Abstract
Low energy ion scattering is a technique to detect the energy of ions which are scattered from a surface. For noble gas ions, it is predominantly sensitive to the topmost surface layer due to strong neutralisation processes. Depending on the combination of projectile ion and target material, the scattering spectra can exhibit contributions resulting from multiple scattering processes in deeper layers when probing ions are re-ionised on the exiting trajectory. These events cause a pronounced continuum located toward lower scattering energies with respect to the direct scattering peak. In a previous work a semi-empirical formula has been given which allows fitting and derivation of quantitative information from the measured spectra [Nelson 1986 J. Vac Sci. Technol. A 4 1567-1569]. Based on the former work an improved formula is derived which has less numerical artefacts and is numerically more stable.
Highlights
Low energy ion scattering from surfaces is a well established method to study the elemental composition of the topmost surface layer
Neutralisation and ionisation processes between noble gas ions and the surface during approach and exit have been studied in great detail and overviews can be found in recent review papers by Monreal [3] or Brongersma [4]
A typical low energy ion scattering spectrum which clearly shows the re-ionisation continuum is depicted in figure 1
Summary
Low energy ion scattering from surfaces is a well established method to study the elemental composition of the topmost surface layer. A considerable number of scattered projectile ions is detected at smaller energy than the single scattering peak, suggesting that these particles underwent a higher energy loss due to multiple binary collisions on their path through the solid surface region. The origin of this continuum is discussed in [4] and more recently [5, 6] deal especially with the role of oxygen for the re-ionisation. With increasing number of collisions the probing particles reach deeper layers and lose more energy This re-ionisation continuum below the scattering peak can even yield depth information [9, 10]. The assumption that re-ionisation is the responsible mechanism for the low energy tails is further supported by the existence of an energy threshold which results from the minimal energy required for a sufficiently close approach of nuclei to allow electronic orbital curve-crossings to occur [4, 11]
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