We have investigated the initial stage of adsorption of a conjugated aromatic compound, 1, 4-bis[β-pyridyl-(2)-vinyl]benzene (P2VB), on the clean Si(100)-2×1 surface and the hydrogen terminated Si(100)-2×1-H surface by ultra-high-vacuum (UHV) scanning tunneling microscopy (STM). We found adsorbed molecules cannot migrate on the chemically active Si(100)-2×1 surface, while they can migrate on the chemically inactive hydrogen terminated Si(100)-2×1-H surface until they are trapped to hydrogen-missing dangling bonds. On the clean Si(100)-2×1, we observed four different adsorption directions. An individual molecule appears as two or three bright spots, the brightness and distance between bright spots varying for different cases. Through structural analysis and bias-voltage-dependent STM images, we conclude that the electronic states of Si dimers modulated by the adsorbed molecules are observed instead of the molecules themselves. A simple estimation by considering only the molecular size and shape reproduces the distribution of four different kinds of adsorption structures we observed
