Vibrational investigation of chemisorbed C-60 by infrared-visible sum frequency generation spectroscopy

Non-linear optical experiments were performed on C-60 and K-doped C-60 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 C-60/Ag(111) shows one resonance at 1445 cm(-1). We assign this peak to the A(g)(2) ''pentagonal pinch'' mode, that is Raman active in the bulk at 1468 cm(-1). The spectrum of a fully K-doped C-60 monolayer exhibits two peaks at 1349 and 1431 cm(-1). The former is still attributed to the A(g)(2) mode while the latter is assigned to the T-1u(4) infrared active mode of C-60.