Channeling Conditions Research Articles

Dynamics of electrons with energies up to several teraelectronvolts in oriented crystals

Stochastic aspects of the dynamics of electrons with energies from 150 GeV to several teraelectronvolts in silicon and germanium crystals oriented by the principal crystallographic directions along the beam axis are studied. The computations take into account the influence of emission of hard γ-quanta on the motion of electrons and also their multiple scattering from atoms of the target crystal. The lifetime of the electrons under channeling conditions are computed, and it is shown that this value (in the units of length) exactly coincides with the effective channeling length introduced earlier by Kumakhov [5].

Fast atom diffraction for grazing scattering of Ne atoms from a LiF(0 0 1) surface

Angular distributions of fast Ne atoms after grazing collisions with a LiF(0 0 1) surface under axial surface channeling conditions are experimentally and theoretically studied. We use the surface eikonal approximation to describe the quantum interference of scattered projectiles, while the atom–surface interaction is represented by means of a pairwise additive potential, including the polarization of the projectile atom. Experimental data serve as a benchmark to investigate the performance of the proposed potential model, analyzing the role played by the projectile polarization.

Transition from axial to planar surface channeling for fast atom diffraction

The diffraction patterns for scattering of fast He atoms from a LiF(0 0 1) surface under a grazing angle of incidence are recorded as function of the azimuthal angle for surface channeling conditions. We observed well defined diffraction patterns which show pronounced changes with the azimuthal rotation of the target surface. Within an azimuth of about 2° the angular distribution turns from a rich pattern to a single peak of zeroth diffraction order only. The data are well reproduced by a semi-classical approach making use of individual pair potentials for the atom surface interaction.

CHARGE AND MASS CHANGING CROSS-SECTIONS OF 158A GeV Pb PROJECTILES ON Al, Cu, Bi, AND Pb TARGETS: FRAGMENTATION PROCESSES

We report here on total and partial charge changing cross-sections of 158A GeV lead projectiles on Al , Cu , Pb and Bi stationary targets using CR-39 detectors. Present results are compared with relevant published results. Deep discussion of nuclear fragmentation processes is provided. Stacks of CR-39 Track Detectors with above-mentioned targets were exposed to 158 A GeV 207 Pb projectiles at the CERN-SPS beam facility at normal incidence with a fluence of about 1500 ions/cm 2. Lengths of etched cones on one face of the CR-39 detectors before and after the target were measured for each stack. These measurements were used for determination of total and partial charge changing cross-sections of 158 A GeV Pb 82+ projectiles on these targets in the charge region 63 < Z < 82. The charge resolution (σZ) achieved in the present experiment is ~0.18e. Mass changing cross-sections are derived from the measured charge changing cross-sections using hyper-geometric relationship. A new method is proposed for separate measurements of electromagnetic and nuclear parts of fragmentation cross-sections using ion channeling and blocking conditions of projectiles in crystalline targets.

Epitaxial growth of GdN on silicon substrate using an AlN buffer layer

We report on the epitaxial growth of the intrinsic ferromagnetic semiconductor GdN on Si (1 1 1) substrates buffered by a thick AlN layer, forming a heteroepitaxial system with promise for spintronics. Growth is achieved by depositing Gd in the presence of unactivated N 2 gas, demonstrating a reactivity at the surface that is sufficient to grow near stoichiometric GdN only when the N 2:Gd flux ratio is at least 100. Reflection high-energy electron diffraction and X-ray diffraction show fully (1 1 1)-oriented epitaxial GdN films. The epitaxial quality of the films is assessed by Rutherford backscattering spectroscopy carried out in random and channelling conditions. Magnetic measurements exhibit a Curie temperature at 65 K and saturation magnetisation of 7 μ B/Gd in agreement with previous bulk and thin-film data. Hall effect and resistance data establish that the films are heavily doped semiconductors, suggesting that up to 1% of the N sites are vacant.

Structure of ultrathin silica films on Mo(112) studied via Classical and Quantum Mechanical Rainbow Scattering of Fast Atoms

AbstractThe structure of ultrathin silica films on Mo(112) has been investigated by grazing scattering of fast He atoms from the film surface. We discuss two recent experimental techniques which allow us to study the arrangements of surface atoms of these films in detail. The methods comprise: (i) classical rainbow scattering under axial surface channeling conditions; and (ii) fast atom diffraction. Our work demonstrates the attractive features of grazing fast atom scattering as a powerful analytical tool in studies on the structure of surfaces. From the analysis of our data we find a clearcut support for a structural model based on the arrangement of film atoms in terms of a 2D network. Copyright © 2010 John Wiley & Sons, Ltd.

Surface channelling and energy losses of 4 keV hydrogen and fluorine ions in grazingscattering on Au(111) and missing row reconstructed Au(110) surfaces

We present the results of an experimental and theoretical study of surface channelling andenergy loss of 4 keV hydrogen and fluorine ions in grazing scattering on a missing rowreconstructed Au(110) surface and a Au(111) surface. We performed measurements ofenergy losses for grazing angle scattering in surface channelling conditions forvarious azimuthal orientations of the crystal. Experimental results are discussedin the light of trajectory calculations of hydrogen and fluorine ions scatteredunder grazing incidence conditions on the surface. A nonlinear model is usedin order to calculate the ion energy loss in these crystalline systems. Ab initioelectronic crystal structure calculations and semi-classical simulations are performedand allow us to delineate various trajectory classes that correspond to differentcontributions in the energy loss spectra for various azimuthal orientations of the surface.

Site-specific electronic configurations of Fe 3d states by energy loss by channeled electrons

Site-specific configurations of Fe 3d electrons in a spinel ferrite were investigated by electron energy loss spectroscopy under electron channeling conditions. Site-specific spectra were extracted by applying a multivariate curve resolution (MCR) technique to the data set. An electronic difference in the Fe sites caused by ligand field splitting of trivalent Fe was probed. This demonstrated the promise of site-specific valence and spin state analysis in spintronics applications of spinel ferrites.

Monte Carlo calculation of energy loss of hydrogen and helium ions transmitted under channelling conditions in silicon single crystal

Abstract In this work, we report on calculations of the electronic channelling energy loss of hydrogen and helium ions along Si〈1 0 0〉 and Si〈1 1 0〉 axial directions for the low energy range by using the Monte Carlo simulation code. Simulated and experimental data are compared for protons and He ions in the 〈1 0 0〉 and 〈1 1 0〉 axis of silicon. A reasonable agreement was found. Computer simulation was also employed to study the angular dependence of energy loss for 0.5, 0.8, 1, and 2 MeV channelled 4He ions transmitted through a silicon crystal of 3 μm thickness along the 〈1 0 0〉 axis.

Channeling effect in electronic spectra produced by grazing impact of fast protons on insulator surfaces

Electron emission due to grazing scattering of fast protons from LiF and KCl surfaces is studied under axial incidence conditions. The differential emission probability is calculated within a distorted-wave formalism, taking into account axial channeled trajectories. For different emission angles, electronic spectra for proton incidence along the two principal crystal axes ([100] and [110]) are compared with those co rresponding to an impact velocity in a random direction, finding effects associated with the channeling conditions.

Channeling effect in electronic spectra produced by grazing impact of fast protons on insulator surfaces

Electron emission due to grazing scattering of fast protons from LiF and KCl surfaces is studied under axial channeling conditions. The differential electron emission probability is calculated within a distorted-wave formalism, taking into account realistic channeled trajectories. For electronic spectra along low-index crystallographic directions, we find that channeling effects on the electron emission yield depend strongly on the penetration rate of projectiles into the solid.

MeV-ion axial surface-channeling from an electron irradiated KCl(0 0 1) surface

Surface-channeling of protons incident with 5 mrad on an electron-irradiated surface is investigated using a 0.55 MeV beam of protons. The target surface is KCl(0 0 1), which is damaged by electron-stimulated desorption with 5 keV electron irradiation. The direction of the incident beam is adjusted along the 〈1 0 0〉 and 〈1 1 0〉 channeling conditions and the channeling-dips of the scattering yields are observed. The irradiation dose-dependence of the minimum yields and widths of the dips is measured. Two dips are compared, i.e., for the 〈1 0 0〉 and 〈1 1 0〉 channelings. By increasing the irradiation dose up to 2 × 10 16 cm −2, the dip around the 〈1 0 0〉 axis becomes opened, but that of the 〈1 1 0〉 axis becomes shallow. The irradiated surfaces are observed to have many overlapped terraces of sub-micron with monolayer steps. The surface morphology deformed by the irradiation effects to truncate trajectories of the 〈1 0 0〉 channeling protons. This roughness of the surface is more effective for the trajectories of the 〈1 1 0〉 channeling protons. The protons incident on the rough surface along the 〈1 1 0〉 axis are not reflected from the atomic row but reflected by a potential of the surface with steps. Results by a simple computer simulation of the trajectories of protons at stepped surfaces also indicate the scattering processes.

Kinetics of double resonant coherent excitation of relativistic multicharged ions in crystals beyond the channeling conditions

The latest experiments carried out by the Tokyo group on the double resonant coherent excitation of Ar16+ and Ar17+ ions in thin silicon targets beyond the channeling conditions have been theoretically analyzed using the density matrix method for the joint description of coherent and incoherent (relaxation) interactions of relativistic multicharged ions in crystals. The calculation results are compared in detail with the experimental data.

Polarization of photons emitted in the process of resonant coherent excitation of relativistic ions under planar channeling conditions

Owing to the recent observation of the strong anisotropy of characteristic X-ray radiation accompanying the resonant coherent excitation of relativistic Fe24+ ions under planar channeling conditions in a silicon crystal, the resonant coherent excitation method can be considered as a candidate for a source of polarized X-ray radiation. The Stokes parameters of the radiation have been calculated using the density matrix approach. The behavior of the polarization characteristics of the radiation in various directions has been explained by considering the properties of the resonance part of the crystal field, which excites an ion and has the form of an elliptically polarized electric field.

Dynamic dependence of the interaction potentials for grazing scattering of fast atoms from metal and insulator surfaces

For scattering of fast atoms from metal and insulator surfaces under axial channeling conditions pronounced peaks in the angular distributions of scattered projectiles are interpreted in terms of rainbow scattering. The angular position of such “rainbow peaks” are closely related to the interaction potential and its corrugation in the topmost surface region. We have scattered N and O atoms, with energies ranging from 10 to 70 keV, from clean and flat Al(0 0 1) and LiF(0 0 1) surfaces along low index axial directions in the surface plane and studied the positions of the rainbow peaks as function of the kinetic energy of the atomic projectiles normal to the surface. For the insulator surface the rainbow angle does not depend on projectile energy for constant normal energy, whereas for the metal surface we find pronounced dynamic effects. We interpret this different behaviour as arising from a projectile energy dependent contribution to the underlying interaction potentials owing to embedding the projectiles into the free electron gas in the selvedge of the surfaces, which is present for the metals but absent for insulators.

Structure of ultrathin oxide layers on metal surfaces from grazing scattering of fast atoms

The structure of ultrathin oxide layers grown on metal substrates is investigated by grazing scattering of fast atoms from the film surface. We present three recent experimental techniques which allow us to study the structure of ordered oxide films on metal substrates in detail. (1) A new variant of a triangulation method with fast atoms based on the detection of emitted electrons, (2) rainbow scattering under axial surface channeling conditions, and (3) fast atom diffraction (FAD) for studies on the structure of oxide films. Our examples demonstrate the attractive features of grazing fast atom scattering as a powerful analytical tool in surface physics.

Resonant coherent excitation of Li-like Ar15+ Ions in a Thin Si crystal

We investigated resonant coherent excitation (RCE) of 445 MeV/u Li-like Ar15+ under the planar channeling and non-channeling conditions. The K-shell and L-shell excitations are observed as decreases in the survival fraction of the Ar15+ ions after passing through a thin Si crystal. RCE under the planar channeling condition provides the opportunity to study a strong static Stark effect on the Li-like system with K-shell or L-shell vacancy. Under the non-channeling conditions, the observed excitation energies and the reported theoretical calculations were in reasonable agreement.

Ion beam triangulation based on electron detection for studies on the structure of 1 ML Mnon Cu(001)

Recent developments in studies on the structure of surfaces based on ion beamtriangulation are discussed. We will outline recent experimental progress in the applicationof this method, which is closely related to the detection of the number of emitted electronsper incident ion during scattering under surface channeling conditions. Key features are thepronounced change of electron emission for the projectile beam aligned along a low indexcrystallographic direction in the surface plane (‘axial surface channeling’) and theinterpretation of data in terms of classical trajectory computer simulations. As arepresentative example we will discuss here studies on the structure of the low temperaturec(8 × 2) Mn/Cu(001) phase.

Assessment of swift-ion damage by RBS/C: Determination of the amorphization threshold

A theoretical strategy is developed to quantitatively assess the ion-beam damage as measured by Rutherford-backscattering spectrometry under channeling conditions (RBS/C) in LiNbO3 crystals. The approach is based on a recent exciton decay model to calculate the concentration of defects generated by the incoming ions as a function of their stopping power. To describe the channeled RBS yield the model takes into account the amorphous (core) as well as the defective halo contributions to the defect tracks caused by the ion impacts. It is concluded that the halo may significantly influence the assessed damage.

Diffraction of fast atoms under axial surface channeling conditions

For scattering of fast atoms under axial channeling conditions from surfaces diffraction effects have been observed in the distributions for scattered projectiles. Basic features concerning this recently observed new phenomenon in ion/atom surface scattering will be demonstrated for collisions of light atoms with well ordered clean and adsorbate covered surfaces of insulators and metals. We will focus our discussion on the scattering of 3He atoms with keV energies from a LiF(001) surface along low indexed axial surface channels and deduce the corrugation of the He–LiF(001) interaction potential from intensity modulations of the diffraction spots. Furthermore, we observe for the first time interference effects also for He+ ions which undergo charge exchange during scattering from the surface. The potential of the method for the investigation of structures of surfaces is discussed.