The adsorption of naphthalene on a Si(100)-2×1 surface at room temperature was investigated using infrared absorption spectroscopy (IRAS) in the multiple internal reflection geometry (MIR). To determine the adsorption configurations on the surface in detail, IRAS spectra in the C-H stretching vibration region were analyzed in comparison with calculations based on the density functional theory. As a result, it was found that naphthalene adsorbs on the surface in different manners depending on the surface coverage of the molecules. At low coverage, a single configuration is favored, in which the 1, 4, 6, 9 carbon atoms of a naphthalene molecule are bound to the dangling bonds of two adjacent Si dimers to form sp3 bonds. At high coverage, on the other hand, the molecules adsorb in several energetically preferred configurations. Furthermore, the effect of coupling of vibration modes between two adjacent molecules adsorbed on the surface was observed at high coverage.
