Auger Neutralization Rates Research Articles

Secondary electron emission influenced by oxidation on the aluminum surface: the roles of the chemisorbed oxygen and the oxide layer

A relationship between the apparent secondary electron yield () and the oxygen coverage/oxide layer thickness on an aluminum cathode is obtained in an experiment under a controlled environment. The apparent secondary electron yield () is deduced from the breakdown voltage between two parallel plate electrodes in a 360 mTorr argon environment using a simple Townsend breakdown model with the assumption that the variation of the apparent secondary electron yield is dominated by the variation of the argon ion induced processes. The oxygen coverage/oxide layer thickness on the aluminum cathode is measured by a semi in situ x-ray photoemission spectroscopy equipment which is directly attached to the discharge chamber. It is found that three phases exist: (1) in the monomonolayer regime, as the oxygen coverage increases from 0 to 0.3, decreases by nearly , (2) as the oxygen coverage increases from 0.3 to 1, keeps nearly constant, (3) as the oxide layer thickness increases from about 0.3 nm to about 1.1 nm, increases by . We propose that, in the submonolayer regime, the chemisorbed oxygen on the aluminum surface causes the decrease of by creating a local potential barrier, which reduces the Auger neutralization rate and the energy gained by the Auger electrons. In the multilayer regime, as the oxide layer grows in thickness, there are three proposed mechanisms which cause the increase of : (1) the work function decreases; (2) resonance neutralization and Auger de-excitation may exist. This is served as another channel for secondary electron production; (3) the kinetic energy of Auger electrons is increased on average, leading to a higher probability for electrons to overcome the surface potential barrier.

Charge exchange processes in He+/Cu scattering at low energy

In this paper we present results on charge exchange of He+ ions at a polycrystalline Cu surface. Monte Carlo simulations were used to calculate the trajectories of projectiles scattered by an angle Θ=136°. By including Auger neutralization and charge exchange in close collisions, energy spectra of the scattered ions as well as ion fraction values were calculated for primary energies in the range 0.5–5keV and compared to experimental results. In the simulations, the Auger neutralization rate Γ and the probabilities of resonant neutralization (PRN) and reionization (PRI) are treated as free parameters. Using well accepted values from literature for these quantities very good agreement between simulations and experimental data was achieved.

Calculation of Auger-neutralization probabilities for He +-ions in LEIS

In Low Energy Ion Scattering (LEIS), Auger-neutralization is an omnipresent charge exchange mechanism, especially when noble gas ions are used as projectiles, with a primary energy below the threshold energy, E th, for collision induced charge exchange processes (neutralization and reionization). Recent experiments revealed a significant dependence of the ion survival probability, P +, on the crystal plane, when He + ions are scattered from a metal surface. This is in contrast to the fact, that the neutralization probability in LEIS is usually assumed to be independent of the chemical environment of the collision partner (absence of matrix effects). In order to investigate this crystal effect, an existent theory on Auger-neutralization (based on a Linear Combination of Atomic Orbitals) is adapted to the LEIS geometry. With this model, Auger-neutralization rates are calculated for a Ag(1 1 0) surface. Trajectories for He particles scattered from this surface into different azimuth directions are obtained by means of Molecular Dynamics simulations. Subsequently, the ion survival probability is calculated and compared to measurements. Good agreement is obtained which gives confidence in the applicability of this model in the LEIS regime. Moreover, it was possible to obtain detailed information on the properties of the neutralization process.

Face-dependent Auger neutralization and ground-state energy shift for He in front of Al surfaces

He atoms and ions with keV energies are scattered under grazing angles of incidence from Al(111), Al(100), and Al(110) surfaces. Fractions of surviving ions and normal energy gains of ${\text{He}}^{+}$ ions prior to neutralization, derived from shifts of angular distributions for incident atoms and ions, are compared to results from three-dimensional Monte Carlo simulations based on theoretically calculated Auger neutralization rates and He ground-state energy shifts. From the good agreement of experimental data with simulations, we conclude a detailed microscopic understanding for a model system of ion-surface interactions. Our work provides further evidence for the recently reported surface Miller index dependence for the neutralization of ${\text{He}}^{+}$ ions at metal surfaces. The study is extended to the face dependence of the He ground-state energy shift.

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.

Hybridization effects on the Auger neutralization process of He+ on Ag(1 1 0) surface

In this article we investigate the effect that hybridization between the 5s- and 4d-orbitals of Ag has in the calculations of Auger neutralization rate and ion survival probability for grazing collisions of He+ with Ag(110) surfaces. We find that it is important to include hybridization to get good agreement with experimental results specially at high incident energies, when the ions get closer to the surface.

Spin dependent screening and Auger neutralization of singly-charged noble gas ions in metals

The screening of Ne+ and Ar+ ions embedded in a paramagnetic free electron gas is calculated using density functional theory within the local spin density approximation, analyzing the spin polarization of the screening cloud. As a second step, the Auger neutralization rates for these ions in a free electron gas are calculated, paying special attention to their dependence on the spin direction of the electron excited in the Auger process. It is observed that the spin polarization of the screening cloud and that of the Auger rates are related. Comparison with previous calculations performed for He+ shows that the spin polarization of the excited electrons is lower in the case of Ne+ and Ar+ than in the case of He+. This result is consistent with the experimental data obtained for the spin polarization of the emitted electrons in the deexcitation of metastable He∗, Ne∗ and Ar∗ atoms in front of metal surfaces.

Role of surface states in Auger neutralization ofHe+ions on Ag surfaces

Recent measurements of He{sup +} ion fractions that survive to a whole scatterig event when they impinge on Ag surfaces have shown two different and interesting effects: (1) a notable difference of surviving ion fraction depending on which crystallographic face of the target surface is studied [Yu. Bandurin et al., Phys. Rev. Lett. 92, 017601 (2004)], and (2) an uncommonly high ion fraction in the very-low-energy range (tens of eV) [S. Wethekam et al., Phys. Rev. Lett. 90, 037602 (2003)]. Apart from the geometry, one of the differences between the surfaces of a crystal can be seen in the electronic structure: while the (111) surface has an occupied surface state near the Fermi level at the {gamma} point the (110) and (100) faces have not. Motivated by these facts, in this work we study the role that the occupied surface state plays on the Auger neutralization rate and we present an estimation of the ion fractions that survive for the different Ag faces.

Study on interactions of He atoms and ions with an Al(1 1 1) surface

He+ ions and He0 atoms with keV energies are scattered under a grazing angle of incidence from an Al(111) surface. Fractions of surviving ions and energy gains of ions prior to neutralization are derived from charge state distributions for scattered projectiles and shifts of angular distributions for impact of charged and neutral projectiles. By comparing the experimental data with theoretical predictions for the interaction potential, the neutralization rate, and the He 1s level shift we reveal deficiencies of the present theoretical understanding for the interaction of He atoms and ions with Al(111), a “model system” for the study of atom–surface interactions. We demonstrate that our data is very sensitive on interaction potentials, the (Auger) neutralization rate, and the He 1s level shift. Our work provides important experimental input for in-depth tests of theoretical models for the interaction of He atoms and ions with Al(111).

Linear combination of atomic orbitals calculation of the Auger neutralization rate ofHe+on Al(111), (100), and (110) surfaces

We develop a theory of the Auger neutralization rate of ions on solid surfaces in which the matrix elements for the transition are calculated by means of a linear combination of atomic orbitals technique. We apply the theory to the calculation of the Auger rate of ${\mathrm{He}}^{+}$ on unreconstructed Al(111), (100), and (110) surfaces, assuming ${\mathrm{He}}^{+}$ to approach these surfaces on high symmetry positions and compare them with the results of the jellium model. Although there are substantial differences between the Auger rates calculated with both kinds of approaches, those differences tend to compensate when evaluating the integral along the ion trajectory and, consequently, are of minor influence in some physical magnitudes like the ion survival probability for perpendicular energies larger than $100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We find that many atoms contribute to the Auger process and small effects of lateral corrugation are registered.

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.

Auger neutralization rate for slowAr+ions in front of KCl(001)

Angular and charge-state distributions are measured for $5--15\text{\penalty1000-\hskip0pt}\mathrm{keV}$. ${\mathrm{Ar}}^{0}$ and ${\mathrm{Ar}}^{+}$ ions reflected from a KCl(001) surface under glazing angle incidence. The ionization of ${\mathrm{Ar}}^{0}$ is almost completely suppressed at incident angles smaller than a critical angle ($11--22\phantom{\rule{0.3em}{0ex}}\mathrm{mrad}$, depending on the ion energy), while a considerable fraction of incident ${\mathrm{Ar}}^{+}$ ions are neutralized. It is demonstrated that a position-dependent Auger neutralization rate can be derived from the observed result without any assumption except for the surface continuum potential. The obtained Auger neutralization rate shows a simple exponential dependence as is usually assumed.

Auger neutralization of He ions on Ag surfaces: surface type and azimuthal orientation dependence

Neutralization of He + ions in grazing incidence scattering on Ag(111) and Ag(110) surfaces is studied. These measurements reveal the existence of an order of magnitude difference in the probability of ion survival on Ag(110) and Ag(111). The effect of changing the direction of ion scattering along the surface azimuthal orientation is investigated. The experimental results are discussed in terms of survival from Auger neutralization, whose rates are derived theoretically. Molecular dynamics simulation of scattered ion trajectories are performed and the surviving ion fractions are then calculated using the theoretical Auger neutralization rates, without adjustable parameters. The calculations agree quite well with the experimental data and show that the observed differences in the neutralization probabilities on these surfaces are related to different extensions of the electron density beyond the surface, resulting from different atomic packing.

Surface Miller Index Dependence of Auger Neutralization of Ions on Surfaces

Neutralization of He+ ions in grazing incidence scattering on Ag(111) and Ag(110) surfaces is studied. These measurements reveal the existence of an order of magnitude difference in the probability of ion survival on Ag(110) and Ag(111). The experimental results are discussed in terms of survival from Auger neutralization, whose rates are derived theoretically. Molecular dynamics simulation of scattered ion trajectories is performed and the surviving ion fractions are then calculated using the theoretical Auger neutralization rates, without adjustable parameters. The calculations agree quite well with the experimental data and show that the observed differences in the neutralization probabilities on these surfaces are related to different extensions of the electron density beyond the surface, resulting from different atomic packing.

Survival of He + ions during grazing scattering from a Cu(1 1 1) surface

He + ions and He atoms with keV energies are scattered under a grazing angle of incidence from an atomically flat and clean Cu(1 1 1) surface. From the comparison of measured charge fractions for specularly reflected charged and neutral projectiles we find evidence for the survival of incident ions from neutralization during the complete scattering event. The survival of ions can only be understood, if the neutralization proceeds around the distance of closest approach, i.e. at the jellium edge. Thus neutralization of ions proceeds closer to the surface than derived in most former evaluations of experimental data where response phenomena are approximated by the concept of classical image charge interactions. From the analysis of our data we derive Auger neutralization rates which compare well with recent theoretical work.

Survival of He+ ions during grazing scattering from a Ag(111) surface.

He+ ions as well as neutral He atoms with keV energies are scattered under a grazing angle of incidence from a clean and atomically flat Ag(111) surface. From a comparison of ion fractions observed after scattering of He+ ions and He atoms we find for energies below some keV small but defined fractions of ions that have survived the complete scattering event with the surface. This feature allows us to clear up the microscopic interaction scenario for Auger neutralization of He+ ions at a Ag(111) surface. The Auger neutralization rates are 2 to 3 orders of magnitude smaller than conventional rates derived from experiments for He+-metal systems and agree with recent calculations.

Adsorbate phase transformations and the coverage-dependent oscillation of electron transfer probabilities

We report a study of the effects of chlorine adsorption on the interaction of positive ions (H+, Ne+, Ar+ with a Ag(111) surface from the submonolayer Cl chemisorption to initial stages of AgCl formation. Cl adsorption on Ag(111) proceeds through different phases and we observed that the neutralization probabilities oscillate in this range, attaining an intermediate minimum at about 2/3 coverage and reach a maximum at full coverage. The subsequent appearance of AgCl phase again leads to a reduction in neutralization. These results are described in terms of changes in Auger neutralization rates due to modifications in the adsorbate density of states.

Theory of Auger neutralization and deexcitation of slow ions at metal surfaces

The contribution of conduction electrons to the Auger neutralization rate of a slow ion at a metal surface has been calculated. We have considered the scattering of He 1 on Al and studied the effect of the ion potential on the neutralization rate. This has been accomplished by taking into account the modification of the wave function of the captured electron induced by the presence of the ion. The effect of the ion potential is shown to increase the Auger neutralization rate. We have also calculated the Auger deexcitation of He* (n52) atoms in front of an aluminum surface, including the distorsion created by the surface on the atom states. In this case, the deexcitation rate is reduced by the mixing of atom and surface states, which also leads to a nonvanishing deexcitation rate of metastable He (2 1 S). @S0163-1829~98!06543-6#

Role of energy-level shifts on Auger neutralization processes: A calculation beyond the image potential

Auger neutralization of slow ${\mathrm{He}}^{+}$ near Al(100) surfaces is analyzed by introducing the effect of the ion on the metal electrons. This is taken into account by including in the calculation the $2s$ and $2p$ levels of He that can be filled by metal electrons when the ion approaches the surface. These levels and the $1s$ level are calculated for short and large distances between the ion and metal atoms. We find that the $1s$ level is completely modified with respect to the image potential shift for short distances, due to the strong interaction among He and Al atoms. Auger neutralization rates are calculated, and we obtain a most probable distance of neutralization of about 3 a.u. between He and the last Al layer. At this distance our theoretical $1s$ level shows an energy shift of about 2 eV, in agreement with experimental findings. We thus conclude that slow ${\mathrm{He}}^{+}$ ions neutralize much closer to the metal surface than previously thought.

Auger transition rates for the neutralization of He + ions in front of an aluminium surface

In a detailed analysis of image charge effects on the trajectories of He + ions with keV energies scattered from a clean Al(111) surface under a grazing angle of incidence we find evidence for contributions from two different Auger processes. From our data we derive Auger neutralization rates for the filling of the 1s hole and the Auger deexcitation rate for the 2s 3S level as function of the distance from the surface. The rates are obtained from the experiments in a straightforward manner and can be considered as a profound basis for the comparison with theoretical calculations.