Localized photothermal infrared spectroscopy using a proximal probe

A near-field thermal probe, as used in scanning thermal microscopy, is used to obtain photothermal Fourier transform infrared (FT-IR) spectra of polym ers, as a first step toward developing FT-IR microscopy at a spatial resolu tion better than the diffraction limit. The signal from the probe after amp lification provides an interferogram, and the resultant spectra are consist ent with those obtained by means of the established technique of attenuated total reflection FT-IR spectroscopy. We have extended this technique to th e analysis of "real-world" industrial samples, both solid (a fungicide in a fine powder form) and liquid (a concentrated surfactant solution). The ove rall shapes of the main peaks or bands reflect the fact that the spectrum i s a convolution of different contributions from both optical and thermal pr operties. To confirm the feasibility of subsurface detection of polymers, w e demonstrate the ability of the technique to perform spectroscopic detecti on of a model polymeric bilayer system, polyisobutylene on top of polystyre ne. A quantitative analysis of the variation of peak height with coating th ickness allows values of thermal diffusion length to be derived. This inves tigation provides a preliminary result for the understanding of the depth s ensitivity of the current setup. Relative intensity distortions are seen, a nd are attributed to photothermal saturation. A complementary technique has been developed that uses tunable monochromatic radiation, using an optical parametric generator as the infrared source. Spectra have successfully bee n obtained using the same localized photothermal detection principle. (C) 2 001 American Institute of Physics.