Role and Mechanism of Island Formation in Chemisorption

Chemisorption on solid surfaces is treated as a combination of a seeding process, whereby molecules adsorb directly onto vacant sites, and an island-growth mechanism initiated by physisorption over occupied sites followed by lateral migration towards vacant sites. It is argued that the islands are compact 2D (Eden) clusters, at marked variance with models which assume random distribution of the chemisorbed molecules. Simple general expressions are derived for the sticking probability and adsorption isotherms. Numerical results are presented for several cases of interest.