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When a beam of ions hits a solid surface part of the projectiles will be scattered back into the vacuum after one or more collisions with target atoms of the top few layers. Measurement of the energy of the backscattered particles can be used to identify the mass of the these atoms. The technique is called Ion Scattering Spectroscopy (ISS). The term encompasses actually several techniques depending on the energy of the primary ion beam. LEIS (Low Energy Ion Scattering) spectroscopy is referred to primary energies in the range of 100 eV to 10 keV, Medium Energy Ion Scattering (MEIS) to a range from 100 to 200 keV, and High Energy Ion Scattering (HEIS) to energies between 1 and several MeV. Often the LEIS technique is called Ion Scattering Spectroscopy (ISS), the term we will use below meaning LEIS, while HEIS technique is best known as Rutherford Backscattering Spectroscopy (RBS).

Backscattering of kilovolt ions
The process can be considered as a series of elastic collisions with the atoms in the top layers. In this (kilovolt) energy range one can assume that the collisions are binary and the atoms are free, so that the ion-atom interaction is described by repulsion interatomic potentials only. According to the classical mechanics, the energy of scattered particle is given by

       E1/Eo = {
(M22-M12sin2q1)1/2 +M1cosq1] / (M1+M2)}2                                          (1)

(both signs hold for M1>M2 and only the positive signs otherwise), where Eo is the initial energy, q1 is the scattering angle, M1 and M2 are the ion and target atom mass respectively. This equation directly relates the energy of the scattered particle to the mass of the target. It can be used also to evaluate the energy after two, three or more collisions.

Advantages and disadvantages
Low energy ion scattering is attractive as a surface-specific technique. Spectra are usually obtained using noble gas ion beams from 0.5 to 5 keV. Due to strong electron affinity of inert-gas ions the probability of electron transfer is very high even in the initial collision with a surface atom. After two or more collisions most ions will be neutralized, so that a detector set to analyze only ions of the same type as those in the incident beam will detect almost entirely ions that have had only one collision with a target atom. Projectiles entering the solid will be discarded since they would need several scattering events to return back to the surface and exit. Unless specific experimental arrangements such as sliding angle incidence are made, multiple collisions will contribute to the background of the energy spectrum only. Thus by measuring the spectrum of backscattered ions and using the equation above one can easily determine the target mass, therefore the kind of atoms on the surface. Note that the energy after two collisions depend on the distance between the scattering atoms, and this energy peak can be used to estimate the interatomic distance on the surface in the scattering plane (incl. the incident and the scattered beams). By rotating the target under the incident beam one can determine the surface arrangement of the atoms in the various azimuthal directions of the top layer.
The intensity of the peaks, or rate at which scattered ions arrive at the detector depends on several parameters. The detected ion intensity,
Ii , is related to the number of atoms of a particular kind on the surface, Nk, through the equation

       Ii = K.Ip.Nk. S . Pi . W . h                                                                                (2)

Here S is the scattering cross section, or the probability that an ion will scatter into the detector when it encounters an atom of type k;
Ip is the current in the primary beam, or the number of ions that strike the sample surface; Pi is the probability that a scattered ion will avoid neutralization at the surface and will retain its charge; W is the acceptance solid angle of the detector. The constant K is added to account for other, perhaps unknown factors, such as shadowing of atoms. The equation above is rarely used for quantitative analysis since major parameters such as Pi are poorly defined. Standard samples or tabulated sensitivity factors are used instead.
The mass resolution determines whether elements with similar masses can be separated from each other. From eq. (1) one can see that E1/Eo is not a linear function of M2, and separation of similar masses for heavier elements becomes difficult (above appr. 40 amu with He+, 60 with Ne+, and 80 with Ar+).
The quantification of surface analysis by using low energy ions is hampered by the uncertainty of the inelastic losses and the neutralization rate depending on ion trajectories. In addition, overlapping peaks and multiple scattering have to be taken into account, and here computer simulations become an indispensable tool.

The practical use of IIS is determined by its extreme sensitivity to only the top surface layer (for standard experimental arrangements) or two monolayers (for grazing incidence). Typical applications include composition of catalytic surfaces, thin film coatings, adhesion, as well as arrangement of surface atoms incl. the localization of adsorbed atoms.

The ISS is available on both AXIS 165 and AXIS ULTRA instruments. He and Ar gas are commonly supplied.

1. Energy spectrum of 1 keV He ions backscattered from solid Cu target taken by using AXIS 165 spectrometer.

2. Energy spectra of 2 keV He ions backscattered from thin PtxRuy layers measured by AXIS-165. The identification of two separate peaks allows to assess the Pt/Ru ratio in the top surface layer. Samples provided by Daniel Quay