Visible region of spectrum

Chance B 1951 Rapid and sensitive spectrophotometry. I. The accelerated and stopped-flow methods for the measurement of the reaction kinetics and spectra of unstable compounds in the visible region of the spectrum Rev. Sci. Instrum 22 619-27... 

A dye molecule has one or more absorption bands in the visible region of the electromagnetic spectrum (approximately 350-700 nm). After absorbing photons, the electronically excited molecules transfer to a more stable (triplet) state, which eventually emits photons (fluoresces) at a longer wavelength (composing three-level system.) The delay allows an inverted population to build up. Sometimes there are more than three levels. For example, the europium complex (Figure 18.15) has a four-level system. 

In Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), a gaseous, solid (as fine particles), or liquid (as an aerosol) sample is directed into the center of a gaseous plasma. The sample is vaporized, atomized, and partially ionized in the plasma. Atoms and ions are excited and emit light at characteristic wavelengths in the ultraviolet or visible region of the spectrum. The emission line intensities are proportional to the concentration of each element in the sample. A grating spectrometer is used for either simultaneous or sequential multielement analysis. The concentration of each element is determined from measured intensities via calibration with standards. 

Usually, the ultraviolet and visible regions of the spectrum are recorded. Many of the most intense emission lines lie between 200 nm and 400 nm. Some elements (the halogens, B, C, P, S, Se, As, Sn, N, and O) emit strong lines in the vacuum ultraviolet region (170-200 nm), requiring vacuum or purged spectrometers for optimum detection. 

The visible region of the spectrum lies between the wavelengths of 0.4 and 0.7 pm. When the temperature of a body is increased, its color changes to-w ard smaller wavelengths—in other words, from the red region of the spectrum to the blue region. 

In contrast to the low-pressure lamps (1—130 Pa) which primarily emit at the resonance line at A = 254nm, high-pressure lamps (lO —10 Pa) also produce numerous bands in the UV and VIS regions (Fig. 16). Table 3 lists the emission lines and the relative spectral energies of the most important mercury lamps (see also [44]). The addition of cadmium to a mercury vapor lamp increases the numbei of emission lines particularly in the visible region of the spectrum [45] so that it i. also possible to work at A = 326, 468, 480, 509 and 644 nm [46]. 

Substances that are intrinsically fluorescent can often be exeited with long-wavelength UV light. They absorb the radiation and then emit, usually in the visible region of the spectrum, so that they appear as bright luminous zones, whieh can frequently be differentiated by color. They, thus, set themselves apart from the multitude of substances that only exhibit absorption. This detection possibility is characterized by high specificity (Sec. 2.3). 

Azo-coupled products are widely used as dyes for textiles because their extended conjugated tt electron system causes them to absorb in the visible region of the electromagnetic spectrum (Section 14.9). / -(Dimethylamino)azobenzene, for instance, is a bright yellow compound that was at one time used as a coloring agent in margarine. 

Spectrophotometry proper is mainly concerned with the following regions of the spectrum ultraviolet, 185-400 nm visible 400-760 nm and infrared, 0.76-15 /tm. Colorimetry is concerned with the visible region of the spectrum. In this chapter attention will be confined largely to the visible and near ultraviolet region of the spectrum. 

The first person to identify a pattern in the lines of the visible region of the spectrum was Joseph Balmer, a Swiss schoolteacher. In 1885, he noticed that the frequencies of all the lines then known could be generated by the expression... 

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