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The complex reaction path of the dissociative chemisorption of Sb 4 on Si(001) involving four distinct types of precursor states is determined using a direct «tracking» method with scanning tunneling microscopy. The energy barriers and the prefactors for the transitions between different states are measured by analyzing the population distribution of Sb clusters as a function of thermal treatment. These precursors are found not to have thermal mobility before dissociation, contrary to the widely held notion about precursor states

A scanning tunneling microscopy (STM) method for studying surface diffusion is developed based on measurements of the displacement distribution of adsorbates by «image-anneal-image» cycles which allow direct observation of the diffusion process while avoiding potential STM tip effects. The method is used to study the anisotropic diffusion of Sb dimers on Si(001). The energy barrier and the prefactor for the faster diffusion across the substrate dimer rows are measured. On the other hand, the diffusion observed by the «image-while-hot» method appears nearly isotropic and also much faster. It is shown that this discrepancy is due to the STM tip influence present in the latter method