Long jumps consist of adatom displacement to a non-nearest-neighbor adsorption site. They may include double, triple, and longer jumps in the same direction as a nearest-neighbor jump would travel, or they may be in entirely different directions as shown in figure 6. They have been predicted by theory to exist in many different systems, and have been shown by experiment to take place at temperatures as low as 0.1 Tm (melting temperature). In some cases data indicate long jumps dominating the diffusion process over single jumps at elevated temperatures; the phenomena of variable jump lengths is expressed in different characteristic distributions of atomic displacement over time (see figure 7). Rebound jumps have been shown by both experiment and simulations to take place in certain systems. Since the motion does not result in a net displacement of the adatom involved, experimental evidence for rebound jumps again comes from statistical interpretation of atomic distributions. A rebound jump is shown in figure 6. The figure is slightly misleading, however, as rebound jumps have only been shown experimentally to take place in the case of 1D diffusion on a channeled surface (in particular, the bcc (211) face of tungsten). Cross-channel diffusion can occur in the case of channeled surfaces. Typically in-channel diffusion dominates due to the lower energy barrier for diffusion of this process. In certain cases cross-channel has been shown to occur, taking place in a manner similar to that shown in figure 8. The intermediate "dumbbell" position may lead to a variety of final adatom and surface atom displacements. Long-range atomic exchange is a process involving an adatom inserting into the surface as in the normal atomic exchange mechanism, but instead of a nearest-neighbor atom it is an atom some distance further from the initial adatom that emerges. Shown in figure 9, this process has only been observed in molecular dynamics simulations and has yet to be confirmed experimentally. In spite of this long range atomic exchange, as well as a variety of other exotic diffusion mechanisms, are anticipated to contribute substantially at temperatures currently too high for direct observation. Cluster diffusion Cluster diffusion involves motion of atomic clusters ranging in size from dimers to islands containing hundreds of atoms. Motion of the cluster may occur via the displacement of individual atoms, sections of the cluster, or the entire cluster moving at once. All of these processes involve a change in the