Infrared radiation reflection spectrometry

INFRARED RADIATION SOURCES FOR TRANSMISSION AND REFLECTION SPECTROMETRY... 

The chalcogenides are all insoluble in water and other common solvents. ZnSe and CdTe have excellent transmission characteristics. The only problem with these materials is their high refractive index, which leads to high front-surface reflectance (see Section 13.2.2), so that transmission spectra of liquids held in cells fabricated from these materials often give rise to interference fringes (see Section 11.1.3). These materials aU make excellent internal reflection elements. AMTIR (amorphous material that transmits infrared radiation) is a mixture of several chalcogenides. Many optical fibers used for mid-infrared spectrometry are made from this material (see Section 15.4). 

Vibrational microspectrometry will undoubtedly be applied to medical diagnosis in the near future. One particularly important application of microspectrometry is for the characterization of tissue samples. Tissue samples can be mounted on a water-insoluble infrared-transparent window such as ZnSe, but these windows are expensive and not conducive to visual examination (e.g., after staining of the tissue). A convenient alternative to transmission spectrometry is the measurement of the transflectance spectrum (see Section 13.5) of tissue samples mounted on low-emissivity glass slides [4]. These slides are transparent to visible light but highly reflective to mid-infrared radiation. 

If a material could be made extremely thin, for example, to the level of a single layer of molecules, this thin layer would transmit almost all of the infrared radiation, so that its infrared transmission spectrum could be measured. In fact, it is possible to measure a mid-infrared transmission spectrum from a thin soap film. It is usually practically difficult, however, to maintain such a thin film without it being supported by a substrate. For a thin film supported on a substrate, its infrared spectmm is often obtained by utilizing a reflection geometry. Two reflection methods are available for measuring infrared spectra from substrate-supported thin films, depending on the dielectric properties of the substrates used. External-reflection (ER) spectrometry, which is the subject of this chapter, is a technique for extracting useful information from thin films on dielectric (or nonmetallic) substrates, while reflection-absorption (RA) spectrometry, described in Chapter 10, is effective for thin films on metallic substrates [1]. In addition to these two reflection methods, attenuated total-reflection (ATR) spectrometry, described in Chapter 13 and emission spectroscopy, described in Chapter 15 may also be useful in some specific cases. 

In a similar manner to that developed with Equations (11.1a) and (11.1 b) for the intensity of the beam of infrared radiation that has passed through the target and reference samples in double-modulation spectrometry, when polarization-modulation spectrometry is applied to a reflection-absorption measurement of a thin film adsorbed onto a metal substrate, the intensity of reflection B (v, f) is expressed in the following form as a function of wavenumber V and time f. 

In infrared emission spectrometry, very weak infrared radiation of emission from a sample itself is to be measured. Consequently, infrared emission measurements have requirements considerably different from transmission and reflection measurements. It is not desirable to use a complicated optical apparatus for emission measurements. 

As mentioned above a spectral profile in infrared spectrometry contains absorbencies at each wavenumber or wavelength of the absorbed radiation. This can be represented by an array of numbers that represent the absorbencies at different wavenumbers or wavelengths. This spectral profile may contain information that reflects the nature of the sample or can be correlated to a physical or chemical property of the system. When measurements are made with this intention, the spectral profiles can be represented by a matrix containing rows of numbers representing the spectral profiles and a column representing the external property (see Fig. 6.1). The relationship between the samples, that is, the relationship between the rows of the matrix, requires some clever mathematical decomposition of the data. 

The other type of measurement that can be made with the microscope in its reflection mode is diffuse reflection (DR) spectroscopy. There are very few applications of mid-infrared microspectroscopy of neat samples because for mid-infrared DR spectrometry, samples should be diluted to a concentration of 0.5-5% with a nonabsorbing diluent, such as KBr powder, to preclude band saturation and severe distortion by reflection from the front surface of the particles. However, this mode has substantial application for NIR measurements, where sample dilution is not needed. Because absorption of NIR radiation by most samples is rather weak, they must be either at least 1mm thick or mounted on a reflective or diffusing substrate, such as a ceramic or Teflon disk. In the latter case, the spectrum is caused by a combination of DR, transflection, and front-surface reflection (with hopefully DR being the dominant process.)... 

Infrared ER spectrometry depends on the reflection characteristics of differently polarized radiation at an interface. Figure 9.4 shows the calculated dependencies of the reflectances... 

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