Grating, diffraction dispersion

The first requirement is a source of infrared radiation that emits all frequencies of the spectral range being studied. This polychromatic beam is analyzed by a monochromator, formerly a system of prisms, today diffraction gratings. The movement of the monochromator causes the spectrum from the source to scan across an exit slit onto the detector. This kind of spectrometer in which the range of wavelengths is swept as a function of time and monochromator movement is called the dispersive type. 

Figure B2.1.1 Femtosecond light source based on an amplified titanium-sapphire laser and an optical parametric amplifier. Symbols used P, Brewster dispersing prism X, titanium-sapphire crystal OC, output coupler B, acousto-optic pulse selector (Bragg cell) FR, Faraday rotator and polarizer assembly DG, diffraction grating BBO, p-barium borate nonlinear crystal.

Typical grating monochromator with inset showing the dispersion of the radiation by the diffraction grating. 

The dispersing element to be described in Section 3.3 splits up the radiation into its component wavelengths and is likely to be a prism, diffraction grating or interferometer, but microwave and millimetre wave spectroscopy do not require such an element. 

Although prisms, as dispersing elements, have been largely superseded by diffraction gratings and interferometers they still have uses in spectroscopy and they also illustrate some important general points regarding dispersion and resolution. 

As in all Fourier transform methods in spectroscopy, the FTIR spectrometer benefits greatly from the multiplex, or Fellgett, advantage of detecting a broad band of radiation (a wide wavenumber range) all the time. By comparison, a spectrometer that disperses the radiation with a prism or diffraction grating detects, at any instant, only that narrow band of radiation that the orientation of the prism or grating allows to fall on the detector, as in the type of infrared spectrometer described in Section 3.6. 

The dispersing elemenf is usually a diffraction grating or an inferferomefer wifh a beamsplitter made from silicon-coafed or germanium-coafed quartz or calcium fluoride. 

Dispersing elements may be either prisms (glass for the visible, quartz for the nearultraviolet) or, more often, diffraction gratings for which a Czemy-Tumer mounting, shown in Figure 3.17, may be used. 

The dispersing element is a diffraction grating preferably used under conditions of grazing incidence (6 in Equation 3.9 about 89°) to improve the reflectance. The grating may also be concave to avoid the use of a focusing mirror. 

Figure 8.28 shows how the X-rays fall on the solid or liquid sample which then emits X-ray fluorescence in the region 0.2-20 A. The fluorescence is dispersed by a flat crystal, often of lithium fluoride, which acts as a diffraction grating (rather like the quartz crystal in the X-ray monochromator in Figure 8.3). The fluorescence may be detected by a scintillation counter, a semiconductor detector or a gas flow proportional detector in which the X-rays ionize a gas such as argon and the resulting ions are counted.

Monochromators for dispersing X-radiation utilize single crystals which behave like a diffraction grating. The spacing of the crystal lattice determines the angles at which radiation is reflected and generally two or more different crystals are required to cover the X-ray region of the spectrum. 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

The outline of the construction of a typical plasma emission spectrometer is to be seen in Figure 8.10. The example shown has an inductively coupled plasma, excitation source, but the outline would be similar were a dc source to be fitted. Different combinations of prisms and diffraction gratings may be used in the dispersion of the emitted radiation, and in the presentation of the analytical signal. Instruments are computerized in operation and make use of automatic sample handling. Sophisticated data handling packages are employed routinely to deal with interferences, and to provide for clarity in data output. 

Figure 2.2 Schematic drawing of an optical emission spectrograph. Light from the sample is focused onto the input slit of the spectrograph and is then dispersed via a prism (or diffraction grating) and recorded on a photographic plate. (Adapted from Britton and Richards, 1969 Fig. 108, by permission of Thames and Hudson Ltd.)...

The ability of a diffraction grating to separate different adjacent wavelengths is known as its dispersion. The angular dispersion of a grating (dr/dA.) is given by differentiation of the above equation at constant / and inversion ... 

Thus, dispersion of the grating increases as d decreases (i.e., as the grating contains more lines per cm). Also, dispersion is not a function of k, and the linear dispersion is therefore a constant, unlike in the case of a prism. The resolving power of a diffraction grating is proportional to the size of the grating and the order of the diffraction used. 

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