Neutralization Of He Research Articles

Different contributions to spin polarization in the Auger neutralization of He+ ions at the Ni (111) surface under the influence of screening length

Theoretical aspects are presented for the spin polarization of electrons excited during the impact of spin-polarized He+ ions on a Ni (111) surface. The screening length effects due to the screened Coulomb (Yukawa) potential on spin polarization are explored within the framework of spin-dependent Auger neutralization process. First-order perturbation theory is used to calculate the Auger rates. The incident He+ ions and the metal electrons are spin-polarized so that we will analyze the spin of empty bound state of He+ ions by two possibilities. One is that the spin of empty bound state is parallel to the majority spin in the metal electrons, and the other is that the spin of empty bound state is parallel to the minority spin in the metal electrons. Results are discussed depending on the spin-dependent matrix elements and the neutralization probability of He+ ions with different contributions.

The Local Interstellar Magnetic Field Observed by Voyager 1 and IBEX

Observations by the Interstellar Boundary Explorer (IBEX) have revealed the presence of the IBEX Ribbon, an almost circular band of increased ENA emissions that is believed to be centered on the direction of the interstellar magnetic field. IBEX measures a range of interstellar species including H, He, and O. IBEX has been able to distinguish primary He coming directly from the interstellar medium from secondary He that is created through neutralization of He+ atoms in the heliosheath. Together with the observations of Lyman-α radiation from SOHO/SWAN, which is resonantly absorbed and re-emitted from interstellar H atoms that move into the heliosphere, the directions of interstellar flow for each of the observed interstellar species (H, primary He, secondary He, and primary O) line-up along a symmetry plane — the interstellar B–V plane — that should contain the interstellar magnetic field direction (B) and the interstellar flow direction (V). The B–V plane also contains the center of the IBEX Ribbon, which supports the concept that the Ribbon center is close to the direction of the interstellar magnetic field. Further, the deflection of the various interstellar species along the B–V plane are consistent with the compression of the heliosheath by the interstellar magnetic field pressure. Lastly, the interstellar magnetic field in the outer heliosheath is directly observed by Voyager 1 (V1). The direction of the interstellar magnetic field observed by V1 is offset from the Ribbon center, consistent with draping around the heliopause, and time variations in the interstellar magnetic field are consistent with the effects of large-scale compression and rarefaction in the outer heliosheath. Thus, we summarize the array of observations that suggest a consistent direction of the interstellar magnetic field, and the effects of the interstellar magnetic field on the structure and time variations in the heliosheath.

Calculation of the Secondary Electron Emission Coefficients of MgO, MgBeO, MgCaO, MgSrO and MgBaO Induced by Auger Neutralization of He$^+$, Ne$^+$, Ar$^+$ and Xe$^+$ Ions

Calculation of the Secondary Electron Emission Coefficients of MgO, MgBeO, MgCaO, MgSrO and MgBaO Induced by Auger Neutralization of He$^+$, Ne$^+$, Ar$^+$ and Xe$^+$ Ions

Kinetic Energy Distribution of Auger Electrons of MgO, CaO, SrO and BaO Induced by Auger Neutralization of He$^+$, Ne$^+$, Ar$^+$ and Xe$^+$ Ions

Kinetic Energy Distribution of Auger Electrons of MgO, CaO, SrO and BaO Induced by Auger Neutralization of He$^+$, Ne$^+$, Ar$^+$ and Xe$^+$ Ions

Mutual neutralization of He(+) with the anions Cl(-), Br(-), I(-), and SF6 (.).

Mutual neutralization (MN) rate coefficients kMN for He(+) with the anions Cl(-), Br(-), I(-), and SF6 (-) are reported from 300 to 500 K. The measured rate coefficients may contain a contribution from transfer ionization, i.e., double ionization of the anion. The large rate coefficient for He(+) + SF6 (-) (2.4 × 10(-7) cm(3) s(-1) at 300 K) is consistent with earlier polyatomic MN results found to have a reduced mass dependence of μ(-1/2). Neutralization of He(+) by the atomic halides follows the trend observed earlier for Ne(+), Ar(+), Kr(+), and Xe(+) neutralized by atomic halides, kMN (Cl(-)) < kMN (Br(-)) < kMN (I(-)). Only an upper limit could be measured for the neutralization of He(+) by Cl(-). Predictions of the rate coefficients from a previously proposed simple model of atomic-atomic MN results are consistent with the present He(+)-halide rate coefficients. The temperature dependences are modestly negative for Br(-) and I(-), while that for SF6 (-) is small or negligible.

Quasi-resonant neutralization of He+ ions at a germanium surface

When low-energy He ions are scattered from a Ge surface, the fraction of positive ions exhibits characteristic oscillations as a function of ion energy. These oscillations are caused by quasi-resonant neutralization (qRN), a process which is active for materials with a narrow band nearly resonant with the unperturbed He 1s-level. In this paper we measure the fraction of He+ backscattered from Ge(100). In conjunction with recently developed theoretical methods, we extract quantitative information on the efficiency of qRN. Our evaluation reveals that qRN is a highly efficient process leading to ion fractions two orders of magnitude lower than in systems for which neutralization is only due to Auger processes.

Effects of the atomic level shift in the Auger neutralization rates of noble metal surfaces

In this work we compare characteristics of Auger neutralization of He+ ions at noble metal and free-electron metal surfaces. For noble metals, we find that the position of the energy level of He with respect to the Fermi level has a non-negligible influence on the values of the calculated Auger rates through the evaluation of the surface dielectric susceptibility. We conclude that even though our calculated rates are accurate, further theoretical effort is needed to obtain realistic values of the energy level of He in front of these surfaces.

Role of He excited configurations in the neutralization of He+ions colliding with a HOPG surface

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Auger neutralization of He+ on Cu surfaces: Simulation of azimuthal scans

Charge exchange by Auger neutralization (AN) plays an important role in surface analysis techniques such as low energy ion scattering (LEIS). Recent advances in the theoretical description of AN have included a model based on a linear combination of atomic orbitals (LCAO) approach, which is able to calculate accurate neutralization probabilities of He+ due to AN in LEIS. Previous investigations have shown that the neutralization probability is strongly influenced by the distance dependent shift of the He 1s level. In this study simulations of He+ scattered from Cu(100) and Cu(110) surfaces at fixed azimuth angles are presented. Additionally, the azimuth dependence of ion- and neutral-yield for He+ scattered from Cu(100) is simulated and compared to experimental data. Calculations were performed using the LCAO model in combination with molecular dynamics simulations. The excellent agreement between simulation and experiment provides evidence that the obtained values for the level shift are a characteristic property of the surface.

Ab initioSimulation of Helium-Ion Microscopy Images: The Case of Suspended Graphene

Helium ion microscopy (HIM), which was released in 2006 by Ward et al., provides nondestructive imaging of nanoscale objects with higher contrast than scanning electron microscopy. HIM measurement of suspended graphene under typical conditions is simulated by first-principles time-dependent density functional theory and the 30 keV He+ collision is found to induce the emission of electrons dependent on the impact point. This finding suggests the possibility of obtaining a highly accurate image of the honeycomb pattern of suspended graphene by HIM. Comparison with a simulation of He0 under the same kinetic energy shows that electron emission is governed by the impact ionization instead of Auger process initiated by neutralization of He+.

Determination of 16O and 18O sensitivity factors and charge-exchange processes in low-energy ion scattering

Quantitative analysis in low-energy ion scattering (LEIS) requires an understanding of the charge-exchange processes to estimate the elemental sensitivity factors. In this work, the neutralization of He+ scattered by 18O-exchanged silica at energies between 0.6 and 7 keV was studied. The process is dominated by Auger neutralization for Ei &amp;lt; 0.8 keV. An additional mechanism starts above the reionization threshold. This collision-induced neutralization becomes the dominant mechanism for Ei &amp;gt; 2 keV. The ion fractions P+ were determined for Si and O using the characteristic velocity method to quantify the surface density. The 18O/16O sensitivity ratio indicates an 18% higher sensitivity for the heavier O isotope.

Atomic and fullerene ions interacting with metal surfaces

Recent progress of studies on the interaction of keV atomic and fullerene ions during grazing scattering from solid surfaces is reported. We focus on electron capture processes for two systems that have been investigated in detail during the last years, the neutralization of He and C60 ions at metal surfaces. Both species can be considered as model systems for Auger neutralization and for resonant neutralization of a large molecule, respectively. Basic experimental concepts and results obtained for scattering from an Al(100) surface are discussed.

Analysis of the Auger neutralization of He+ at Cu surfaces in low energy ion scattering

Recently, strong crystal effects in P+ were observed for He+ and Cu in the Auger neutralization regime, with differences in the ion fraction by up to a factor of three non-equivalent Cu surfaces. In this contribution, it is shown that these findings can quantitatively be described within the jellium model assuming perpendicular velocity scaling of P+.

Azimuth-dependent Auger neutralization ofHe+onAg(111)and (110) surfaces

We present a detailed theoretical analysis of the role played by s and d electrons in Auger neutralization processes of He{sup +} at Ag(111) and Ag(110) surfaces. We calculate crystal-lattice-site Auger neutralization rates as a function of the perpendicular distance between ions and surfaces. We find that the rate is very insensitive to the lateral position for large values of the perpendicular distance because the contribution of the delocalized s electrons dominates in this case. In contrast, the contribution of d electrons dominates at short perpendicular distances and the strong spatial localization of these electrons causes a similar strong dependence of the Auger rate with lateral position. We perform molecular dynamic simulations of scattered ion trajectories, which, used together with the Auger neutralization rates, allow us to obtain the theoretical ion fraction that we compare with our measurements. This parameter-free theory is able to reproduce the magnitude of the ion survival probability and its dependence with the azimuthal angle of incidence for both surfaces of Ag, thus showing the important role played by localized electrons in Auger neutralization of He.

Spin-dependent electron excitation and emission in the neutralization of He+ ions at paramagnetic surfaces

The interaction of He+ ions with surfaces has been widely used as a tool to probe different properties of the surface. There are recent experiments that measure the spin polarization of electrons emitted during the neutralization of spin polarized He+ ions impinging on the Al(001), Au(001) and Cu(001) surfaces. The experimental results show that (i) the spin polarization of the total emitted electron yield is about 30% for the three surfaces, being the preferable spin state that of the electron bound to the He+ ion and (ii) the spin polarization of electrons emitted at higher energies is high (around 70% for Al(001) and 50% for Au(001) and Cu(001)) but decreases as electrons with lower energies are included. Here, we apply our local model to understand in a quantitative way these measurements. First, we calculate the energy distribution of electrons excited in the Auger neutralization of a He+ ion. Next, we describe the transport and escape of these excited electrons to obtain the energy distribution of emitted electrons. The theoretical results allow us to explain the similarities and the differences found in these surfaces.

Auger Neutralization of He Ions at an Al(100) Surface Using Isotope Effect

He atoms and ions of the isotopes 3He and 4He are scattered with keV energies under a grazing angle of incidence from a flat and clean Al(100) surface. For the two isotopes we investigate Auger neutralization of incident He+ and He2+ ions via fractions of surviving ions. Pronounced effects for the different isotopes are observed which can be attributed to different time scales concerning the neutralization process of He ions in front of a metal surface. From the analysis of the data obtained for singly and doubly charged ions we find evidence that charge fractions for scattering of He+ ions from an Al surface result predominantly from a direct (Auger) electron capture event.

The dynamics of He + ion neutralization at a xenon film: Energy- and spin-resolved studies

The neutralization of He + ions with energies in the range 10–500 eV at an adsorbed xenon layer is examined both by analyzing the energy distribution of electrons ejected from the surface and by use of spin-labeling techniques, specifically the use of electron-spin-polarized He + ions coupled with measurement of the ejected electron polarization. The data indicate that neutralization proceeds via an Auger process similar to that which occurs at a clean high-work-function metal surface. At the higher ion energies, however, kinetic ejection becomes important and provides an increasing contribution to the total electron yield.

Studies of electron transfer dynamics in particle‐surface interactions

AbstractA review of recent work on electron transfer processes leading to neutralization of positive ions and formation of negative ions is presented. Experiments were performed on simple jellium‐like metal surfaces, transition metals, and specific characteristics of electronic band structure were investigated for some cases, revealing differences in electron transfer processes between surfaces of the same element. It was shown that electronegative adsorbates like oxygen and chlorine provoke strong changes in electron transfer rates, which are akin to promotion and poisoning of reactions at surfaces. The results of experiments on Auger neutralization of He+ ions on Ag surfaces and H− and F− formation on Ag and Pd surfaces are presented and compared with recent theoretical treatments of these processes. The effect of adsorbates is illustrated by the example of chlorine adsorption on silver. Directions where further theoretical effort would considerably advance the understanding of these phenomena are delineated.

Spin dependence in the neutralization of He + ions in metals: An analysis of different contributions

We study the spin polarization of the Auger electrons produced during the neutralization of He + ions in a free electron gas. In this process, one metal electron decays to the unoccupied state and a second electron is promoted to a continuum excited state. Although the spin of the decaying electron is fixed, both spins are allowed for the excited one. The states of the electrons involved in this Auger capture process are described by the spin-dependent Kohn–Sham orbitals obtained from density functional theory and the local spin approximation. The Auger capture rates indicate a strong polarization of the excited electron. In a paramagnetic free electron gas, there are two mechanisms accounting for this effect, the spin-dependent screening and the interference between indistinguishable processes when the involved electrons are in the same spin state. In a spin-polarized medium, the difference in the density of spin-up and spin-down electrons is a new ingredient to be considered. As a result, the excited electrons preferably come from the majority band, even in the case of He + ions with spin opposite to that of the majority band embedded in a low spin-polarized free electron gas.

Spin effects in the screening and Auger neutralization of He+ ions in a spin-polarized electron gas

The screening of a He+ ion embedded in a free electron gas is studied for different spin-polarizations of the medium. Density functional theory and the local spin density approximation are used to calculate the induced electronic density for each spin orientation, i.e. parallel or antiparallel to the spin of the electron bound to the ion. Since both the He+ ion and the electron gas are spin-polarized, we analyze in detail the spin state of the screening cloud for the two different possibilities: the spin of the bound electron can be parallel to either the mayority spin or the minority spin in the medium. Finally, the spin-dependent Kohn–Sham orbitals are used to calculate the Auger neutralization rate of the He+ ion. The polarization of the Auger excited electron is influenced by the spin-polarization of the medium. The results are discussed in terms of the spin-dependent screening and the indistinguishability of electrons with the same spin state.