Electronic tuning of dynamical charge transfer at an interface: K doping of C60/Ag(111)

High-resolution vibrational spectroscopy of fullerene thin films is achieved by sum-frequency generation and infrared absorption. The Ag(2) mode of C60 gains strong infrared activity when the molecule is adsorbed on Ag(111). K-doping allows to demonstrate the process of adsorbate/substrate dynamical charge transfer, by showing, for the first time, the quenching of the mode softening and infrared activity upon tuning of the admolecule electronic properties. These observations enable the quantitative evaluation of the coupling strength of the Ag(2) vibration to the t1u orbital of C60.