Vibrational properties and charge transfer of adsorbed on Si(111) and Si(100) surfaces

We report here the measurements of vibrational excitation spectra and the temperature dependence of (Formula presented) molecules adsorbed on Si(111)-(Formula presented) and Si(100)-(Formula presented) surfaces using high-resolution electron-energy-loss spectroscopy in combination with scanning tunneling microscopy. A quantitative determination of the molecular charge state is obtained by the lower energy shifts of vibrational modes of (Formula presented) upon adsorption. On the Si(111)-(Formula presented) surface, the amount of charge transfer from the surface to a (Formula presented) molecule is estimated to be (Formula presented) electrons at the coverage lower than 0.25 monolayer (ML) and (Formula presented) electrons as an average at 1 ML. On the contrary, no indication of charge transfer is observed on the Si(100)-(Formula presented) surface at a coverage lower than 1 ML. The difference in the charge state on both surfaces is attributed to the difference in the nature of surface states. After annealing 1 ML (Formula presented) film at 500 K, the strong interaction between the surface Si atom and a (Formula presented) molecule is indicated by the softening of vibrational modes on both surfaces.