In the SFG technique, a pulsed visible green laser beam (532 nm) is overlapped on a surface with a tunable, pulsed infrared (IR) laser beam (2 to 10 Ám). The two laser pulses combine to produce light at the sum of their frequencies (440 to 500 nm) by a nonlinear process. The light emitted at the sum frequency is detected by a photomultiplier and the spectrum is obtained by tuning the IR beam over the frequency range of interest. (Figure 1)
Figure 1. Overlap of the infrared (IR) and visible laser beams on a polymer surface to produce the sum-frequency beam.
An amorphous media, such as a polymer, does not generate a sum frequency signal because the bulk is randomly oriented, and second-order mixing is thus forbidden. However, polar ordering can occur at the surface of a polymer to reduce interfacial energy, resulting in a net orientation of molecules, and allowing a sum frequency response. Therefore, SFG is intrinsically surface specific.
Enhancement of the SFG signal occurs when either the IR or visible beam is in resonance with a vibrational mode or electronic state, respectively. Molecular-level information about the interfacial species can be derived from the vibrational spectrum obtained by tuning the IR beam. This allows both the identity and orientation of surface groups or molecules to be determined.