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Description
Low-energy ion scattering (LEIS) is an ion beam analysis technique with utmost surface sensitivity [1]. When used with electrostatic analysers for ion detection (esaLEIS), the resulting high solid-angle coverage allows for spectra acquisition in short time scales and at low sample exposure to the incoming ion flux. However, in esaLEIS, only charged particles are detected, limiting quantitative analyses unless the charge fractions of probing ions are known.
Here, we present a combined experimental and numerical approach to analyse LEIS spectra obtained from He⁺ scattering off a CaSiO$_3$ sample with energies from 1–3 keV. Experiments were carried out using a commercially available setup (ionTOF Qtac). Simulations were performed with the binary collision approximation codes SDTrimSP [2] and IMINTDYN [3]. The former enables the calculation of the equilibrium surface composition after sputter cleaning, while the latter is capable of directly calculating LEIS spectra under consideration of the experiment geometry. Furthermore, it can separate the simulated spectra by scattering partner (sample species) and scattering type (whether the ion scattered once, twice, or multiple times).
While the experimental spectra contain only charged ions, the simulations do not account for neutralisation and/or re-ionisation processes. A comparison therefore enables to extract the charge fraction of the probing He after scattering. The singly scattered particles make up most of the characteristic peaks in the LEIS spectra used for elemental identification. The double and multiple scattering events, while suppressed by roughly an order of magnitude, determine the shape of the spectral background. Beyond this deeper understanding of the spectral shapes, our results on the charge fractions potentially aid future quantification of samples with similar chemical environments in LEIS.
[1] H.H. Brongersma et al., Surf. Sci. Rep. 62 (2007) 63–109.
[2] A. Mutzke et al., (2019).
[3] H. Hofsäss, A. Stegmaier, Nucl. Instrum. Methods Phys. Res. B 517 (2022) 49–62.