Electron-phonon couplings at C_(60) interfaces studied by doubly resonant infrared-visible sum-frequency generation spectroscopy

摘要

The very symmetric C_(60) molecule constitutes a remarkable model system for studying electron-phonon couplings at interfaces.Indeed,despite its size,the molecule possesses a simple vibrational spectrum,that comprises only four infrared-active (4 T_(lu)) and 10 Raman-active (2 A_g+8 mug) modes.Pure C_(60) is a semiconductor having an electronic band gap of about 2.2 eV.The lowest unoccupied molecular orbital (LUMO) can accept up to six electrons as it is of t_(lu) symmetry.In the fullerides,the LUMO is filled gradually with electrons from intercalated alkali donors (K,Rb,...).The electronic properties of these materials are strongly affected by electron-phonon couplings,and superconductivity occurs in many A_3C_(60) compounds (up to 29 K for Rb_3C_(60)).Electron transfer to C_(60) arises also from adsorption on metallic surfaces (1,2).