Collision cascading, sequence

Collision cascade Ion impact-induced collision sequences in which momentum transfer occurs... 

It should be remembered that F was derived on the basis of assuming an amorphous solid. The same is true for Sigmund s sputtering theory. However, it is the suspicion of the present author that the sequence of collisions, which are present in every cascade, tends to make the random assumption more realistic. Therefore, it is possible that this theory can be applied with some degree of confidence to polycrystalline materials. It cannot, of course, be applied to single crystal materials where chaneling and related phenomena occur. 

The treatment of secondary-electron emission presented in this section essentially follows the ideas and organization of Sickafus which in turn was strongly influenced by Wolff . Upon entering a solid, an energetic electron is subjected to a sequence of elastic and inelastic scattering events. The collisions produce a cascade of moving electrons. The intersection of the cascade with the surface results in the emission of electrons into the gas phase. Therefore, the externally-observed, energy-... 

In addition to recoil mixing, other ballistic phenomena are possible during ion irradiation and implantation. For example, enhanced atomic mixing can occur when multiple displacements of target atoms result from a single incident ion. In the multiple displacement process, an initially displaced target atom (primary recoil) continues the knock-on-atom processes, producing secondary recoil atom displacements which in turn displace additional atoms. The multiple displacement sequence of collision events is commonly referred to as a collision cascade. 

Here the asterisk denotes translational excitation, and Equation 1 represents a cascading sequence of momentum transferring collisions. 

A collision cascade, as the name suggests, describes a sequence of many collisions involving many atoms within the solid. Each collision is, however, considered separate from each other. As a result, these can be considered a linear sequence of collisions. Ejection of surface-bound atoms, ions, and/or molecules will ensue if, at any time during the collision cascade, there exists sufficient energy transfer (momentum) in the direction away from the solid such that the atom, ion, or molecule can escape the solid surface. Note The molecules observed may be... 

These trends, which are fully supported by empirical data, confer with the supposition that the sputtered population arises from a sequence of many collisions (the collision cascade) with each collision acting independently of all other collisions (linear collision cascade) (Eckstein 1989, Rabalais 1994, Gnaser 1999). 

The primary reason why various simulations of sputtering resulting from momentum transfer, as described within the linear cascade model, are successful lies in the fact that the collision events can be treated using classical arguments, that is, Newtonian mechanics as opposed to quantum mechanics. Owing to the insignificant wavelength of ions ( 10" A), quantum mechanics is not needed. In addition, calculations are simplified for isotropic linear cascades as these represent a linear sequence of independent collision events that occur over time scales much shorter than lattice vibrations. 

The extremely short lifetimes of such excitations are, however, expected to preclude the possibility that the above processes could affect the formation/survival of secondary atoms/ions formed within the same linear collision cascade sequence... 

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