The chemistry of interfaces plays a fundamental role in adhesion. Many analytical methods, such as SIMS, XPS, AES and SEM, can be used to study surfaces after adhesive failure. Only a few techniques exist, however, for the non-destructive, in situ chemical analysis of buried interfaces. Infrared-visible sum-frequency generation (SFG) is one such technique. Shen has demonstrated recently' that infrared spectra of interfaces can be obtained by sum-frequency spectroscopy, even in the presence of bulk phases on either side. SFG is a powerful technique still in its infancy; here, we demonstrate the application of SFG to the analysis of metals coated with monolayer films. These films are useful model systems and also have practical importance as adhesion promoters and wetting modifiers.In infrared-visible sum-frequency generation, a fixedfrequency visible laser and a tunable infrared laser are overlapped at an interface and the light emitted at the sum of the incident frequencies, u , , , , is detected (Scheme 1). In our experiments, laser light at 532 nm is provided by a frequencydoubled Nd : YAG laser, and the tunable infrared radiation is generated by difference frequency mixing of the Nd : YAG fundamental and a near-IR, Nd:YAG-pumped dye laser. The photons emitted at the sum frequency are separated from scattered light by a filter and monochromator, and detected with a photomultiplier tube. The intensity, S,,, , of the emitted sum frequency is given by' (photons per pulse) where Li are Fresnel field factors, I is the intensity of a laser, A the area of overlap of the laser beams, T the pulse duration and $, ) , , ( -us,,; wvis, urn) the second-order non-linear susceptibility of the sample.
