Quantum-mechanical model for valence-electron emission from metal surfaces (original) (raw)

Electron emission in grazing-ion–surface collisions

For ions impinging grazingly on a solid surface, the electron emission from the inner shells of solid atoms is investigated by employing a semiclassical formalism. The emission rate is expressed in terms of probabilities of atomic ionization, which are evaluated with the continuum-distorted-wave-eikonal-initial-state approximation, taking into account the full dependency on the impact parameter. The model is applied to the calculation of the differential yield of ejected electrons for fast protons colliding with an aluminum surface. Inner-shell emission is compared with the electron emission from the valence band of the metal, considering different ejection angles. Calculated energy spectra of emitted electrons are in good agreement with the available experimental data.

Angular dependence of electron emission induced by grazing-ion–surface collisions

In this work, electron emission spectra produced by impact of fast protons on Al(111) surfaces are theoretically and experimentally studied. Contributions coming from the different electronic sources of the metalatomic inner shells and valence band-are analyzed as a function of the angle of electron emission. In the forward direction, the inner-shell ionization process is the dominant mechanism. The valence emission, instead, becomes important when the ejection angle is separated from the specular-reflection direction. In both angular regions, theoretical and experimental values are in reasonable agreement. The energy shift and broadening of the convoy electron peak at glancing observation angles are well described by the present model, which takes into account the influence of the induced surface field on the ionized electron.

Band-structure-based collisional model for electronic excitations in ion-surface collisions

Energy loss per unit path in grazing collisions with metal surfaces is studied by using the collisional and dielectric formalisms. Within both theories we make use of the band-structure-based ͑BSB͒ model to represent the surface interaction. The BSB approach is based on a model potential and provides a precise description of the one-electron states and the surface-induced potential. The method is applied to evaluate the energy lost by 100 keV protons impinging on aluminum surfaces at glancing angles. We found that when the realistic BSB description of the surface is used, the energy loss obtained from the collisional formalism agrees with the dielectric one, which includes not only binary but also plasmon excitations. The distance-dependent stopping power derived from the BSB model is in good agreement with available experimental data. We have also investigated the influence of the surface band structure in collisions with the Al͑100͒ surface. Surface-state contributions to the energy loss and electron emission probability are analyzed.

Grazing Ion-Surface Collisions

Electron emission after grazing ion-surface collisions is studied for high impact velocities. We have focused on glancing angles of electron emission where the dominant mechanism is the ionization from atomic bound states. To describe this process, we introduce a quantum model called field distorted-wave (FDW) approximation, which takes into account the effect of the surface interaction on the electronic transition.

Influence of the surface band structure on electron emission spectra from metal surfaces

Electron distributions produced by grazing impact of fast protons on Mg(0001), Cu(111), Ag(111), and Au surfaces are investigated, focusing on the effects of the electronic band structure. The process is described within the band-structure-based approximation, which is a perturbative method that includes an accurate representation of the electron-surface interaction, incorporating information of the electronic band structure of the solid. For all the studied surfaces, the presence of partially occupied surface electronic states produces noticeable structures in double-differential-energy-and angle-resolved-electron emission probabilities from the valence band. For Mg, Cu, and Ag these structures remain visible in electron emission spectra after adding contributions coming from core electrons, which might make possible their experimental detection, but for Au they are hidden by inner-shell emission.

Energy and electron spectra after grazing-ion–surface collisions

For ions that impinge grazing on solid surfaces, binary collisions with the free-electron gas are investigated by means of a modified specular-reflection ͑MSR͒ model. The proposed MSR theory is applied to the calculation of the energy lost by fast protons after colliding with an aluminum surface. We also employ the MSR binary theory to study the energy spectra of the emitted electrons. The contribution coming from atomic inner shells, calculated with the continuum-distorted-wake-eikonal-initial-state approximation, is added to the emission probability from the valence band. The total results obtained with the MSR model are in reasonable agreement with available experimental data for 100 keV protons and large ejection angles of the electron. In contrast instead, the usual specular-reflection binary model shows a prominent structure at low electron energies, which greatly overstimates the experimental spectrum.

Electron emission from the interaction of multiply charged ions with a Au(110) surface

Surface Science, 1986

We have measured energy distributions of electrons produced during slow glancing collisions of H-like and He-like N and O projectiles with a clean Au(llO) surface. For the case of the H-like incident projectiles, two peaks, at 250 and 350 eV, and 250 and 490 eV for incident N +6 and O +7 , respectively, are observed. Both peaks are associated with Auger transitions to the K-shell vacancy brought into the collision by the incident H-like multicharged ions: from the higher lying shells of the projectile in the case of the high energy peaks, and from the target inner shells in the case of the low energy peaks.

Electron emission in 10–100 keV H+ and He+ grazing ion-surface collisions

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1994

We have measured energy distributions of electrons ejected during grazing lo-100 keV H+ and He+ ions scattering from Si surfaces for a broad range of electron observation angles. The distributions have contributions strongly dependent on the directions of incidence and observation. For observation regions around the specular reflection of the ions we have studied the angular dependence of the electron structure resulting from electron transfer to the continuum of the effective ion potential. Far from the ion scattering plane we have observed the Si Auger electron spectrum. We discuss the differences between the Auger peaks obtained by electron and grazing proton bombardment of Si surfaces partially covered with 02 and AI.

The role of atomic collisions in kinetic electron emission from Al surfaces by slow ions

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2007

We measured energy distributions of electrons emitted in the interaction of slow Kr + and Na + with Al surfaces. The data allow to correlate emission intensities with spectral signatures of electron excitation processes. Our results indicate that electron promotion processes leading to the excitation of Al target atoms plays the dominating role in kinetic electron emission from Al surfaces by slow ions. In the case of Kr + ions, electron promotion occurs in close atomic collisions between recoiling target atoms. For Na + projectiles, a significant contribution to Al excitation comes also from a vacancy transfer process in asymmetric collisions involving ions that have survived neutralization in the interaction with the surface.