Fourier Transform Infrared Spectrometry (FT-IR), with its high sensitivity, allows the detection of weak infrared (IR) radiation emitted by samples. Published papers on emission spectrometry demonstrate that this technique is a powerful tool for studying vibration spectra of surface species, such as emission spectra measurements from metal surface species. For example, workers have observed the emission spectrum of oleic acid on aluminum foil; emission spectra of both thick and thin films of silicone grease on aluminum sheets have been measured; and the orientation of lubricant molecules in operating bearings and the formation of polymeric deposits from jet fuel on a heat exchanger surface have been studied.
The main problem in extending the emission technique to surface analysis is the strong background emission which is superimposed on a weak emission from the surface. The resultant low signal-to-noise ratio (S/N) is one reason why the great potential of emission IR spectrometry has not yet been fully exploited. Various methods have been suggested to overcome this problem. One approach is to reduce the background by cooling the spectrometer and sample chamber in liquid nitrogen. This approach, though expensive and cumbersome, has worked well and has provided sensitivities down to fractions of a monolayer of organic adsorbates on metal surfaces. Another approach tried to remove most of the background is by polarization-modulation, making use of the orientation of solid samples and, therefore, the polarization of their emitted IR radiation. This technique has its disadvantages due to the very small signal intensity. Still another way suggested to increase the S/N is by increasing the collected radiation. For instance, calculations have been made of the angular distribution of the intensity of the emission from species adsorbed on metal surfaces. These calculations indicate that the coherent emission from the oscillator dipole normal to a metal surface has an intensity maximum when the viewing angle is between 70 and 80 degrees from the normal, and is approximately zero in the direction normal to the surface. This is because the dipoles are oriented perpendicular to the surface and results from, it was suggested, the destructive interference between the emitted and reflected light from the interface to the metal surface. Using a viewing angle of 70 degrees, good quality measurements have been made of the emission spectra species from metal surfaces. Measurements have also been reported combining the large viewing angle with the polarization modulation technique for aluminum oxide on aluminum spectra measurements.
The techniques described above are complicated and require expensive equipment. To my knowledge, no one in the field has devised an accessory which is sufficiently adaptable to be employed in most standard spectrometers to enable a user to reflect radiation from an IR radiating sample into the spectrometer. Nor, to my knowledge, has anyone provided a configuration that can collect much of the radiant flux or will provide spatial resolution for neighboring source elements.