Neutralization rate for slow Ar ions in front of KCl(0 0 1)

Charge state distributions of reflected ions are measured when 5 keV Ar^q+(q = 0−2) ions are incident on a clean KCl(0 0 1) surface at grazing angle, θ_i. Although the charge state distribution does not depend on the incident charge state at larger θ_i, significant dependence of the charge state distribution on incident charge state is observed at smaller θ_i. The ionization of Ar⁰ is completely suppressed at θ_i < 20 mrad, while large neutralization probability is observed for Ar⁺ incidence. These features allow us to derive the position-dependent neutralization rate of Ar⁺ in front of KCl(0 0 1). The obtained neutralization rate decreases exponentially with distance from the surface as it is usually assumed.     

Neutralization of keV Ne and Ar ions during grazing scattering from an Al(1 1 1) surface

Charge fractions after scattering of Ne⁺ ions, Ne⁰ atoms and Ar⁺ ions with keV energies under a grazing angle of incidence from an atomically clean and flat Al(1 1 1) surface are studied. For incoming Ne⁺ ions we observe defined ion fractions in the scattered beams, whereas for incident Ne⁰ atoms ion fractions are more than one order of magnitude smaller. This experimental result provides clear evidence for a survival of Ne⁺ ions over the whole scattering event. From the dependence of ion fractions on the perpendicular energy component we derive neutralization rates as function of distance from the surface. These rates compare well with recent theoretical calculations for the system He⁺–Al(1 1 1). For incident Ar⁺ ions no survival of ions is found and upper limits for the survival probability and lower limits for the neutralization rate are determined.     

Charge transfer in grazing scattering on clean and oxygen covered Cu(1 1 0) surfaces: velocity effects

We have measured negative (F⁻) and positive (Na⁺, Li⁺) charge fractions produced by grazing scattering from the corrugated metallic Cu(1 1 0) surface. A wide range of incident energies and scattered angles were explored in order to analyse the effects of the parallel and perpendicular velocities of the projectiles on the charge transfer process. The obtained dependences of F⁻ formation and of Na⁺ and Li⁺ neutralization with the parallel velocity are in good agreement with previous experimental and theoretical studies, which incorporate parallel velocity effects via the “shifted Fermi sphere” model. The surface structure is then modified by adsorption of oxygen. The dependence of charge fractions with oxygen adsorption and the effects of velocity in the case of the Cu–O surface are analysed.     

Surface spin polarization in Fe(1 1 0) and Ni(1 1 0)

The magnetism of Ni(1 1 0) and Fe(1 1 0) surfaces was investigated by electron capture spectroscopy. He⁺ and He²⁺ ions impinged on the Fe(1 1 0) and Ni(1 1 0) surfaces under grazing incidence, and the degree of polarization of the light emitted by the neutralized projectiles was analyzed. Our measurements show that in Ni(1 1 0) minority electrons have a higher density of states at the Fermi energy than majority electrons, opposed to the Fe(1 1 0) case. From a comparison of our measurements we estimate the ratio between captured minority and majority electrons in Ni(1 1 0) to be similar as the ratio between captured majority and minority electrons in Fe(1 1 0).     

He scattering on a TiO2(1 1 0) surface

We present measurements of energy-losses and neutralisation of helium ions on a TiO₂(1 1 0) surface at low keV energies. Measurements were performed in the 1–4 keV energy range. The target azimuthal orientation dependence of total scattering intensity profiles, energy-losses and probability of electron capture or loss were determined. On the basis of Marlowe simulations it is concluded that ion scattering intensities give indications about defects regarding surface bridging O atoms. Variations in energy losses for different scattering directions are reported. Ion fraction measurements were made using both incident ions and neutrals. Similar ion fractions were found indicating that He is efficiently neutralised and ions result from reionisation. These ion fractions are found to depend on the azimuthal orientation and have a minimum along the main crystallographic directions.     

Low-energy ion scattering and sputtering at grazing ion bombardment of clean and oxygen covered Ag(1 1 0) surface

The ion scattering and sputtering processes at low energy grazing N⁺ and Ne⁺ ion bombardment of clean and oxygen covered Ag(1 1 0) surface have been investigated by computer simulation in the binary collision approximation.

The spatial, angular and energy distributions of scattered, sputtered particles and desorbed molecules of oxygen as well as their yields versus the angle of incidence have been calculated. In these distributions the some characteristic peaks were observed and analysed. It was found that an adsorption layer plays a role of the additional surface barrier, i.e. it reflects leaving target atoms back to crystal. The azimuth angular dependencies of Ag sputtering yield and non-dissociative O₂ desorption yield at grazing incidence have been calculated. It was shown that these dependencies correlate the crystal orientation.     

Channeling regimes in ion surface scattering

We report on surface channeling experiments of singly charged ions on single crystal surfaces of Pt(1 1 0) and Pd(1 1 0). Using a time-of-flight system installed in forward direction we analyze the energy distribution of the scattered projectiles. By variation of the primary energy and the angle of incidence we investigate effects of the perpendicular energy on the channeling features. The perpendicular energy is defined as E = E₀sin²ψ with ψ the angle of incidence. In combination with precise azimuthal rotations of the crystal, we are sensitive to axial channeling and obtain information about the limits of axial surface channeling. From a comparison with detailed trajectory calculations we find that axial channeling effects are most pronounced for a perpendicular energy between 5 and 20 eV. As a result, we obtain an exemplary channeling map for the interaction of nitrogen ions with the (1 × 2) reconstructed Pt(1 1 0) surface identifying different channeling regimes.     

Electron emission induced by H and H incident on thin carbon foils: influence of charge changing processes

The forward and backward electron emission yields γ_F and γ_B have been calculated by Monte Carlo simulation for protons (H⁺) and hydrogen atoms (H⁰) (with energies between 25 keV and 5 MeV) incident on thin amorphous carbon foils. Direct electron excitations by the incident projectiles as well as electron excitations resulting from charge exchange processes undergone by H⁺ or H⁰ have been taken into account. For the latter, Auger and Shell processes have been considered. Subsequent electron transport has been considered in order to calculate the forward and backward electron emission yields γ_F and γ_B.     

Large conductivity enhancement in polycrystalline 12CaO · 7Al2O3 thin films induced by extrusion of clathrated O ions by hot Au implantation and ultraviolet light illumination

Large enhancement in electrical conductivity from <10⁻¹⁰ S cm⁻¹ to 4 × 10⁻² S cm⁻¹ was achieved in polycrystalline 12CaO · 7Al₂O₃ (p-C12A7) thin films by hot Au⁺ implantation at 600 °C and subsequent ultraviolet (UV) light illumination. Although the as-implanted films were transparent and insulating, the subsequent UV-light illumination induced persistent electronic conduction and coloration. A good correlation was found between the concentration of photo-induced F⁺-like centers (a cage trapping an electron) and calculated displacements per atom, indicating that the hot Au⁺ implantation extruded free O²⁻ ions from the cages in the p-C12A7 films by kick-out effects and left electrons in the cages. These results suggest that H⁻ ions are formed by the Au⁺ implantation through the decomposition of preexisting OH⁻ ions. Subsequent UV-light illumination produced F⁺-like centers via photoionization of the H⁻ ions, which leads to the electronic conduction and coloration.     

Charge state distribution and its equilibration of 2 MeV/u sulfur ions passing through carbon foils

Charge state distributions of 2.0 MeV/u sulfur ions of various initial charge states (6, 10, 11, 13+) after passing through 0.9, 1.5, 2.0, 3.0, 4.7, 6.9 and 10 μg/cm² carbon foils have been studied. It is observed that the processes involving the L-shell electrons are equilibrated within the target thickness of 5 μg/cm² and the charge equilibration over this thickness is ruled by the K-shell processes. Measured charge state distributions do not flat off to establish equilibrium within the measured thicknesses, but the mean charge states almost saturate to 12.4 for all initial charge states examined. Calculation with ETACHA code, developed by Rozet et al. [Nucl. Instr. and Meth. B 107 (1996) 67], is employed, although the present impact energy is lower than the assumed region for the code.     

Optical absorption of metallic Zn nanoparticles in Zn ion implanted sapphire

Zn⁺ ion implantation (48 keV) was performed at room temperature up to a fluence of 5 × 10¹⁷ cm⁻² in -Al₂O₃ single crystals. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and optical absorption spectroscopy were utilized to characterize the optical properties, chemical charge states and the microstructure of embedded metallic Zn nanoparticles, respectively. XPS analysis indicated that implanted Zn ions are in the charge state of metallic Zn⁰. TEM analysis revealed the metallic Zn nanoparticles of 3–10 nm in the as-implanted sample at a fluence of 1 × 10¹⁷ cm⁻². A selected area electron diffraction (SAD) pattern indicates the random orientation of the Zn nanoparticles. A clear absorption peak appeared gradually in the optical absorption spectra of the as-implanted crystals, due to surface plasma resonance (SPR) of Zn nanoparticles. The wavelength of the absorption peak shifted from 260 nm to 285 nm with the increasing ion fluence, ascribed to the growth of Zn nanoparticles.     

Effect of electric charge accumulation on low energy ion implantation in insulators

Charge accumulation at the surface of insulators during low energy ion implantation is related to two processes: ion impinging on the sample and secondary electron emission. Samples composed of a piece of Si (having the size of the ion beam) fixed on the centre of polyethylene (PE) coupons have been implanted with 2.2 keV H₂ ions to a fluence of 2 × 10¹⁶ H/cm². ERD (Elastic Recoil Detection) depth profiles of the implanted ions are shallower with an increase of the PE coupon size. The relative critical Si/PE size to repel all the incident ions is around 1.1 × 1.1 cm²/2.5 × 2.5 cm². The potential of the secondary electron suppressor has been varied from −500 V to +500 V. It changes the secondary electron distribution around the implanted area and, consequently, affects the accumulation of charges at the sample surface. When the potential is 0 V, a uniform ion implantation with little effect of charge accumulation for all sizes of PE coupons is obtained. A two-dimension model has been performed and gives a good explanation for the mechanism of the electric charge neutralisation.     

Skipping motion of 30 keV H ions on single crystal surface of KCl

The angular distributions and the energy spectra of scattered H⁺ ions are studied at glancing angle incidence of 30 keV H⁺ ions on the (001) surface of KCl single crystal. Discrete energy loss spectra of the scattered ions are observed. The observed exit angle distributions of the ions which have lost anomalous large amounts of energy at various angles of incidence do not depend on the angle of incidence. From the observed results, it is concluded that skipping motion occurs on the surface of KCl.     

Effect of spin polarization of Ni(1 1 0) surface on Auger neutralization for grazing scattering of He ions

The survival of ions during grazing scattering of keV He⁺ ions from a clean Ni(1 1 0) surface is studied as function of target temperature. We observe ion fractions in the scattered beams of typically 10⁻³ which show a slight increase with temperature of the target surface. From computer simulations of projectile trajectories we attribute this enhancement for ion fractions to effects of thermal vibrations of lattice atoms on the survival of ions in their initial charge state. Based on concepts of Auger neutralization, we discuss the role of the spin polarization of target electrons on charge transfer. We do not find corresponding signatures in our data and conclude that in the present case of Ni(1 1 0) the spin polarization has to be small.     

Broadening effects in Auger neutralization of 130–430 eV Ar ions at Al surfaces

We present a model for electron emission from Al surfaces by Auger neutralization of 130–430 eV Ar⁺ ions, that includes the singular response of the metal conduction band to the abrupt change of the surface potential caused by electron capture by the incident ion. This effect, previously identified in X-ray studies, produces a broadening that plays a significant role in reproducing the higher energy part of the experimental electron kinetic energy distributions.     

Resonant ion stimulated desorption of O from the TiO2(1 1 0) surface: Effects of oxygenation and ion bombardment

The O⁺ desorption from reduced, oxygenated, and ion-bombarded TiO₂(1 1 0) surfaces has been investigated during He⁺ irradiation. The O⁺ desorption is initiated by creation of an antibonding O 2s core hole state via quasi-resonant charge exchange with the He⁺ 1s state, followed by the intra-atomic Auger decay of the O 2s hole. Upon oxygenation of the reduced TiO₂(1 1 0) surface, the O⁺ yield increases by one order of magnitude. The O₂ molecule is dissociated at the vacancy site of bridging oxygen and the oxygen atoms either fill a vacancy site or chemisorb at a fivefold-coordinated Ti⁴⁺ site as an adatom. The latter is detected with much higher efficiency than the former. The O⁺ yield is increased during He⁺ bombardment of the reduced TiO₂(1 1 0) surface due to formation of lower coordinated oxygen atoms. The oxygen species thus formed by ion bombardment or oxygenation are unstable on the surface and tend to diffuse into bulk vacancy sites or higher coordination surface sites even at room temperature.     

Color center annealing and ageing in electron and ion-irradiated yttria-stabilized zirconia

We have used X-band electron paramagnetic resonance (EPR) measurements at room-temperature (RT) to study the thermal annealing and RT ageing of color centers induced in yttria-stabilized zirconia (YSZ), i.e. ZrO₂:Y with 9.5 mol% Y₂O₃, by swift electron and ion-irradiations. YSZ single crystals with the 1 0 0 orientation were irradiated with 2.5 MeV electrons, and implanted with 100 MeV ¹³C ions. Electron and ion beams produce the same two color centers, namely an F⁺-type center (singly ionized oxygen vacancy) and the so-called T-center (Zr³⁺ in a trigonal oxygen local environment) which is also produced by X-ray irradiations. Isochronal annealing was performed in air up to 973 K. For both electron and ion irradiations, the defect densities are plotted versus temperature or time at various fluences. The influence of a thermal treatment at 1373 K of the YSZ single crystals under vacuum prior to the irradiations was also investigated. In these reduced samples, color centers are found to be more stable than in as-received samples. Two kinds of recovery processes are observed depending on fluence and heat treatment.     

Irradiation by swift heavy ions: Influence of the non-equilibrium projectile charge state for near surface experiments

When dealing with surface and/or thin film modifications of materials under irradiation by swift heavy ions, special attention should be given to the choice the incident projectile charge state delivered by the accelerator. This charge state should be of the order of the effective charge defined in the electronic energy loss theory. Under some hypothesis, this effective charge is assumed to be equivalent to the mean charge state deduced from measurements of ion charge state distribution. To reach such a charge state, it is necessary to introduce before the irradiated sample a thin stripper foil of carbon, the thickness of which is in between 5 and 1500 μg/cm² for beam energies ranging between 1 and 44 MeV/u, respectively.     

Carbon ion-implanted optical waveguides in Nd:YLiF4 crystal: Refractive index profiles and thermal stability

We report on the optical planar waveguides in Nd:YLiF₄ laser crystals fabricated by 6.0 MeV C³⁺ ion implantation at doses of 1 × 10¹⁵ or 2.5 × 10¹⁵ ions/cm², respectively. The refractive index profiles, which are reconstructed according to the measured dark mode spectroscopy, show that the ordinary index had a positive change in the surface region, forming non-leaky waveguide structures. The extraordinary index is with a typical barrier-shaped distribution, which may be mainly due to the nuclear energy deposition of the incident ions into the substrate. In order to investigate the thermal stability of the waveguides, the samples are annealed at temperature of 200–300 °C in air. The results show that waveguide produced by higher-dose carbon implantation remains relatively stable with post-irradiation annealing treatment at 200 °C in air.