Dynamic charge transfer at the C60/metal interface studied by infrared absorption and sum-frequency generation

Fullerene thin films are probed by infrared absorption and sum-frequency generation. These techniques together determine the infrared and/or Raman character of adsorbate vibrations. The normally Raman active Ag(2) mode of C60 gains strong infrared activity upon adsorption on Ag(111). This unexpected infrared activity of the mode is shown to result from a dynamic charge transfer occurring between the substrate and the adsorbate. Through K-doping of the monolayer, tuning of the admolecule electronic properties and subsequent control of the interfacial dynamic charge transfer (IDCT) process are obtained. In particular, the predicted concomitant quenching of the IDCT and of the accompanying infrared activity and softening of the Ag(2) mode is observed. A quantitative evaluation of the coupling strength of the Ag(2) vibration to the t1u orbital of C60 is provided.