Spectroscopic components radiation sources

New metliods appear regularly. The principal challenges to the ingenuity of the spectroscopist are availability of appropriate radiation sources, absorption or distortion of the radiation by the windows and other components of the high-pressure cells, and small samples. Lasers and synchrotron radiation sources are especially valuable, and use of beryllium gaskets for diamond-anvil cells will open new applications. Impulse-stimulated Brillouin [75], coherent anti-Stokes Raman [76, 77], picosecond kinetics of shocked materials [78], visible circular and x-ray magnetic circular dicliroism [79, 80] and x-ray emission [72] are but a few recent spectroscopic developments in static and dynamic high-pressure research. 

In principle, a set-up for spectral studies consists of three components a radiation source, an analyser and a detection system. In many modern techniques the system under investigation is subjected to different types of static or oscillatory fields and the influence of these fields on the system is studied in order to obtain a more complete picture of the system. Resonance methods are of special importance since they provide high accuracy hi the determination of small energy spHttings. The basic arrangement of a spectroscopic set-up is shown in Fig. 1.2. 

The basic elements of any emission spectroscope are (1) a source that contains the substance to be studied and is capable of energizing that substance so that it can emit its characteristic radiation, (2) a dispersing device to resolve the emitted radiation into its component frequencies, and (3) a detector that can measure the intensity of the radiation at the various frequencies. The choice of devices for each of these elements depends on the region of the spectrum under investigation. 

The double-beam system is used extensively for spectroscopic absorption studies. The individual components of the system have the same function as in the single-beam system, with one very important difference. The radiation from the source is split into two beams of approximately equal intensity using a beam splitter, shown in Fig. 2.28. One beam is termed the reference beam, the second beam, which passes through the sample, is called the sample beam. The two beams are then recombined and pass through the monochromator and slit systems to the detector. This is illustrated schematically in Fig. 2.28. In this schematic, there is a cell in the reference beam that would be identical to the cell used to hold the sample. The reference cell may be empty or it may contain the solvent used to dilute the sample, for example. This particular arrangement showing the monochromator after the sample is typical of a dispersive IR double-beam spectrophotometer. There are many commercial variations in the optical layout of double-beam systems. 

A schematic block diagram of the instrumentation used for AAS is shown in Figure 6.3. The components are similar to those used in other spectroscopic absorption methods as discussed in Chapters 2 and 5. Light from a snitable source is directed through the atomizer, which serves as the sample cell, into a wavelength selector and then to a detector. The detector measnres how much light is absorbed by the sample. The sample, usually in solution form, is introdnced into the atomizer by some type of introduction device. The atomizer converts the sample to gas-phase ground-state atoms that can absorb the incident radiation. 

Absorption, emission, fluorescence, and diffraction of X-rays are all applied in analytical chemistry. Instruments for these applications contain components that are analogous in function to the five components of instruments for optical spectroscopic measurement these components include a source, a device for restricting the wavelength range of incident radiation, a sample holder, a radiation detector or transducer, and a signal processor and readout. These components differ considerably in detail from the corresponding optical components. Their functions, however, are the same, and the ways they combine to form instruments are often similar to those shown in Figure 7-1. 

---