Characterization of the C60/Ag(111) interface by sum-frequency generation

Non-linear optical experiments were performed on C60 and K-doped C60 monolayers grown on Ag(111). We have recorded the visible SFG signal resulting from the frequency mixing of one tunable infrared and one visible laser beam. The SFG process is forbidden in the bulk for centrosymmetric systems and thus provides the fingerprint of interfacial vibrations. The monolayer spectrum of pure C60/Ag(111) shows one resonance at 1445 cm-1. We assign this peak to the Ag(2) "pentagonal pinch" mode, that is Raman active in the bulk at 1468 cm-1. The spectrum of a fully K-doped C60 monolayer exhibits two peaks at 1349 and 1431 cm-1. The former is still attributed to the Ag(2) mode while the latter is assigned to the T1u(4) infrared active mode of C60.