Dispersing elements

Light sources can either be broadband, such as a Globar, a Nemst glower, an incandescent wire or mercury arc lamp or they can be tunable, such as a laser or optical parametric oscillator (OPO). In the fomier case, a monocln-omator is needed to achieve spectral resolution. In the case of a tunable light source, the spectral resolution is detemiined by the linewidth of the source itself In either case, the spectral coverage of the light source imposes limits on the vibrational frequencies that can be measured. Of course, limitations on the dispersing element and detector also affect the overall spectral response of the spectrometer. 

When dispersing elements are used, the resolution of the speetrometer is detennined by the entranee slit widtir, the exit slit width, the foeal length and the dispersing element itself Resolving power is defined as... 

Even while Raman spectrometers today incorporate modem teclmology, the fiindamental components remain unchanged. Connnercially, one still has an excitation source, sample illuminating optics, a scattered light collection system, a dispersive element and a detechon system. Each is now briefly discussed. 

The two essential elements of an electron spectrometer are the electrodes that accelerate electrons and focus them into a beam and the dispersive elements that sort electrons according to their energies. These serve the fimctions of lenses and prisms in an optical spectrometer. The same parameters are used to describe these elements in an electron spectrometer as in an optical spectrometer the teclmology is referred to as electron optics. 

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Although prisms, as dispersing elements, have been largely superseded by diffraction gratings and interferometers they still have uses in spectroscopy and they also illustrate some important general points regarding dispersion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

Table 3.1 summarizes the details of typical sources, absorption cells, dispersing elements and detectors used in different regions of the electromagnetic spectrum. 

Region Source Absorption cell window Dispersing element Detector... 

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. 

An alternative type of spectrometer is the energy dispersive spectrometer which dispenses with a crystal dispersion element. Instead, a type of detector is used which receives the undispersed X-ray fluorescence and outputs a series of pulses of different voltages that correspond to the different wavelengths (energies) that it has received. These energies are then separated with a multichannel analyser. 

Microscopes are also classified by the type of information they present size, shape, transparency, crystallinity, color, anisotropy, refractive indices and dispersion, elemental analyses, and duorescence, as well as infrared, visible, or ultraviolet absorption frequencies, etc. One or more of these microscopes are used in every area of the physical sciences, ie, biology, chemistry, and physics, and also in their subsciences, mineralogy, histology, cytology, pathology, metallography, etc. 

A dispersive element for spectral analysis of PL. This may be as simple as a filter, but it is usually a scanning grating monochromator. For excitation spectroscopy or in the presence of much scattered light, a double or triple monochromator (as used in Raman scattering) may be required. 

Beyond the complexities of the dispersive element, the equipment requirements of infrared instrumentation are quite simple. The optical path is normally under a purge of dry nitrogen at atmospheric pressure thus, no complicated vacuum pumps, chambers, or seals are needed. The infrared light source can be cooled by water. No high-voltage connections are required. A variety of detectors are avail-... 

Abstract The basic principles of astronomical spectroscopy are introduced and the main types of dispersing element surveyed. The principles behind two modem spectroscopic techniques, multiple object and integral held spectroscopy, are also discussed. 

Finally, we need to consider the use of prisms as dispersing elements. Although capable only of low dispersion, they have the advantage that, because they do not rely on interference effects, there are no multiple orders to overlap... 

---