Surface and depth-profile analysis using FTIR spectroscopy

Summary

The introduction of Fourier Transform techniques and the increasing use of computers in infrared spectroscopy has made new techniques of investigation available to the spectroscopist, such as photoacoustic spectroscopy (PAS) and IR microscopy. These methods now complement the techniques of specular reflectance and attenuated total reflectance. Thin films on metals, sometimes with a thickness of much less than a micron, can be studied by various specular reflectance methods. The physical basis of the attenuated total reflectance technique (ATR) leads to a penetration of the radiation in the order of a few microns. It is, therefore, especially suitable for the investigation of surfaces and of layers close to the surface. By changing the modulation frequency of the IR radiation, i.e. the mirror velocity of the FTIR spectrometer, photoacoustic spectroscopy (PAS) can be employed to study layers at various depths below the surface of a sample. Therefore, this technique allows depth-profile analysis, so that PAS reveals itself as a complementary method to attenuated total reflectance spectroscopy. Samples with inhomogeneous profiles, e.g. laminated polymer films, can often be prepared as microtome slices perpendicular to the layered structure. Using infrared microscopy it is now possible to investigate different regions of the cross-section easily. The size of the regions that can be studied in this way may be as small as a few microns.