The migration and thermal desorption of sub-monolayer coverages of Cs on (100) 2 x 1 were studied by using biased secondary electron imaging. The results indicated the existence of 3 different bonding regimes as a function of coverage. These were found at coverages of less than 1/6 of a monolayer, between 1/6 and 1/2 of a monolayer, and between 1/2 and 1 monolayer. When the coverage was less than 1/6 of a monolayer, the diffusion could best be described in terms of strongly repulsive dipole interactions. The complex diffusion profiles which were observed at coverages of between 1/6 and 1/2 of a monolayer were explained by incorporating a 2-phase model into coupled rate and diffusion equations. The dilute phase represented all of the structures where individual adatoms were separated by at least 2 unoccupied sites, while the condensed phase represented structures in which adatoms were on neighboring sites. The model predicted the existence of large activation energies for adatom transfer between the 2 phases, and a difference of 0.04eV between the energies which were required in order for an adatom to move from the dilute phase to the condensed phase and  vice versa.

R.H.Milne, M.Azim, R.Persaud, J.A.Venables: Surface Science, 1995, 336, 63-75