Ion scattering spectroscopy (ISS) or low energy ion scattering spectroscopy (LEIS) is a surface technique which involves probing the outermost surface layers of a solid using elastic collisions of ions with the surface layer. Originally involving high energy (MeV) ions,¹ eventually the technique was refined to utilise lower energy ions to be more readily available in laboratories.^2-4 It was the discovery that ion-surface collisions could be described kinematically by a simple two body elastic collision model that allowed for facile determination of surface composition:

Where, E₀ = E_k of primary ion, E₁ = E_k of primary ion following a scattering process at 90° to initial velocity, m₁ = mass of primary ion, m₂ = mass of target atom.

Standard trigonometry was later used to expand this calculation to accommodate angles other than 90° and allow for more freedom in experimental design of instrumentation.³ The extreme surface sensitivity of ISS allows it to offer additional structural information, complimentary to that of pure XPS analysis and has found it use in many fields including catalysis, thin films, electronics and sensor materials.

What can it tell us?

Since ISS is much more surface sensitive than XPS, using the two in synergy permits additional information regarding sample morphology. For example, determining surface coverages and terminating layers in thin-films, monitoring structural changes following reactions or processes or determining growth modes during deposition or surface growth syntheses.

ISS also works very nicely with TPD for the study of heterogeneous catalysts, where surface speciation is crucial to activity. For example figure 2 shows ISS as a function of temperature for small and large cluster Pd catalysts following exposure to CO. Comparisons of these changing ISS ratios with CO TPDs permitted the observation that low temperature CO desorption in the TPD was due to CO adsorbed onto Pd-TiO₂ peripheral sites, rather than directly on-top of Pd clusters.⁵

ISS can be challenging to obtain experimentally, since sample surfaces are required to be very flat.

Both the Kratos SUPRA and Thermo NEXSA systems at HarwellXPS central hub have the capacity to record ISS.

References

1. Brown, S. (1951). “Fowler, and Lauritsen.” Phys. Rev 82: 159.
2. Datz, S. and C. Snoek (1964). “Large-angle, single-collision scattering of argon ions (40-80 keV) from metals.” Physical Review 134(2A): A347. Read it online here.
3. Smith, D. P., Scattering of Low‐Energy Noble Gas Ions from Metal Surfaces. Journal of Applied Physics 1967,38 (1), 340-347. Read it online here.
4. D.P.Smith (1966). Bull. Am. Phys. Soc. 11.
5. Kaden, W. E., et al. (2012). “CO adsorption and desorption on size-selected Pdn/TiO2 (110) model catalysts: Size dependence of binding sites and energies, and support-mediated adsorption.” The Journal of chemical physics 136(20): 204705. Read it online here.