Femtosecond sum frequency generation at the metal–liquid interface

We present a study of the C–H and C–N vibrational modes of acetonitrile molecules at the interface between the bulk liquid and a gold film using both time and frequency domain femtosecond sum frequency generation. Acetonitrile is adsorbed on the gold surface with the N atom towards the interface. Due to the surface roughness there are different adsorption sites, leading to an inhomogeneous distribution of vibrational frequencies of the C–N stretch vibration of different molecules. This is observed most clearly in the time domain SFG measurements, while the frequency domain measurements are much less sensitive to the inhomogeneity. In contrast to the C–N stretch mode, the C–H stretching modes are unaffected by the surface and display homogeneous line broadening.