Vibrational sum frequency generation spectroscopy of fullerene at dielectric interfaces

Vibrational sum frequency generation (VSFG) spectroscopy was used to measure the interfacial spectra of fullerene thin films on dielectric substrates that are commonly used in IR spectroscopy. The VSFG spectra on SiO₂ and CaF₂ exhibit notably different intensities for the F_1u and A_g vibrational modes. This difference is attributed to different interfacial surface charges and C₆₀/surface interactions. DFT calculations were performed to model the influence of a unidirectional electrostatic perturbation on the IR and Raman activities. The VSFG activities were then calculated for comparison to the interfacial second-order susceptibilities obtained from multilayer interference fitting of the experimental spectra. We find that the negative surface charge of CaF₂ substrates causes a larger perturbation of fullerene than native silica surfaces, which leads to a stronger influence on the VSFG spectra.