The dynamics of activated surface diffusion

Abstract

An elementary process occurring on surfaces is diffusion. The dynamics is simplest whenthe concentration of adsorbates is sufficiently small that interaction between adsorbatescan be ignored. But even for this tracer diffusion process, much remains to beuncovered. Here, we present the interplay between experimental measurement of tracerdiffusion and its theoretical interpretation, which leads to good estimates of theinteraction of the adparticle with the surface. We show how the results fromthree different experimental techniques—field ion microscopy, scanning tunnellingmicroscopy and quasielastic helium atom scattering—can be interpreted. Usingthe generalized Langevin equation as a model for the diffusion dynamics, weshow how the turnover theory for activated diffusion may be used to describe themeasured time evolution of the adparticle distribution on the surface. The differentactivation energy measured for hopping over single or double lattice lengths is shownto come from the added energy loss to the surface, as the particle moves overthe longer path. We discuss some of the issues which are not yet clear; theseinclude quantum effects, such as the quantum suppression of diffusion, vibrationallyassisted diffusion, multidimensional effects and diffusion in the presence of externalfields.