Neutralization effects in impact collision ion scattering spectroscopy: 6Li+ versus 4He+

Impact collision ion scattering spectroscopy (ICISS) was employed to analyze a Cu(110) surface utilizing 5 keV ⁶Li⁺ and 3.5 keV ⁴He⁺ projectiles. The experimental data revealed large differences between the polar-angle dependence of the scattered yield of Li and He ions, despite the fact that the scattering cross sections for the two projectiles were approximately equal. The differences in the scattering behavior are attributed primarily to the Auger neutralization of backscattered He. A model for quantitatively fitting the neutralization behavior is discussed.     

Electron-spin-dependent 4He+ ion scattering on Bi surfaces

We studied low-energy (～ 1.55 keV) electron-spin-polarized ⁴He⁺ ion scattering on a Bi(111) ultrathin film epitaxially grown on a Si(111) substrate. We observed that the scattered ion intensity differed between the incident He⁺ ions with up and down spins even though Bi is a non-magnetic element. To analyze the origin of this spin-dependent ion scattering (the spin asymmetry), we investigated the detailed relationship between the spin asymmetry and the incident angle, the azimuthal angle, the scattering angle, and the incident energy. All the data indicate that the spin asymmetry originates from the scattering cross section owing to the non-central force in the He⁺–Bi atom binary collision. The non-central force is most likely attributed to the spin–orbit coupling that acts transiently on the He⁺ 1s electron spin in the binary collision.     

Trajectory and collision related neutralisation in low energy He+ ion scattering

A recent Surface Science Letter by MacDonald and Martin (Surface Sci. 111 (1981) L739) presents new experimental data on the neutralisation of He⁺ ions scattered from a Ni surface, which is interpreted in terms of a large collisional contribution to the neutralisation, not compatible with the usual Auger neutralisation picture. With the aid of new calculations, it is shown here that these data are consistent with an Auger neutralisation model involving local ion-atom charge exchange.     

Spin-dependent low-energy 4He+ ion scattering from nonmagnetic surfaces

We investigated electron-spin-polarized (4)He(+) ion scattering on various nonmagnetic surfaces at kinetic energies below 2 keV. It was observed that the scattered He(+) ion yield depends on the He(+) ion spin. We interpret this spin-dependent scattering in terms of the spin-orbit coupling that acts transiently on the He(+)1s electron spin in the He(+)-target binary collision. This interpretation qualitatively explains the relationship between the spin-dependent scattering and the scattering geometry, incident velocity, and magnetic field arrangement. This is the first study to report spin-orbit coupling caused by projectile electron spin in ion scattering.     

The determination of shadowing critical angles in impact collision ion scattering spectroscopy

Critical angles for shadowing in low-energy ion scattering spectroscopy are calculated in the momentum approximation for the Thomas-Fermi-Molière potential and the Ziegler-Biersack-Littmark potential. In the relevant parameter range the results can be fitted by a formula containing five constants depending on the potential only. For a fixed projectile-target combination at a given energy the distance dependence of the critical angle is described by a simple power law. The comparison of the calculated results with experimental data shows that the inclusion of the effect of thermal vibrations on the critical angles improves the agreement between calculations and experiment significantly.     

Crystal effects in the neutralization of He+ ions in the low energy ion scattering regime

Investigating possible crystal effects in ion scattering from elemental surfaces, measurements of the positive ion fraction P+ are reported for He+ ions scattered from single and polycrystalline Cu surfaces. In the Auger neutralization regime, the ion yield is determined by scattering from the outermost atomic layer. For Cu(110) P+ exceeds that for polycrystalline Cu by up to a factor of 2.5, thus exhibiting a strong crystal effect. It is much less pronounced at higher energies, i.e., in the reionization regime. However, there a completely different angular dependence of the ion yield is observed for poly- and single crystals, due to massive subsurface contributions in nonchanneling directions.     

Measuring concentration depth profiles at liquid surfaces: Comparing angle resolved X-ray photoelectron spectroscopy and neutral impact collision scattering spectroscopy

Measuring concentration depth profiles is important for analyzing surfaces. The surface excess, the change in concentration from the surface to the bulk and separation of the constituents are some of the features that can be derived from concentration depth profiles and are most important in particular for analyzing liquid surfaces. Angle resolved X-ray photoelectron spectroscopy (ARXPS) and neutral impact collision ion scattering spectroscopy (NICISS) are methods used for determining concentration depth profiles at liquid surfaces. Here we compare concentration depth profiles determined with both methods from the constituents of a solution of the ionic surfactant tetrabutylphosphonium bromide in the polar solvent formamide. ARXPS is performed with a laboratory X-ray source and NICISS with 4.5 keV helium ions. Agreement is found in the surface excess and in the shape of the cation concentration depth profiles. Disagreement was found in the shape of the anion concentration depth profiles. The separation of charges as found with NICISS for projectiles with low kinetic energies could not be reproduced. The advantages of each of the methods and the criteria for selecting the projectile energy in NICISS are discussed.     

Low sputter damage of metal single crystalline surfaces investigated with medium energy ion scattering spectroscopy

It was observed clearly that the sputter damage due to Ar⁺ ion bombardment on metal single crystalline surfaces is extremely low and the local surface atomic structure is preserved, which is totally different from semiconductor single crystalline surfaces. Medium energy ion scattering spectroscopy (MEIS) shows that there is little irradiation damage on the metal single crystalline surfaces such as Pt(111), Pt(100), and Cu(111), in contrast to the semiconductor Si(100) surfaces, for the ion energy of 3–7 keV even above 10¹⁶–10¹⁷ ions/cm² ion doses at room temperature. However, low energy electron diffraction (LEED) spots became blurred after bombardment. Transmission Electron Microscopy (TEM) studies of a Pt polycrystalline thin film showed formation of dislocations after sputtering. Complementary MEIS, LEED and TEM data show that on sputtered single-crystal metal surfaces, metal atoms recrystallize at room temperature after each ion impact. After repeated ion impacts, local defects accumulate to degrade long range orders.     

分子尺度上水和金属表面的相互作用

虽然水普遍存在,但人们在分子尺度上对水与固体表面的相互作用的理解却仍然处在初始阶段.文章简述了20年来人们对水在贵金属表面的吸附和浸润过程进行微观尺度上研究的进展,分析和讨论了水和表面作用的一般规律和所获得的经验教训,特别着重讨论了对上世纪80年代人们提出的经典双层冰模型的修正.     

MCD spectroscopy of transition metal ions in fluoride crystals

Absorption spectra of crystals of KZn _x Fe_1-x F₃ (0·006 < 1 - x < 0·07) have been measured in order to obtain information about Fe²⁺-F^--Fe²⁺ pairs. However, by means of high resolution MCD spectroscopy, it has been established that very small amounts of Fe³⁺ are present in the crystals. The absorption spectra then show features which are due to (a) Fe²⁺, (b) Fe³⁺ and (c) Fe²⁺, Fe³⁺ pairs. In addition to the characteristic Jahn-Teller split ⁵ T _2g → ⁵ E_g band of Fe²⁺, there is a weak, spin-allowed, vibronically assisted transition near 40 000 cm^-1, which is assigned to ⁵ T _2g (d⁶) → ⁵ A _1g (d⁵s). The MCD of this band is consistent with this assignment. The remainder of the visible and near ultra-violet absorption intensity is associated with Fe²⁺/Fe³⁺ pairs. Most of the bands are due to excitations localized on the Fe²⁺ though some weaker ones are assigned as excitations to ⁴ A _1g on the Fe³⁺ of the pair. There is also a strong, broad underlying absorption probably due to t _2g → t _2g charge transfer in the pair.     

分子尺度上水和金属表面的相互作用

虽然水普遍存在,但人们在分子尺度上对水与固体表面的相互作用的理解却仍然处在初始阶段.文章简述了20年来人们对水在贵金属表面的吸附和浸润过程进行微观尺度上研究的进展,分析和讨论了水和表面作用的一般规律和所获得的经验教训,特别着重讨论了对上世纪80年代人们提出的经典双层冰模型的修正.     

Low-energy 3He+ ion scattering spectroscopy studies of D2 adsorption on W(211)

Direct physical evidence for occupation of a trough site by the β₂ state of deuterium adsorbed on W(211) has been obtained by angle-resolved ³He⁺/D(ads) ion-scattering spectroscopy (ISS) in combination with LEED/Auger. The W(211) surface is composed of close-packed 〈111〉 rows of W atoms separated by a wide channel. Previous thermal desorption studies have shown two clearly resolved hydrogen states: β₁ desorbing with a temperature maximum of ca. 60°C, and β₂ at ca. 400°C. Analyses of flash desorption, work-function, adsorption kinetics, stoichiometry and mixed adsorption further indicated that the more tightly bound β₂ form occupies a deep-trough position. In the present study, ISS polar-angle profiles were taken at an incident energy of 306 eV. With only the β₂ state populated, a ³He⁺ beam parallel to the close-packed 〈111〉 rows was found to scatter from D(ads) with a cutoff angle close to grazing incidence while for the perpendicular direction D(ads) scattering is observed only for angles greater than 18° away from grazing incidence. These measurements are consistent with the corrugated W(211) geometry and with the proposed β₂-D trough-site model.     

EPR spectroscopy of weak exchange interactions between Co2+ ions in ZnO

Electron paramagnetic resonance (EPR) is used to identify the next nearest neighbour Co²⁺ pairs coupled by spin–spin interaction in Co‐doped ZnO single crystals grown by the hydrothermal technique. These dimer centers are described by a spin Hamiltonian with exchange coupling terms written as interaction between identical effective spins S^eff = 1/2 of the lowest ground state Kramers doublets of the two Co²⁺ ions. The exchange parameters of weakly ferromagnetically coupled next nearest neighbor Co²⁺ pairs are estimated. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Corrugating and anticorrugating static interactions in helium-atom scattering from metal surfaces

We perform a density-functional-theory calculation of the static repulsive potential of He scattering off a noble and a simple metal surface. The classical turning point of He on Cu(111) is found to be closer to the metal when the adatom is at top than at bridge site (anticorrugating effect). The potential of He on Al(111) is instead corrugated. By comparing the results of the two systems, we conclude that the He-metal anticorrugating effect occurs when the kinetic energy difference for He at top and bridge sites is larger than the electrostatic one, and an induced localized dipole on He is formed.     

Element-selective spin polarization analysis of surfaces by spin-polarized ion scattering spectroscopy

We discuss the validity of surface spin polarization analysis with element selectivity using spin-polarized ion scattering spectroscopy (SP-ISS). We examined the control of the incident ⁴He⁺ spins and successfully conducted magnetic hysteresis measurement on an Fe(100) surface. The spin polarization of the Fe(100) surface exposed to O₂ atmosphere measured by spin-polarized ion neutralization spectroscopy was consistent with that reported by spin-polarized metastable de-excitation spectroscopy. The element selectivity of SP-ISS is discussed in terms of ion neutralization, re-ionization, and multiple scattering.     

Computer simulations and rainbow patterns of alkali ion scattering from metal surfaces

In a previous article [Surface Sci. 172 (1986) 90] the necessity of 3-dimensional computer simulations has been shown for the modelling of the scattering of 10–100 eV K⁺ from W(110). The measured triple differential cross section shows a complicated peak structure which can only be understood with the help of trajectory calculations. A simple model potential is given that can describe the scattering at 35 eV. The calculated trajectories show from which impact regions in the surface unit cell the peaks in the spectra originate. This mapping also allows a rainbow analysis for a triple differential cross section.     

First order Raman scattering analysis of transition metal ions implanted GaN

Transition Metal (TM) ions V, Cr, Mn and Co were implanted into GaN/sapphire films at fluences 5×10¹⁴, 5×10¹⁵ and 5×10¹⁶ cm⁻². First order Raman Scattering (RS) measurements were carried out to study the effects of ion implantation on the microstructure of the materials, which revealed the appearance of disorder and new phonon modes in the lattice. The variations in characteristic modes 1GaN i.e. E₂(high) and A₁(LO), observed for different implanted samples is discussed in detail. The intensity of nitrogen vacancy related vibrational modes appearing at 363 and 665 cm⁻¹ was observed for samples having different fluences. A gallium vacancy related mode observed at 277/281 cm⁻¹ for TM ions implanted at 5×10¹⁴ cm⁻² disappeared for all samples implanted with rest of fluences. The fluence dependent production of implantation induced disorder and substitution of TM ions on cationic sites is discussed, which is expected to provide necessary information for the potential use of these materials as diluted magnetic semiconductors in future spintronic devices.     

The characteristic spectral lines of target atoms in the impact of ~(40)Ar~(q+) ions on metal surfaces

1 Introduction It has been found that a large number of ions and atoms can be sputtered; and elec-trons and X-ray can be emitted in the impact of slow highly charged ions (SHCI) onmetal surfaces. It has also been shown that a slow highly charged ion can deposit anamount of potential energy ranging from tens to hundreds of keV within a nanometer-sized volume on femtosecond time scale during impinging on a solid surface. Theequivalent power density is about 1014 W/cm2 and bombardment craters …