Electromagnet spectrophotometer

Chapter 4, presents details of the absorption and reflectivity spectra of pure crystals. The first part of this chapter coimects the optical magnimdes that can be measured by spectrophotometers with the dielectric constant. We then consider how the valence electrons of the solid units (atoms or ions) respond to the electromagnetic field of the optical radiation. This establishes a frequency dependence of the dielectric constant, so that the absorption and reflectivity spectrum (the transparency) of a solid can be predicted. The last part of this chapter focuses on the main features of the spectra associated with metals, insulators, and semiconductors. The absorption edge and excitonic structure of band gap (semiconductors or insulator) materials are also treated. 

In a typical spectrophotometer, a dissolved compound is exposed to electromagnetic radiation with a continuous spread in wavelength. The radiation passing through or absorbed is recorded on a chart against the wavelength or wave number. Absorption peaks are plotted as minima in infrared, and usually as maxima in ultraviolet sp>ectroscopy. 

When a photon passes close to a molecule, there is an interaction between the electromagnetic field associated with the molecule and that associated with the radiation. If, and only if, the radiation is absorbed by the molecule as a result of this interaction, can the radiation be effective in producing photochemical changes (Grotthus-Draper law, see, e.g., Finlay son-Pitts and Pitts, 1986). Therefore, the first thing we need to be concerned about is the probability with which a given compound absorbs uv and visible light. This information is contained in the compounds uv/vis absorption spectrum, which is often readily available or can be easily measured with a spectrophotometer. 

Despite seven decades of technical and scientific progress, the original Hammett method has not become obsolete. The colorimeter has been replaced by modern spectrophotometers that can be operated at selected wavelengths extending the spectra beyond visible into the ultraviolet region of the electromagnetic spectrum. The experimental variable, which is wavelength-dependent, is the optical density D. D is related to the concentration by the Beer-Lambert law [Eq. (1.23)]. 

Spectroscopy is another method used to detect art forgeries. In spectroscopy, electromagnetic radiation is applied to the substance or substances under consideration. The resulting spectra are recorded to identify the chemical composition of the substance or substances. When a spectrophotometer is used, the radiation, usually visible and invisible ultraviolet waves, can be applied to a solution in a tube, and the absorption of radiation is noted. For this type of analysis, a spectrophotometer must be available. 

Color measurements use the part of the electromagnetic spectrum that is sensed by the human eye and brain. This region is approximately 400-700 nm. The colors of the rainbow are associated with specific wavelengths of visible light, as listed in Table 3.14. If colored filters such as RGB (red, green, and blue) are used, the device is called a colorimeter. If a grating or prism is used, the device is called a spectrophotometer. 

In an absorbing medium, /cabs / 0, this wave will be attenuated as e" abs rrf. The imaginary part k of the complex index of refraction, nKf + Kabs, is a measure of the attenuation of an electromagnetic wave as seen, for example, in a spectrophotometer. We speak of k k(o>) as the absorption spectrum of light. In the limit of high absorption (k3bS... 

The spectrometer to be used need cover only the visible portion of the electromagnetic spectrum. Preferably it should be a double-beam instrument, to allow for compensation for absorption by the cell windows. However, a single-beam instrument may be used if the absorption spectrum of an equivalent set of cell windows is obtained separately, so that the absorbances of the windows can be subtracted from those of the iodine-containing cell. The spectrophotometer must have a cell compartment large enough to contain the... 

Spectroscopy is the measurement of electromagnetic radiation absorbed, scattered, or emitted by chemical species. Because different chemical species and electromagnetic radiation interact in characteristic ways, it is possible to tailor instrumentation to detect these interactions specifically and quantitatively. A simple absorption spectrophotometer, depicted schematically in Figure 12.2, contains components that are common to many spectroscopic devices and are representative of many of the basic principles of instrumentation found in analytical biochemistry. 

In forward optic spectrophotometers, electromagnetic radiation of a specified wavelength is passed through sample solutions. Because radiation from source is polychromatic (consists of light of all wavelengths), the various wavelengths of radiation must be separated... 

Spectrometric Analysis. Spectroscopy has been extensively used for polymer and copolymer analysis. (59-69). The kind of information available from different spectroscopic techniques as well as the instrumentation required depends on the region of the electromagnetic spectrum in which absorption is taking place. Recent investigations (63) on the use of spectrophotometers for copolymer analysis have shown that the response from spectrophotometers is sometimes sensitive to the microstructure of the polymer molecules and that calibration of spectrophotometers with absolute measurements on the microstructure (i.e. NMR) may be necessary in order to obtain reliable quantitative information on concentration and copolymer composition determinations. 

Spectrometric methods Methods based on the absorption, emission, or fluorescence of electromagnetic radiation that is related to the amount of analyte in the sample. Spectrophotometer A spectrometer designed for the measurement of the absorption of ultraviolet, visible, or infrared radiation. The instrument includes a source of radiation, a monochromator, and an electrical means of measuring the ratio of the intensities of the sample and reference beams. 

In the spectrophotometric method, color is assessed by means of a standard absorption spectrophotometer. In this case, a description of the hue, intensity and brightness of a non-turbid colored solution is possible in the visible region (400-700 mn) of the electromagnetic spectrum. Since pH will have a significant impact on the color of certain dyes, the pH of the dye solution employed must be measured and reported. Details regarding the method are provided in section 2120 C of the Standard Methods book, and can be summarized as follows ... 

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