Prism Materials

The change of index of refraction with change in wavelength dn/dX is a property of the prism material and thus cannot be changed except by a change in the prism itself as used in the spectrometer. 

A wide variety of glasses possessing different indices of refraction is available for use in the visible region. For example, a light crown glass has an index of refraction of 1.5170 at 5893 A and a dense flint glass has an index of refraction of 1.6499 at the same wavelength. Salt (NaCl) crystals are used frequently since they are transparent in the infrared, but they are subject to severe deterioration unless carefully protected from moisture. 

Liquid prisms have been used for special purposes, the liquid being enclosed in a hollow prism. Most liquids, however, possess high temperature coefficients for their indices of refraction and temperature gradients within the prism can be troublesome. 

Quartz also will rotate the plane of polarization of a beam of plane polarized light. This effect occurs even along the optical axis of the quartz, but can be compensated in the construction of the prism. 

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The material of the prism is important in infrared spectroscopy, since it must be transparent to infrared light. The material most frequently used for analysis in the middle wavelength region is sodium chloride. Prism materials for the analysis of short and long wave infrared light are usually potassium bromide, cesium bromide, and cesium iodide. 

The refractive index of the prism material depends upon the wavelength of the incident light, so the angle of deviation becomes a function of wavelength... 

The factor d4>ldn is determined by geometrical considerations, but the factor dnjdX is characteristic of the prism material and it is called the dispersion. Typical prism materials are quartz glass, NaCl, KBr, or Csl. Quartz is used in the ultraviolet region, glass is used in the visible region, and the other three materials are used in the medium infrared region. 

When using the MIR technique for the analysis of aqueous solutions, germanium is taken as the prism material (angle of incidence (3) 19 to 20 infrared-transmitting between 3000 and 300 cm" ). It should be noted that the MIR technique yields good results only if used in clean, carefully controlled conditions. 

For many years, the prism formed the basis of most commercial monochromators. The spectral purity of the refracted radiation is determined by the dispersion characteristics of the prism material. The principle of operation is illustrated in Figure 3. 

Table 1. Penetration depth at a wavelength of 3.0 p,m and an angle of incidence of 45 for typical prism materials and for a sample with refractive index equal to 1.5...

Prism material Refractive index Angle of incidence (in degrees) Penetration depth (in p,m) at 3-0 = 3.0 p-m Penetration depth (in p,m) at A-o = 10.0... 

Many vibrational-rotational transitions of molecules such as H2O or CO2 fall within the range 3—10 xm, causing selective absorption of the transmitted radiation. Infrared spectrometers therefore have to be either evacuated or filled with dry nitrogen. Because dispersion and absorption are closely related, prism materials with low absorption losses also show low dispersion, resulting in a limited resolving power (see below). 

When passing through a prism, a light ray is refracted by an angle 0 that depends on the prism angle 6, the angle of incidence a, and the refractive index n of the prism material (Fig. 4.15). The minimum deviation 0 is obtained when the ray passes the prism parallel to the base g (symmetrical arrangement with a =a2 = a). In this case, one can derive [4.5]... 

According to (4.20a), the theoretical maximum resolving power depends solely on the base length g and on the spectral dispersion of the prism material. Because of the finite slit width b > the resolution reached in practice is somewhat lower. The corresponding resolving power can be derived from (4.11) to be at most... 

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