Boltzmann transport equation radically changes the angular and energy spectra of the ions and cascade atoms obtained when the slowdown is ignored. Moreover, slowdown is the factor responsible for the anisotropy of the angular distributions of low-energy primary ions and cascade atoms.     

Multiple scattering and neutralization aspects of low energy noble gas ions incident on a Cu (110) crystal

Experiments have been done with 1 keV Ne⁺ ions bombarding a Cu (110) single crystal in a (111) plane. From the measured energy spectra of the scattered ions the minimum and maximum scattering angles for multiple scattering on the surface are determined. These minimum and maximum scattering angles were also calculated using a computer model. The parameters for an interatomic potential function between ion and metal atom are determined by comparison of the experimental and calculated scattering angles. A neutralization model for an energetic ion at an atomic surface is given. The reliability of this model is tested by comparison of the results of computer calculations and the measured peak intensity distributions as a function of the scattering angle.     

Simulated reaction dynamics of F atoms on partially fluorinated Si(100) surfaces

We have investigated the influence of translational excitation on the reactivity of atomic fluorine with the Si(100) surface via molecular dynamics simulations using a first-principles-derived interaction potential. Surface reactivity is contrasted for both clean and partially fluorinated surfaces with the results of previous simulations of F₂ molecules impinging on Si(100) surfaces, indicating many similarities between the dynamics of F atoms and F₂ molecules. Mechanisms for the reaction are proposed based on reactivity trends and scattered product energy and angular distributions, including evidence for the existence of a precursor-mediated adsorption pathway for low incident energy F atoms on partially fluorinated surfaces.     

Sputtering studies with the Monte Carlo Program TRIM.SP

The Monte Carlo Program TRIM.SP (sputtering version of TRIM) was used to determine sputtering yields and energy and angular distributions of sputtered particles in physical (collisional) sputtering processes. The output is set up to distinguish between the contributions of primary and secondary knock-on atoms as caused by in- and outgoing incident ions, in order to get a better understanding of the sputtering mechanisms and to check on previous theoretical models. The influence of the interatomic potential and the inelastic energy loss model as well as the surface binding energy on the sputtering yield is investigated. Further results are sputtering yields versus incident energy and angle as well as total angular distributions of sputtered particles and energy distributions in specific solid angles for non-normal incidence. The calculated data are compared with experimental results as far as possible. From this comparison it turns out that the TRIM.SP is able to reproduce experimental results even in very special details of angular and energy distributions.     

Modeling of neutral-atom scattering at the surface of a crystal for the case of grazing incidence

The features of the angular distributions of accelerated neutral atoms at grazing angles of incidence on the Al(001) surface are studied using the mathematical modeling method. The interaction of accelerated atoms with crystal-lattice atoms and the electronic properties and atomic structure of the Al(001) surface are calculated using the electron-density-functional method. The angular distributions of scattered atoms are modeled by taking into account their interaction with several atomic layers in the crystal lattice and atomic displacements during thermal oscillations. The influence of crystal surface-layer relaxation on rainbow scattering, i.e., the difference between the distances of planes on the surface and in the volume, is established. The possibilities of using the effect of rainbow scattering to study the structural features of a crystal surface are discussed.     

Temperature effect in antiproton channeling

The temperature dependence of axially channeled antiprotons in a Si crystal is studied using computer simulation. The number of channeled particles and the energy and angular distributions are calculated for antiprotons with an energy of 300 keV in the temperature range of 100 to 1000 K in the 〈100〉, 〈110〉, and 〈111〉 channels. The roles of the potential and atomic thermal vibrations are studied thoroughly. It is found that dechanneling depends on the crystal temperature. There exists an optimal temperature, which is dependent on the axis direction, for which dechanneling is minimal.     

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.     

Influence of ion bombardment induced radiation damages in the crystal lattice on the angular regularlties in ion scattering

The paper examines the angular, spatial and energy distribution of ions scattered by various faces of silicon and germanium single crystals. It is shown that, due to the orderly arrangement of atoms in the crystal lattice, the orientation effects in the angular, spatial and energy distributions are observed at temperatures exceeding the annealing temperature of ion bombardment induced radiation damage. At high temperatures of the sample a reduction of the anisotropy in the angular, spatial and energy distributions of scattered ions was observed which is due to the changing transparency of the crystal resulting from the thermal vibrations of atoms in the lattice.

Results of investigations testify to the possibility of using the orientation dependences of phenomena occurring during the interaction of ions with crystals for determining the annealing temperature of ion bombardment damage and for studying the kinetics of the process of ion bombardment. It is shown that further studies along the same lines as conducted by the authors of this paper could develop a technique for the quantitative estimation of ion bombardment damage in crystals and also in thin epitaxial films.     

Relaxation of the edge atoms of a stepped Cu(410) surface

The edge atom position of the stepped Cu(410) surface has been investigated with low energy ion scattering (LEIS) using 10–20 keV H⁺. It is possible to determine the atomic position by utilizing atomic shadowing effects. The scattered particles are analysed with a time-of-flight spectrometer. This means that we can detect scattered hydrogen leaving the copper surface in the neutral state as well as in the charged state, so the influence of neutralization of the scattered particles along different outgoing trajectories is eliminated. We find an inward relaxation of the edge atoms; assuming that this relaxation takes place in the (100) plane it amounts to about 8% of the interlayer spacing.     

Coulomb explosion of a cluster with a complex ion composition

The problem of the Coulomb explosion of a homogeneous cluster with light and heavy ions has been analytically solved. The space-time and spectral distributions of accelerated ions have been obtained. The characteristics of scattered light ions are determined as functions of the atomic composition of the cluster. It has been shown that sources of monoenergetic ions can be created using the interaction of high-power ultrashort laser pulses with molecular clusters.     

Sputtering of Cu atoms by Ar ions

Sputtering of Cu single-crystal, polycrystal and amorphous targets by 5 keV Ar ions has been studied by the binary collision lattice simulation code Cosipo. The sputtering yields, angular distributions, energy distributions and the space distributions of the original positions of the sputtered Cu atoms have been calculated. The results are discussed within the framework of cascade generations and surface structure.     

Experimental and computer study of ion scattering from single crystals

The spatial distribution of reflected particles was experimentally studied in the case of oblique incidence of 4.5 keV Ar ions on a (001) Cu face along the (110) and (100) planes. The distribution in ejection angles of the reflected particles was found to have several peaks and minima. To explain the results of these reflection experiments a computer study was performed. The shape of the calculated spatial distribution curves appears to be very sensitive to variations of the constants in the interaction potentials. Some features of the spatial distribution of the reflected ions have been shown to be connected to the motion of ions in surface semichannels. The spatial distribution of the neutral component of scattered and sputtered particles was also studied in the experiments. A predominant yield of the sputtered particles was observed not only in the low-index but also in high-index directions.