Low-energy ion scattering

Low-energy ion scattering spectroscopy (LEIS), sometimes referred to simply as ion scattering spectroscopy (ISS), is a surface-sensitive analytical technique used to characterize the chemical and structural makeup of materials.

LEIS involves directing a stream of charged particles known as ions at a surface and making observations of the positions, velocities, and energies of the ions that have interacted with the surface.

LEIS is closely related to both medium-energy ion scattering (MEIS) and high-energy ion scattering (HEIS, known in practice as Rutherford backscattering spectroscopy, or RBS), differing primarily in the energy range of the ion beam used to probe the surface.

While much of the information collected using LEIS can be obtained using other surface science techniques, LEIS is unique in its sensitivity to both structure and composition of surfaces.

Additionally, LEIS is one of a very few surface-sensitive techniques capable of directly observing hydrogen atoms, an aspect that may make it an increasingly more important technique as the hydrogen economy is being explored.

LEIS systems consist of the following: Several different types of events may take place as a result of the ion beam impinging on a target surface.

Some of these events include electron or photon emission, electron transfer (both ion-surface and surface-ion), scattering, adsorption, and sputtering (i.e. ejection of atoms from the surface).

As the name suggests, LEIS is primarily concerned with scattering phenomena.

Due to the energy range typically used in ion scattering experiments (> 500 eV), effects of thermal vibrations, phonon oscillations, and interatomic binding are ignored since they are far below this range (~a few eV), and the interaction of particle and surface may be thought of as a classical two-body elastic collision problem.

Measuring the energy of ions scattered in this type of interaction can be used to determine the elemental composition of a surface, as is shown in the following: Two-body elastic collisions are governed by the concepts of energy and momentum conservation.

Using trigonometry we are able to determine Similarly, we know In a well-controlled experiment the energy and mass of the primary ions (E0 and mx, respectively) and the scattering or recoiling geometries are all known, so determination of surface elemental composition is given by the correlation between E1 or E2 and my.

Higher energy scattering peaks correspond to heavier atoms and lower energy peaks correspond to lighter atoms.

While obtaining qualitative information about the elemental composition of a surface is relatively straightforward, it is necessary to understand the statistical cross-section of interaction between ion and surface atoms in order to obtain quantitative information.

The two-body collision model fails to give quantitative results as it ignores the contributions of coulomb repulsion as well as the more complicated effects of charge screening by electrons.

This is generally less of a problem in MEIS and RBS experiments but presents issues in LEIS.

Coulomb repulsion occurs between positively charged primary ions and the nuclei of surface atoms.

Additionally, when using noble gas ion beams there is a high probability of neutralization on impact (which has strong angular dependence) due to the strong desire of these ions to be in a neutral, closed shell state.

In a simple Coulomb repulsion model, the resulting region of “forbidden” space behind the scattering center takes the form of a paraboloid with radius

This interaction results in another “shadowing cone” now called a blocking cone where ions scattered from the first nucleus are blocked from exiting at angles below

Focusing effects again result in an increased flux density near

In both shadowing and blocking, the "forbidden" regions are actually accessible to trajectories when the mass of incoming ions is greater than that of the surface atoms (e.g. Ar+ impacting Si or Al).

In this case the region will have a finite but depleted flux density.

For higher energy ions such as those used in MEIS and RBS the concepts of shadowing and blocking are relatively straightforward since ion-nucleus interactions dominate and electron screening effects are insignificant.

However, in the case of LEIS these screening effects do interfere with ion-nucleus interactions and the repulsive potential becomes more complicated.

Importantly, due to the lower energy ions used LEIS is typically characterized by large interaction cross-sections and shadow cone radii.

For this reason penetration depth is low and the method has much higher first-layer sensitivity than MEIS or RBS.

Overall, these concepts are essential for data analysis in impact collision LEIS experiments (see below).

The de Broglie wavelength of ions used in LEIS experiments is given as

Because of this, the effects of diffraction are not significant in a normal LEIS experiment.

Depending on the particular experimental setup, LEIS may be used to obtain a variety of information about a sample.

Image of a Kratos Axis-165 system equipped with XPS, ISS, and AES, from Alberta Centre for Surface Engineering and Science (ACSES).
General experimental setup for LEIS.
Diagram of an electrostatic analyzer in the hemispherical geometry. Only ions of a selected energy pass through to the detector.
Diagram of various ion-surface interactions (non-exhaustive). (1) Incoming ion; (2) Scattering; (3) Neutralization and scattering; (4) Sputtering or recoiling; (5) Electron emission; (6) Photon emission; (7) Adsorption; (8) Displacement. LEIS is unique in its high sensitivity to the first surface layer in a sample.
Repulsive scattering by a point particle.
Shadowing and blocking effects in two dimensions. No ions will be detected at angles below Primary ions are approaching from the upper left.
ICISS geometry and its relevance to structural characterization of surfaces. The direction and length of the surface-subsurface bond may be determined from an intensity vs. plot. Red: determining the shape of the shadow cone; Green: determining surface-subsurface spacing and direction with a known shadow cone shape.
A graph of intensity versus angle of incidence for scattering from a subsurface atom in the ICISS geometry. The directionality of the surface-subsurface bond (see diagram above) may be deduced from . The length of this bond may be deduced from and when the shape of the shadow cone is known.