Physisorption versus chemisorption in inelastic electron tunneling spectroscopy: Mode position, intensity, and spatial distribution

Local vibrational spectra of meta-dichlorobenzene molecules adsorbed on different parts of the Au(111) reconstruction are investigated using a low-temperature scanning tunneling microscope. The spectra show substantial variations on subnanometer length scale. While for the molecule physisorbed on either the hcp or the fcc domain of the reconstruction only low-energy modes are beyond the detection limit, higher-energy modes are observed for the molecule chemisorbed at the elbow site. The different adsorption strengths of the molecules manifest themselves in an energy shift of the modes. These shifts are used to identify through which part the molecule is bonded to the surface.