Preferential bonding orientations of ferrocene on surfaces

We have measured the molecular orientation and bonding of adsorbed ferrocene on Ag(100) and Cu(100) using angle-resolved photoemission spectroscopy (ARPES). The results for molecular adsorption on Ag(100) are complemented by high-resolution electron energy loss spectroscopy (HREELS) measurements and ab initio calculations for the ferrocene vibrational modes. The measurements indicate that ferrocene adsorbs on Ag(100) with the molecular axis perpendicular to the surface. In contrast, as indicted using ARPES and scanning tunneling microscopy, ferrocene adsorbed on the Cu(100) surface is oriented with the molecular axis parallel with the surface. Model calculations allow us to assign all of the observed vibrational modes for the weakly bound molecular ferrocene on Ag(100)-both dipole and impact scattering modes have been observed.