Shadowing and blocking are important concepts in almost all types of ion-surface interactions and result from the repulsive nature of the ion-nucleus interaction.  As shown at right, when a flux of ions flows in parallel towards a scattering center (nucleus), they are each scattered according to the force of the Coulomb repulsion.  This effect is known as shadowing.  In a simple Coulomb repulsion model, the resulting region of “forbidden” space behind the scattering center takes the form of a paraboloid with radius  at a distance L from the scattering center.  The flux density is increased near the edge of the paraboloid.
Blocking is closely related to shadowing, and involves the interaction between scattered ions and a neighboring scattering center (as such it inherently requires the presence of at least two scattering centers).  As shown, ions scattered from the first nucleus are now on diverging paths as they undergo interaction with the second nucleus.  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.  Also, multiple scattering events are very likely which complicates analysis.  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).