In-situ nonlinear optical spectroscopy of electron-phonon couplings at alkali-doped C60/Ag(111) interfaces

We use doubly resonant, infrared-visible sum-frequency generation spectroscopy (DR-SFG) to probe vibrational and electronic properties of C60 and K-doped C60 monolayers adsorbed on Ag(111) single crystal under ultra-high vacuum (UHV). We recorded the interface SFG spectra for five visible wavelengths. We observe a strong dependence of the SFG intensity of the totally symmetric Ag(2) mode of the fullerene while scanning the visible wavelength, due to the DR-SFG phenomenon. The SFG intensity of the Ag(2) mode is the strongest at 488 nm and at 532 nm for the pure and fully doped monolayers, respectively. These results demonstrate the occurrence of electron–phonon couplings at the C60/Ag(111) and saturated K/C60/Ag(111) interfaces. They enable us to determine the energy of the coupled electronic transition and to link the electronic resonance to the hu (HOMO) to t1g (LUMO + 1) transition of C60.