Ultraviolet and visible absorption spectroscopy

neutral molecules, and ions (simple and assoeiated) ean exist in several possible electronic states this is the basis of visible and UV absorption spectroscopy. Transitions between these energy states occur by the absorption of discrete energy quanta A which are related to the frequency v of the light absorbed by the well-known relation 

A historic use of the Beer-Lambert law was by Bjerrum, who studied the absorption spectra of dilute copper sulfate solutions and found that the molar absorptivity was independent of the concentration. Bjerrum concluded that the only species present in dilute copper sulfate solutions are free, unassociated copper and sulfate ions and not, as was thought at the time, undissociated copper sulfate molecules that dissociate into ions to an extent that depends on the concentration. For if any undissociated molecules were present, then the molar absorptivity of the copper sulfate solution would have been dependent on the concentration. 

In recent years, it has been found that the molar absorptivity of concentrated copper sulfate solutions does show a shght concentration dependence. This concentration dependence has been attributed to ion-pair formation occurring through the operation of Coulombic forces between the copper and sulfate ions. This is perhaps ironic because Bjerrum s concept of ion pairs is being used to contradict his conclusion that there are only free ions in copper sulfate solutions. Nevertheless, there is a fundamental difference between the erroneous idea that a copper sulfate crystal dissolves to give copper sulfate molecules, which then dissociate into free ions, and the modem point of view that the ions of an ionic crystal pass into solution as free solvated ions which, under certain conditions, associate into ion pairs. 

The method of visible and UV absorption spectroscopy is at its best when the absorption spectra of the free ions and the associated ions are quite different and known. When the associated ions cannot be chemically isolated and their spectra studied, the type of absorption by the associated ions has to be attributed to electronic transitions known from other well-studied systems. For example, there can be an electron transfer to the ion from its immediate environment (charge-transfer spectra), i.e., from the entities associated with the ion or transitions between new electronic levels produced in the ion under the influence of the electrostatic field of the species associated with the ion (crystal-field splitting). Thus, there is an influence of the environment on the absorption characteristics of a species, and this influence reduces the clarity with which spectra are characteristic of species rather than of their environment. Herein lies what may be considered a disadvantage of visible and UV absorption spectroscopy. 

The development of photodetectors enabled the human eye to be replaced by a much more sensitive detector of light intensity. The evolution of modem colorimeters and of spectrophotometers capable of operation in both the ultraviolet and visible regions of the spectrum has been discussed.217,218 The phenomenon of fluorescence was first employed for quantitative analysis in the 1930s, when the first filter fluorimeters were constructed. An article has outlined the development of fluorescence analysis up to 1980.219 Lasers have now been employed long enough in analytical chemistry for a historical account to be given.220 

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The spectroscopy experiments are further subdivided into atomic spectroscopy found in Table XII, infrared and Raman spectra found in table XIII, visible and ultraviolet absorption spectroscopy found in table XIV, and luminescence spectroscopies found in table XV. 

Remote visible and ultraviolet absorption and emission spectroscopy... 

J. L. West and R. Ondris-Crawford, Characterization of polymer dispersed liquid crystal shutters by ultraviolet/visible and infrared absorption spectroscopy, J. Appl Phys., 70, 3785 (1991). 

Figure 6-1. The following electromagnetic spectrum indicates the regions commonly used for ultraviolet/visible and infrared absorption spectroscopy.

The section on Spectroscopy has been expanded to include ultraviolet-visible spectroscopy, fluorescence, Raman spectroscopy, and mass spectroscopy. Retained sections have been thoroughly revised in particular, the tables on electronic emission and atomic absorption spectroscopy, nuclear magnetic resonance, and infrared spectroscopy. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon ICP, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-29, and phosphorus-31. 

Emission spectroscopy is confined largely to the visible and ultraviolet regions, where spectra may be produced in an arc or discharge or by laser excitation. Absorption spectroscopy is, generally speaking, a more frequently used technique in all regions of the spectrum and it is for this reason that we shall concentrate rather more on absorption. 

The symmetry of the LB films was determined by polarized ultraviolet-visible (UV-Vis) absorption spectroscopy, optical rotation, and second-harmonic generation. All studies showed that the constructed LB films are anisotropic in the plane of the film and that the symmetry of the film is C2 with the twofold rotation axis perpendicular to the film plane. For example, when the SH intensity is plotted as a function of the azimuthal rotation angle (rotation around an axis perpendicular to the plane of the film), the twofold symmetry becomes evident (Figure 9.23). Isotropic films generate an SH signal independent of the azimuthal rotation angle. On the other hand, the LB... 

ELEMENTAL ANALYSIS BY ABSORPTION AND EMISSION SPECTROSCOPIES IN THE VISIBLE AND ULTRAVIOLET... 

Fergusson et al. were the first to report the existence of binary compounds with a general formula Se Sg in these melts. They carried out an extensive investigation by X-ray powder diffraction and by absorption spectroscopy in the infrared, visible, and ultraviolet regions over the whole composition range of molten mixtures of sulfur and selenium cooled down to 20 °C. They also examined phases obtained by recrystallization of the cooled melts from carbon disulfide. All phases were isomorphic with one of the allotropes of Sg and SCg indicating that the structures also consist of cyclic eight-membered molecules ... 

The photochemical and thermal stabilities of Ru complexes have been investigated in detail [8,153-156]. For example, it has been reported that the NCS ligand of the N3 dye, cri-Ru(II)(dcbpy)2(NCS)2 (dcbpy = 2,2 -bipyridyl-4,4 -dicarboxylic acid), is oxidized to produce a cyano group (—CN) under irradiation in methanol solution. It was measured by both ultraviolet-visible (UV-vis) absorption spectroscopy and nuclear magnetic resonance (NMR) [8,153]. In addition, the intensity of the infrared (IR) absorption peak attributed to the NCS ligand starts to decrease at 135°C, and decarboxylation of N3 dyes occurs at temperatures above 180°C [155]. Desorption of the dye from the 2 surface has been observed at temperatures above 200°C. 

Various kinds of visible and ultraviolet spectroscopy—absorption, excitation, emission—are well suited for monitoring certain reaction intermediates. These techniques are complementary to X-ray in that they provide excellent sensitivity and time resolution but low structural precision. 

Commercial cylindrical quartz cells can be adapted for gas-phase work as illustrated in Fig. 9.18. Such a cell finds use in the near infrared for the determination of overtone vibrational frequencies, and also in visible and ultraviolet spectroscopy. A much less expensive cell which is adequate for most gases may be constructed from Pyrex along the lines of the cell shown in Fig. 9.18. Quartz windows may then be attached by epoxy resin. A cell which is filled from a conventional vacuum line will generally contain mercury vapor which absorbs at 2537 A. Once the origin of this absorption is recognized, it causes little difficulty because of its narrow bandwidth. 

The spectra (absorption or emission) of atoms are much sharper than those of molecules, because every electronic energy level in a molecule has a rich complement of vibronic levels and rotational sublevels (Fig. 3.15). In the late nineteenth century these smaller features could not be resolved in visible-ultraviolet spectroscopy, so, in ignorance of all the quantum effects explained decades later, the sharper spectra of atoms were called "line spectra," while the broadened spectra of molecules were called "band spectra." Cooling the molecules to 77 K or 4.2 K does resolve some of the vibronic substructure, even in visible-ultraviolet absorption spectroscopy. 

In 1964, the spin echo experiment was extended to the optical regime by the development of the photon echo experiment (3,4). The photon echo began the application of coherent pulse techniques in the visible and ultraviolet portions of the electromagnetic spectrum. Since its development, the photon echo and related pulse sequences have been applied to a wide variety of problems including dynamics and intermolecular interactions in crystals, glasses, proteins, and liquids (5-8). Like the spin echo, the photon echo and other optical coherent pulse sequences provide information that is not available from absorption or fluorescence spectroscopies. 

The FT direct absorption spectra [28] of OCIO provide an example of the capabilities of FT spectroscopy in the visible and ultraviolet regions for the study of short-lived species. In Figure 14, part of the near-UV absorption spectrum is... 

There are various kinds of spectroscopy visible and ultraviolet (UV) absorption spectroscopy, Raman and infrared spectroscopy, nuclear magnetic resonance spectroscopy, and electron-spin resonance (ESR) spectroscopy. A brief description of the principles of these techniques and their application to the study of ions in solution follows (see also Section 2.11). 

Spectroscopic Techniques. In absorption spectroscopy, Vanadium(V), a d° system, shows no transitions in the visible region. The yellow color of some V complexes can be ascribed to the tail of an intense absorption in the ultraviolet. Absorption spectroscopy is more useful for systems (3d ), where electronic energies of transitions generally correlate well with ligand type and with electron spin resonance (ESR) parameters. 

Using ultraviolet/visible (UV/Vis) absorption spectroscopy, it is possible to measure the protein concentration using Beer s Law A = e c, where A is the measured absorbance of a solution, e is the absorptivity of the protein, is the pathlength of the cell used to determine the absorbance, and c is the protein concentration. Proteins typically exhibit two strong, broad absorption bands in the UV/Vis part of the spectrum. The first and most intense band is centered at 214 nm and arises from absorption of light by the peptide backbone. The second absorption band is typically found at 280nm. This band arises from absorbance from the aromatic side chains of Trp, Tyr, and Phe. Disulfide bonds may exhibit weak absorption in this range as well. 

Historically important in the development of modern atomic theory was the recognition that although polyatomic molecules show more or less broad bands of absorption and emission in the visible and ultraviolet regions of the spectrum, the characteristic light absorption or emission by individual atoms occurs at fairly narrow lines of the spectrum, which correspond to sharply defined wavelengths. The line spectrum of each element is so uniquely characteristic of that element that atomic spectroscopy can be used for precise elementary analysis of many types of chemically complex materials. 

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