Asymmetric one-dimensional random walks.

For one-dimensional surface diffusion in the presence of fields, movement of particles has to be considered as a random walk in which jumps to the right occur at a rate different from jumps to the left. Moments of the displacement distribution are worked out for such a one-dimensional walk to nearest-neighbor sites as well as by longer jumps to second nearest neighbors. The actual distribution of displacements, and how it changes as the asymmetry of the jump rates changes, is also examined, as this provides important information about the participating jump processes. We show that deriving the third moment gives a clear indication of the asymmetry in the random walk.

Atom incorporation at edge defects in clusters.

Theoretical estimates indicate that atoms originally deposited on top of a surface cluster have a significantly lower energy barrier to incorporation at edge defects compared to attachment at straight steps of sizable clusters. We have carried out the first test of these findings, by field ion microscopic observations of the behavior of Ir atoms on Ir(111) clusters with and without defects in the edges. The clusters Ir18, Ir19, Ir20, Ir55, and Ir63 have been examined. Our observations suggest that defects do not provide especially low energy paths for incorporation, which generally occur in straight steps.

Long jump rates in surface diffusion: W on W(110).

We have reexamined the diffusion of W adatoms on W(110) using the field ion microscope. The diffusivity is in good agreement with previous results and reveals no unusual features, but from observations of the distribution of displacements, corrected for diffusion during temperature transients, we have for the first time been able to measure the temperature dependence of rates for nearest-neighbor and double jumps, as well as for vertical and horizontal transitions. Activation energies and frequency factors for all the long jumps are significantly larger than for single jumps, and a simple model is proposed to account for our observations.

Long jumps in surface diffusion.

Diffusion on metal surfaces is often viewed as the movement of atoms jumping between nearest-neighbor sites. However, statistical predictions suggest that at elevated temperatures longer jumps should become important. Now, with instruments such as the field ion microscope and the scanning tunneling microscope, which reveal individual adsorbed atoms, it has become possible to examine diffusion in some detail. Here we summarize the experimental efforts that have revealed the contributions of long jumps in various diffusion processes.

Non-nearest-neighbor jumps in 2D diffusion: Pd on W(110).

A variety of jumps has in the past been identified in diffusion of atoms on 1D channeled surfaces. To establish the jump processes important in diffusion on a 2D surface, the movement of individual Pd atoms has been examined on W(110). From the distribution of displacements of Pd at high temperatures, double jumps are found along the close-packed <111>. For the first time, sizable differences are also observed between the mean-square displacements along x and y, which demonstrate unexpected contributions from jumps along <110>, but not along <001>. These jumps proceed over activation barriers higher than for single jumps, under conditions predicted from previous work with Pd on the channeled W(211).