smaller than the energy of desorption for diffusion to occur, otherwise desorption processes would dominate. Importantly, equation 1 tells us how strongly the jump rate varies with temperature. The manner in which diffusion takes place is dependent on the relationship between Ediff and kBT as is given in the thermodynamic factor: when Ediff < kBT the thermodynamic factor approaches unity and Ediff ceases to be a meaningful barrier to diffusion. This case, known as mobile diffusion, is relatively uncommon and has only been observed in a few systems. For the phenomena described throughout this article, it is assumed that Ediff >> kBT and therefore Γ << ν. In the case of Fickian diffusion it is possible to extract both the ν and Ediff from an Arrhenius plot of the logarithm of the diffusion coefficient, D, versus 1/T. For cases where more than one diffusion mechanism is present (see below), there may be more than one Ediff such that the relative distribution between the different processes would change with temperature. Random walk statistics describe the mean squared displacement of diffusing species in terms of the number of jumps N and the distance per jump a. The number of successful jumps is simply Γ multiplied by the time allowed for diffusion, t. In the most basic model only nearest-neighbor jumps are considered and a corresponds to the spacing between nearest-neighbor adsorption sites. The root mean squared displacement goes as: The diffusion coefficient is given as: where for 1D diffusion as would be the case for in-channel diffusion, for 2D diffusion, and for 3D diffusion. Regimes There are four different general schemes in which diffusion may take place. Tracer diffusion and chemical diffusion differ in the level of adsorbate coverage at the surface, while intrinsic diffusion and mass transfer diffusion differ in the nature of the diffusion environment. Tracer diffusion and intrinsic diffusion both refer to systems where adparticles experience a relatively homogeneous environment, whereas in chemical and mass transfer diffusion adparticles are more strongly affected by their surroundings. Tracer diffusion describes the motion of individual adparticles on a surface at relatively low coverage levels. At these low levels (< 0.01 monolayer), particle interaction is low and each particle can be considered to move independently of the others. The single atom diffusing in figure 1 is a nice example of tracer diffusion. Chemical diffusion describes the process at higher level of coverage where