CAICISS is an acronym for coaxial impact-collision ion scattering spectroscopy. The technique is a derivative of the generic technique of ion scattering spectroscopy (ISS), and in particular of low-energy ion scattering spectroscopy (LEIS). LEIS is a powerful technique for analysis of surface atomic structure, because it uses a simple classical model of a beam of ions (charged atoms) hitting the surface layer of atoms in a solid. If the direction of the ion beam is altered, then a variation in the number of rebounding ions occurs, as the surface atoms place the sub-surface atoms in shadow. A similar effect called blocking occurs, as ions which do manage to penetrate to deeper layers and rebound cannot escape without colliding with the surface layers on the way out. These shadowing and blocking effects ensure that ISS only looks at the top layers of a surface, i.e that there is surface specificity. By using simple billiard ball physics, the relative positions of the atoms in a surface can be found.
ISS can be subdivided into groups roughly by the energy of the incident ions. High energy ions (>10keV) obey Rutherford's scattering law, as the dominant interaction between the ion and the atoms in the surface is internuclear Coulombic repulsion. This technique, known as Rutherford backscattering spectroscopy (RBS) makes the modelling straightforward, but since the ions have high energy, they travel quickly and with large momenta, resulting in narrow shadow and blocking cones, and poorer surface specificity. RBS is better suited to depth profiling studies than surface crystallography.
In low energy ion scattering spectroscopy (LEIS), with energies <5keV, the shadow and blocking cones are thicker and the technique more surface specific. However, this is at the expense of more difficult modelling, as the lower energy makes neutralisation of the ions (by the electrons in the solid) and multiple scattering events more likely. Medium energy ion scattering spectroscopy (MEIS) uses ions of intermediate energy, and therefore has properties between these extremes. MEIS can be used for both depth profiling and crystallographic studies, although it is more difficult to achieve surface specificity, attainable only by the use of carefully-aligned experimental geometries.
In LEIS, both shadowing and blocking are important. However, the combination of effects can cause problems, as the variation in the numbers of backscattered cannot always be attributed to one of the two effects without ambiguity. This makes evaluation of the atomic structure difficult, and so it is desirable to be able to design experiments in which one of the effects does not occur. If the scattering angle is close to 180°, the blocking effect rarely occurs, and furthermore, the collisions observed must be close to impact (i.e. head-on). This special case of ISS is known as impact-collision ion scattering spectroscopy or ICISS, and allows more straightforward analysis of surface structure, as the physics of the collisions and shadowing are simpler.
In the past, true 180° backscattering was not used for ICISS as it is technically difficult. Generally, ICISS uses scattering angles in the range 140° to 170°, and the assumption that this difference is negligible is not wholly unreasonable. However, by using an annular (ring-shaped) detector mounted around the incoming ion trajectory, the measured scattering angle may be kept to within 1° of true backscattering. This coaxial arrangement uses a time-of-flight detector, which determines the energy of the backscattered ions by measuring the time taken for them to rebound, and using simple conservation of energy and momentum relationships. Hence, this refinement of the technique is known as coaxial impact-collision ion scattering spectroscopy (CAICISS) or sometimes as time-of-flight impact-collision ion scattering spectroscopy (TOF-ICISS).