LEIS: A reliable tool for surface composition analysis?

Abstract

Different single and polycrystalline surfaces of Cu and Ag have been investigated by Time-Of-Flight low-energy ion scattering using 4 He + ions. The fraction of ions that survived single scattering from the outermost surface layers, P + , was measured in different neutralization regimes. At low energies, a distinct difference in P + was observed for non-equivalent Cu crystal surfaces for projectiles backscattered in a single collision. The polycrystalline surface was found to exhibit similar neutralization behaviour as the (111) single crystal surface. At higher energies, P + shows a strong dependence on the angular orientation of the single crystal. The impact of these findings on quantitative surface composition analysis by LEIS is discussed.

Band structure effects in Auger neutralization of He ions at metal surfaces

“…In the calculation the difference between Ag(111) and polycrystalline Ag arises from the different scattering geometry, which is used for Ag(111) in order to achieve maximum surface sensitivity. 27 One can see that in contrast to the experimental findings, the calculated data exhibit a pronounced physical matrix effect. For Ag(111) and polycrystalline Ag, the difference in AN rates amounts to ∼30%, whereas for Ag(110) a discrepancy in of ∼10% can be deduced.…”

“…+ scattered from Ag(110) (full squares), Ag(111) (full diamonds), and polycrystalline Ag (full asterisks), calculated with a constant upward shift of the He 1s level by 2 eV. Open symbols correspond to experimental data 26,27 ; straight lines represent single-exponential fits. level shift with the He level set to −20.5 eV w.r.t.…”

“…[22][23][24] Recent experiments for He + ions scattered from Cu and Au crystals in the AN regime of LEIS exhibited a pronounced dependence of the neutralization probability on the surface orientation, similarly as discussed above for grazing collisions. 25,26 Analog investigations for Ag crystals did not result in a pronounced orientation dependence; 26,27 therefore, in a recent effort the LCAO model was adopted to also describe AN in the large angle scattering geometry. First results obtained for He + scattered from Ag(110) in different azimuth directions were in good agreement with experimental data.…”

Auger neutralization and ionization processes for charge exchange between slow noble gas atoms and solid surfaces

“…33 (a) and (b) show the comparison of experimental [163,164] and calculated ion fractions of He + scattered from polycrystalline Ag and polycrystalline Au, respectively, for different values of the energy level position of He with respect to the Fermi level. The polycrystalline samples, were approximated as a surface with randomly oriented (111) domains [165], and, consequently, the ion fractions were obtained as an average over trajectories scattered from a (111) surface with normal incidence and arbitrary azimuth exit directions.…”

Determination of 16O and 18O sensitivity factors and charge-exchange processes in low-energy ion scattering

“…9,11,12 Since several processes may occur during the scattering event, a quantitative prediction of the neutralization behaviour (e.g., P þ dependence on the primary ion energy) is required for the selection of the optimal experimental parameters to perform a reliable quantitative LEIS analysis while avoiding timeconsuming calibrations. 6,13,14 In this work, we used the characteristic velocity method in order to study the neutralization behaviour of 4 He þ scattered from 16 O and 18 O and to determine the elemental sensitivity factors. A silica sample has been chosen as a reference material since the preferential sputtering can be neglected at the low ion doses used for the analysis.…”