Search Results (43 total)

We report on measurements of electron emission spectra from surfaces of highly oriented pyrolytic graphite (HOPG) excited by 1–5 keV He⁺ and Li⁺ which, for He⁺, exhibit a previously unreported high-energy structure. Through a full quantum dynamic description that allows for the calculation of neutralization and electron-hole pair excitation, we show that these high-energy electrons can arise from autoionization of excitons formed by electron promotion to conduction band states close to the vacuum level. The same calculation explains the observed absence of high-energy excitons for Li⁺ on HOPG.

Kondo resonances for transition-metal-atom impurities in a metal host have been analyzed by means of an ionic model as a function of the impurity d-orbital occupancy. The ionic Hamiltonian has been obtained by making use of the atomic Hund first rule. This Hamiltonian has been solved using a Green-function equation of motion method up to second order in the transition metal–host interaction. We find Kondo temperatures that decrease with the atomic total spin, the largest one appearing for charge fluctuations d⁰↔d¹ and d¹⁰↔d⁹, in good agreement with the experimental evidence.

We present a time-dependent quantum mechanical calculation of the charge transfer process in He⁺∕Al collision, where the resonant neutralization to the ground and first excited states of He is taken into account in a correlated way according to a Coulomb blockade effect. Our results provide an explanation to the discrepancies still found between theory and experiments in low energy ion scattering for this system, as well as allow us to understand the presence of high energy electrons in ion induced secondary electron emission spectra.

The density of states on atoms interacting with solid surfaces is a required physical quantity for the understanding of processes related to scanning tunneling and photoemission spectroscopy, single-atom conductance, and emission and scattering of atoms from surfaces. In this work, we present a model calculation that allows including the localized aspects of atomic interactions and the extended features of the surface, together with alternative treatments of the Coulomb repulsion terms in the atom site. The effects of the spin fluctuation statistics treated up to a second order in the atom-surface coupling term are especially explored in this case. This approximation is comparatively analyzed with the exact results available in a model system of four levels and then used in the description of hydrogen interacting with an Al surface. Effects due to finite bandwidth and energy dependence of the local surface density of states are discussed.

The collision process of H⁺ with clean and AlF₃ covered Al(111) surfaces is theoretically studied for large scattering angles and different ion incoming energies. The energy distributions of the charge fractions are analyzed as a function of the backscattered particle energy. Important differences between the two surfaces are found: in the case of pure Al the outgoing hydrogen particles are predominantly neutral, with a 10% of negative ions, while in the case of AlF₃ an important positive charge fraction is observed. The theoretical calculation reproduces the experimental trends, and shows that these results are strongly related with the electronic structures in each case. In both surfaces a resonant mechanism is responsible for the charge exchange, but while in the pure Al case only the valence band states are involved, in the AlF₃ case the promotion of the projectile energy level by the interaction with the core surface states inhibits the electron capture from the valence band.

Ion scattering spectroscopy with time-of-flight analysis has been used to study the deposition of a thin AlF₃ insulating film on an Al(111) sample, and to measure the ion fractions for 3–30 keV H⁺ projectiles scattered off both the metallic and the insulating surface. The total ion fraction measured for the clean surface at a scattering angle of 108° is Γ∼12%, composed mainly of negative ions. For AlF₃ film thickness greater than 2 ML, the ion fraction increases, being in this case mainly composed of positive ions (Γ⁺=35%, Γ⁻=3%). These changes are interpreted in terms of a competition of resonant electron capture and loss processes between surface and hydrogen electron states.

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 He⁺ on unreconstructed Al(111), (100), and (110) surfaces, assuming 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 eV. We find that many atoms contribute to the Auger process and small effects of lateral corrugation are registered.

This work is mainly devoted to the description of processes that involve the interaction between an atom and a surface, in which a strong Coulomb repulsion on the atomic site (U) limits the charge exchange to one electron (infinite-U limit). In this limit, the Anderson Hamiltonian for a many-fold (N) of states localized on the atomic site can be represented in terms of auxiliary bosons and physical operators in the mixed boson-electron space can be defined. In this work the Hamiltonian is solved by defining appropriate Green’s functions for physical operators. Then we solved the equations of motion of these Green’s functions, up to a second order in the atom-surface coupling, either for the stationary case or for a real time-dependent problem. We show that our approach reproduces the known exact results in the nondegenerate (N=1) case, and for N≻1 gives excellent agreement with exact calculations and approximations valid for large N (the 1∕N expansion). Finally, the accurate description of dynamical processes is shown by the comparison with the exact results available for a small four-level system. In this case we also compare with results obtained by using the noncrossing approximation and with the usual spinless model calculation.

The generalized forward spin polarizabilities γ₀ and δ_LT of the neutron have been extracted for the first time in a Q² range from 0.1 to 0.9  GeV². Since γ₀ is sensitive to nucleon resonances and δ_LT is insensitive to the Δ resonance, it is expected that the pair of forward spin polarizabilities should provide benchmark tests of the current understanding of the chiral dynamics of QCD. The new results on δ_LT show significant disagreement with chiral perturbation theory calculations, while the data for γ₀ at low Q² are in good agreement with a next-to-leading-order relativistic baryon chiral perturbation theory calculation. The data show good agreement with the phenomenological MAID model.

We have measured the spin structure functions g₁ and g₂ of ³He in a double-spin experiment by inclusively scattering polarized electrons at energies ranging from 0.862 to 5.058 GeV off a polarized ³He target at a 15.5° scattering angle. Excitation energies covered the resonance and the onset of the deep inelastic regions. We have determined for the first time the Q² evolution of Γ₁(Q²)=∫₀¹g₁(x,Q²)dx, Γ₂(Q²)=∫₀¹g₂(x,Q²)dx, and d₂(Q²)=∫₀¹x²[2g₁(x,Q²)+3g₂(x,Q²)]dx for the neutron in the range 0.1≤Q²≤0.9  GeV² with good precision. Γ₁(Q²) displays a smooth variation from high to low Q². The Burkhardt-Cottingham sum rule holds within uncertainties and d₂ is nonzero over the measured range.

A theoretical calculation that accounts for a fairly complete description of the resonant charge-exchange process occuring in the H⁺ scattering by metal surfaces is presented. Realistic trajectories defined by the binary collision model are considered. The interaction with nuclei and electrons of the all surface atoms that the projectile can see along its trajectory is calculated within a mean-field approximation, and in this form the contributions of the short-range interaction terms to the energy level shift are well contemplated. The long-range contributions and the motion of the projectile respect to the surface reference frame are also taken into account to define the level shift. All these ingredients are incorporated into a quantum mechanical description of the time evolution. The negative ion fractions calculated in this form show an excellent agreement with the experimental data for three different incoming energies and for a wide range of exit angles.

In this work we solve the dynamics of the Newns-Anderson Hamiltonian supplemented with Auger terms and analyze the case of He⁺ scattered off an Al (100) surface. The dynamical solution is compared with results of calculations based on much simpler approximations. We prove that resonant and Auger processes can be treated separately and independently in this case and that charge exchange between He and Al proceeds via resonant and Auger exchange of electrons between the promoted molecular orbital of He and the conduction band states of Al.

We present data on the inclusive scattering of polarized electrons from a polarized ³He target at energies from 0.862 to 5.06 GeV, obtained at a scattering angle of 15.5°. Our data include measurements from the quasielastic peak, through the nucleon resonance region, and beyond, and were used to determine the virtual photon cross-section difference σ_1/2-σ_3/2. We extract the extended Gerasimov-Drell-Hearn integral for the neutron in the range of four-momentum transfer squared Q² of 0.1–0.9   GeV².

In this paper, we present an alternative way to build the effective one-electron picture of a many-atom interacting system. By simplifying the many-body general problem we present two different options for the bond-pair model Hamiltonian. We have found that the successive approximations in order to achieve the effective description have a dramatic influence on the result. Thus, only the model that introduces the correct renormalization in the diagonal term due to the overlap is able to reproduce, even in a quantitative fashion, the main properties of simple homonuclear diatomic molecules. The success of the model resides in the accurate definitions (free of parametrization) of the Hamiltonian terms, which, therefore, could be used to describe more complex interacting systems such as polyatomic molecules, adsorbed species, or atoms scattered by a surface.

The interaction of He⁺ with a typical metal surface (Al or Pd) is described, analyzing in detail the different mechanisms that contribute to the neutralization of the projectile when backscattered from the surface. Auger and resonant neutralization processes are considered and analyzed including a detailed quantum-mechanical description of the He-metal interaction, for projectile energies between 100 eV and 3 keV. We show that the promotion of the He-1s level, due to its interaction with the metal-atom-core orbitals, is the crucial mechanism making resonant processes operative. We find, however, that resonant processes are much more important for Al than for Pd. In Al, both Auger and resonant processes are equally important for neutralization of the ion, while for Pd we find that Auger is the dominant mechanism, making the He/Pd system the ideal case for which Hagstrum’s exponential law appears to be practically valid for all velocities. We also find qualitative agreement with experimental data, which we consider a satisfactory result in view of the fact that our theory is a complex ab initio calculation free of adjustable parameters.

A theoretical calculation that allows for a fairly complete description of the charge-exchange and surface electronic excitation processes occurring in the H⁺ and He⁺ scattering by ionic surfaces is presented. The interaction parameters required to describe the collisional process are calculated by using a model Hamiltonian that has proved to provide a systematic good description of the properties like binding energy, equilibrium distance, and vibrational frequency of several dimers and atom-surface systems. The formalism is applied to the comparative study of the scattering of H⁺ and He⁺ by the fluorine atom of a LiF surface. The ion survival probabilities by elastic and inelastic processes are calculated, and the general trends of the experimental findings are reproduced. Very satisfactory results concerning the electron-hole pair excitations in the He⁺ scattering are obtained when the charge fluctuation on the active F site is considered. The role of the surface core states is found to be decisive for the e-h pair excitation by He⁺ scattering, and for the neutralization of H⁺ projectiles.

The negative ion formation for large angle hydrogen scattering by a LiF surface is studied within a time-dependent Hartree-Fock approximation of the Anderson’s model Hamiltonian. The effect of the Madelung potential on the projectile energy level shifts is modeled by including the point-charge field of the LiF semicrystal in the calculation of the interaction Hamiltonian parameters. Comparisons between results obtained by a linear-chain model and a description of the solid target based on a cluster approach, allow to infer the effects of the local environment on the charge exchange process. It is found that H^- formation is enhanced in the hydrogen scattering by the alkali Li⁺ ions when the F^- nearest neighbors are included. The scattering of H⁺ within a spinless picture is also analyzed for both descriptions of the target. The H-1s energy level variations along the trajectory, and the quasimolecular states originated by the adiabatic interaction between the projectile state and the core states of the target, mainly determine the H⁺ neutralization probability and also the electronic excitations in the solid.

A bond-pair model Hamiltonian developed previously for systems consisting of interacting atoms is applied to describe atom-surface interactions. By proposing a mixed basis set involving localized adatom orbitals {φ_α} and extended surface states {φ_k}, and by application of a mean-field approximation, the Hamiltonian is reduced to the form of the single-particle Anderson model. The resulting model Hamiltonian is free from adjustable parameters. These parameters include both the effects of electronic interactions between the atom and the solid and those arising from the lack of orthogonality between the adsorbate and substrate orbitals. The nonlocal exchange contributions are treated consistently within the Hartree-Fock method, while valencelike and corelike band states are also taken into account. This model is applied to consider the interaction of hydrogen with metals (Al, Li, and Na). The results for chemisorption are in good agreement with those obtained by other theoretical approaches based on either the density functional theory or embedding cluster methods, as well as with existing experimental data. In addition, the calculation of the shifts and widths of the adsorbate levels in an ample range of separation distances are also in good agreement with those obtained by using atomic physics techniques.

The Keldysh Green’s-function formalism is used to evaluate the atomic occupation number of a projectile colliding with a metal surface. This formalism has an advantage that allows us to handle simultaneously Auger, resonant, and plasmon-assisted exchange processes along with the interference between them. A time-dependent Hamiltonian containing Auger-like and resonantlike terms, and the electron-electron Coulomb potential in the solid, is proposed. The atomic self-energies are calculated up to a second order in the interaction potential. An effective resonant amplitude is defined, and Auger self-energies are presented where the plasmon-assisted processes are included through a surface response function. Finally, some numerical results for a proton colliding with an Al surface by using a simplified description of the solid response function are presented, where an analysis in terms of the incidence angle and the energy level is shown.

Charge exchange and inelastic excitation processes have been analyzed in the scattering of low-energy He⁺ from metallic and ionic surfaces. An Anderson-like Hamiltonian is proposed, where the parameters are defined taking into account the electronic band structure of the surface as well as the atomic nature of the interaction between the projectile and the target atoms. The time-dependent collisional process is solved by using a Green-function formalism, which allows us to calculate not only the charge-state probabilities but also the one-electron interband excitations in the solid. Competitive effects of the hybridizations among the localized state at the projectile site and the localized and extended surface states are contemplated. In this way we can explain the observed energy dependences of the neutralization probability, as well as the occurrence of energy-loss processes due to the excitation of valence and core surface electrons induced by the collision.

Charge-transfer mechanisms in low-energy helium-scattering spectroscopy are analyzed by using an Anderson-like description of the time-dependent collisional process, which allows us to include several electronic bands of extended and localized nature in the solid. The Hamiltonian parameters are obtained from a Hartree-Fock self-consistent-field calculation of the He-target atom dimeric system. We examine in particular cases such as Ca and Ga linear chain substrates. We found that at velocities large enough, the localized state in the solid contributes to the He⁺ neutralization, showing the characteristic oscillatory behavior of the nonadiabatic charge exchange between localized states, in agreement with other calculations. In the range of low velocities we found that if the hybridization between the He orbital and the localized states in the solid is able to produce the formation of an antibonding state having a predominant weight of the He-1s orbital, this promotes the charge exchange between the Helium and the extended bandstates of the solid.

The performance of a recently proposed ab initio description of chemisorption problems, obtained by reducing the total many-body Hamiltonian to a superposition of bond-pair Hamiltonians, is tested. This scheme, which in its Hartree-Fock version leads to an independent-particle model of the tight-binding form, is applied to analyze the interaction curves for the hydrides of the first-row elements as a function of internuclear distances. By fixing the occupation numbers for the orthonormal spin orbitals at large internuclear separations, its relationship with results from both the valence-bond and the molecular-orbital methods is established. It is shown that the model works fairly well in predicting binding energies, equilibrium distances, dipole moments, and vibrational frequencies for the whole series of the hydrides. It was also found that if a consistent expansion of the intervening parameters is made up to second order in the atomic-orbital overlaps, reasonable results are obtained for the hydrides forming bonds of nearly ionic character: LiH, OH, and FH.

Dynamical processes involving charge exchange betwen atoms and solid surfaces are studied within an Anderson-Newns model. We performed a perturbative treatment of the correlation term starting from a time-dependent Hartree-Fock basis, and calculated the probabilities of the final atomic charge states by using the Green-function formalism for irreversible processes. We analyzed the negative-ion fraction, assuming an Anderson symmetric case, and considering the electronic correlation effects up to a second perturbative order, for two ‘‘extreme’’ model systems: the two-level one, and a solid within the wide-band approximation.

Total-energy calculations have been performed for hydrogen atoms embedded in a small Be cluster. The effect of Li impurities has also been studied, and it was found that the presence of Li favors the nucleation of protons inside the cluster.

The ion-velocity dependence of the ionization probability for an atom ejected from a surface is examined by using a quantum approach in which the coupled motion between electrons and the outgoing nucleus is followed along the whole trajectory by solving the stationary Schrödinger equation. We choose a very-small-cluster-model system in which the motion of the atom is restricted to one dimension, and with energy potential curves corresponding to the involved channels varying appreciably with the atom position. We found an exponential dependence on the inverse of the asymptotic ion velocity for high emission energies, and a smoother behavior with slight oscillations at low energies. These results are compared with those obtained within a dynamical-trajectory approximation using either a constant velocity equal to the asymptotic ionic value, or expressions for the velocity derived from the eikonal approximation and from the classical limit of the current vector. Both approaches give similar results provided the velocity is allowed to adjust self-consistently to potential energies and transition-amplitude variations. Strong oscillations are observed in the low-emission-energy range either if the transitions are neglected, or a constant velocity along the whole path is assumed for the ejected particle.