Double matrix effect in Low Energy Ion Scattering from La surfaces

Abstract

Low Energy Ion Scattering (LEIS) has been performed on several lanthanum-based surfaces. Strong subsurface matrix effects-dependence of surface scattered He+ ion yield on the composition of subsurface layer-have been observed. The ion yield of He+ scattered by La differed by a factor of up to 2.5 for different surfaces, while only the La peak was visible in the spectra. To study these effects and enable surface quantification, He+ ion yields have been measured in a range of incident He+ energies from 1000 to 7500 eV for LaB6, La2O3, oxidized La and pure La surfaces. The investigation showed that as many as two simultaneous matrix effects are present, each one driven by a separate charge exchange mechanism. The first one is a resonant neutralization from the conduction band of La to an excited state of the He+ ion. It depends on the work function of the surface, which is lowered significantly when La interacts with O or B. The second mechanism is quasiresonant charge transfer between bound La levels and He 1s, which creates characteristic oscillations in the energy dependence of ion yields. The exact structure of the oscillations depends on small changes in binding energies of interacting La levels. This is the first time quasiresonant charge transfer is proven to be present in La. It is likely that La 5p orbitals participate in this resonance, which can be the first clear observation of a resonance between p and s orbitals in LEIS. This type of resonance was previously believed to be absent because of strong damping. We also demonstrated that despite the complex matrix effect precise measurements over a wide energy range allow quantification of the atomic composition of La-based surfaces.

Surface Restructuring of Thin-Film Electrodes Based on Thermal History and Its Significance for the Catalytic Activity and Stability at the Gas/Solid and Solid/Solid Interfaces

“… 21 , 24 , 51 In this work, elemental quantification has not been attempted because depth profiles may display preferential sputtering for heavier elements such as La. 52 Therefore, depth profiles in this work provide qualitative information and is used only to compare the surface chemical evolution among the three films.…”

Oxidation of metal thin films by atomic oxygen: A low energy ion scattering study

“…The evolution of oxide thickness with exposure time was obtained by LEIS static depth profiling (static DP), with details described in subsection 2.1. LEIS measurements were performed in an ION-TOF GmbH Qtac 100 high sensitivity LEIS spectrometer, described in details elsewhere 19 . A He + ion beam at 3 keV energy and 2.5-3.5 nA current, measured before each spectra acquisition in a Faraday cup, was chosen for characterization.…”

“…The tail is formed by primary ions that penetrated into the solid, backscattered on a sub-surface atom and are emitted from the target in an ionized state into the direction of the analyzer. Due to the low energy applied in LEIS, noble gas ions are neutralized on penetrating the sample 16, 19,21,22 . The formation of a tail will be determined by the finite probability of these scattered neutrals to be reionized upon leaving the sample, namely the reionization probability 16,23,24 .…”

“…Only ions scattered from Mo can form the Mo related tail, as ions scattered from light elements lose more energy. Therefore, the ion yield energy in a LEIS tail signal depends both on the reionization probability and the distribution of the respective atoms inside the film 16,19 . Due to the stopping of particles by the target, these backscattered ions will present an energy loss with respect to the energy of the surface peak.…”

“…The straggling is a result of the statistical fluctuation of the energy-loss processes that particles are subjected to as they travel through matter 28 . The matrix effect appears from changes in neutralization according to changes in the composition of the material 16, 19,29 .…”