SFG-VS involves overlapping a visible laser beam and an IR laser beam at the interface of interest. The interaction of these beams at the interface generates a third beam at the sum frequency of the two input beams. The intensity of this sum frequency signal is directly related to the vibrational modes of the molecules at the interface. By tuning the IR laser to different frequencies, one can obtain a vibrational spectrum that is specific to the molecules on the surface.
