We investigate adsorption of Si atoms on the Si(111)7×7 surface by STM experiments and by molecular orbital calculations. In experiments where Si atoms are deposited from an STM tip, not only “staying” Si adsorbates but also noise-like “diffusing” Si ones are observed. To identify their geometries, stable positions, transition states and electron density iso-surfaces for extra Si atoms on cluster models are obtained using molecular orbital calculations. Calculated stable geometries for both one and two Si atoms agree well with the observed STM images of the staying Si adsorbates. For one Si atom, however, calculated energy barrier around a rest atom is only several tenth eV, while it is more than 1 eV for two Si atoms. Therefore, we propose that the staying Si and diffusing Si adsorbates correspond to two Si atoms and one Si atom, respectively.
