Maria Silvia Gravielle

Influence of the polarization in grazing scattering of fast helium atoms from LiF(001) surfaces

Fil: Gravielle, Maria Silvia. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Astronomia y Fisica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomia y Fisica del Espacio; Argentina

Study of the induced potential produced by ultrashort pulses on metal surfaces

The relevance of the induced potential for photoelectron emission from metal surfaces resulting from grazing-incidence ultrashort laser pulses is studied. To describe this process we introduce a distorted-wave method, which includes the perturbation on the emitted electron produced by both the laser and the induced fields. The method is applied to an Al(111) surface, and the results are compared with the numerical solution to the time-dependent Schrodinger equation. We find that our approach reproduces the main features of emission spectra well, accounting properly for effects originated by the induced potential.

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

(Received 22 October 2013; revised manuscript received 25 February 2014; published 21 April 2014)ElectrondistributionsproducedbygrazingimpactoffastprotonsonMg(0001),Cu(111),Ag(111),andAu(111)surfaces are investigated, focusing on the effects of the electronic band structure. The process is described withinthe band-structure-based approximation, which is a perturbative method that includes an accurate representationof the electron-surface interaction, incorporating information of the electronic band structure of the solid. For allthe studied surfaces, the presence of partially occupied surface electronic states produces noticeable structuresin double-differential—energy- and angle-resolved—electron emission probabilities from the valence band. ForMg, Cu, and Ag these structures remain visible in electron emission spectra after adding contributions comingfrom core electrons, which might make possible their experimental detection, but for Au they are hidden byinner-shell emission.DOI: 10.1103/PhysRevB.89.155421 PACS number(s): 34

Production of excitons in grazing collisions of protons with LiF surfaces: An onion model

In this work we evaluate the production of excitons of a lithium fluoride crystal induced by proton impact in the intermediate and high energy regime (from 100 keV to 1 MeV). A simple model is proposed to account for the influence of the Coulomb grid of the target by dressing crystal ions to transform them in what we call onions. The excited states of these onions can be interpreted as excitons. Within this model, total cross section and stopping power are calculated by using the first Born and the continuum distorted-wave (CDW) eikonal initial-state (EIS) approximations. We found that between 7 and 30 excitons per incident proton are produced in grazing collisions with LiF surfaces, becoming a relevant mechanism of inelastic transitions.

Binary contribution to the stopping in fast ion–surface collisions

Abstract The mechanism of the binary collision with the free-electron gas in the proximity of a surface is investigated by employing a semiclassical formalism. The surface dynamic screening of the projectile is described with two well-known models commonly used in the literature: the parallel-dispersion (PD) and specular-reflection (SR) models. The Mermin–Lindhard dielectric function is used to represent the dielectric response of the bulk material. The formalism is applied to the calculation of the energy loss by fast protons colliding with an aluminum surface. Results are compared with those obtained with the dielectric formalism, and energy-loss distributions are analyzed. It is concluded that the models used to describe the dynamic screening potential do not seem to give a reliable description of the binary mechanism.

Influence of the projectile charge state on electron emission spectra from a Cu(111) surface

Double differential electron emission distributions produced by grazing impact of fast dressed ions on a Cu(111) surface are investigated focusing on the effects of the electronic band structure. The process is described within the Band-Structure-Based (BSB) approximation, which is a perturbative method that incorporates information of the electronic band structure of the solid. Differences in the behavior of the emission spectra for He+q, Li+q, Be+q and C+q projectiles with different charge states q are explained by the combined effect of the projectile trajectory and the projectile charge distribution.

Influence of surface electronic states on electron emission from metallic targets

We study the electron emission produced by swift protons impinging grazingly on a Beryllium (0001) surface. This material presents a technological interest in modern fusion reactors. We found that surface states play an important role in the double differential energy- and angle- resolved electron emission probabilities, but the effect disappears when angular distributions are integrated to obtain electron energy spectra.

Band-Structure Based model for photoelectron emission from metal surfaces

Photoelectron emission spectra induced by grazing incidence of intense and ultrashort laser pulses on a metal surface are studied within a distorted-wave formalism. The proposed aproximation, named Band-Structure Based-Volkov (BSB-V) approach, includes a precise description of the surface potential, incorporating information of the band structure of the solid. Results are compared with the numerical solution of the time-dependent Schrodinger equation and with values derived from simpler theoretical models.

Channeling effect in electronic spectra produced by grazing impact of fast protons on insulator surfaces

Electron emission due to grazing scattering of fast protons from LiF and KCl surfaces is studied under axial incidence conditions. The differential emission probability is calculated within a distorted-wave formalism, taking into account axial channeled trajectories. For different emission angles, electronic spectra for proton incidence along the two principal crystal axes ([100] and [110]) are compared with those co rresponding to an impact velocity in a random direction, finding effects associated with the channeling conditions.

3d-shell contribution to the energy loss of protons during grazing scattering from Cu(111) surfaces

Motivated by a recent experimental work [S. Lederer and H. Winter, Phys. Rev. A 73, 054901 (2006)] we study the contribution of the 3d shell electrons to the energy loss of 100 keV protons scattered off from the Cu(111) surface. To describe this process we use a multiple collision formalism, where the interaction of the projectile with 3d electrons is described by means of a sequence of single encounters with atoms belonging to the first atomic layer. In order to compare the theoretical energy loss with the experimental data, we add the contribution of valence electrons, which is evaluated in linear response theory using a response function that incorporates information on the surface band structure. For completeness, the energy lost by protons is also calculated within a jellium model that includes 3d and valence electrons with equal footing. Fair agreement between theory and experiment exists when the 3d shell is taken into account in the calculation.