Absorption, of light

Light absorption promotes an electron from the ground state to an excited state. The presence of 18, tt electrons in the aromatic core of the porphyrin consequently generates a UV-visible spectrum that displays several absorption bands corresponding to rather energetically inexpensive n-n transitions. Our attention should focus mainly on two types of absorption bands, the Soret or B band, and 

The promotion of one electron to an excited state upon light absorption generates an electron-hole pair in the frontier orbital system of the porphyrinic chro-mophore. This electron-hole pair is an exciton, and its generation introduces a nonpermanent (transitory) dipole in the chromophore. The energy transfer process is strongly related to the interactions between excitons in multiple chromophore assemblies. 

The way in which light is ahsorhed was studied hy Johann Heinrich Lambert (1728-1777), who first expressed it in the differential form, 

The intensity of the incident light l0 is reduced to an intensity i on passing through the solution. 

The absorbance of the solution A depends on the concentration c, the extinction coefficient of the chromophore , and the path length of the cuvette d as given in Eqn. 7.5. 

in terms of the more commonly used decadic logarithms 

The absorbance A can thus be used to determine concentration, provided that the extinction coefficient is known. Consider, as an example, a solution of tryptophan that has an absorbance of 0.550 at 280nm in a 0.5 cm path length cuvette, and that the molar (decadic) extinction coefficient of tryptophan in water at this wavelength is 5600 M-1 cm-1. The concentration of tryptophan can be evaluated from the expression 

The absorbance of the aromatic amino acids can be used for rapid and reliable determinations of protein concentration, provided that its amino acid composition is known (see Sect. 5.1.2.3). Spectrophotometric determination of nucleic add concentrations requires information not only about base composition but also about conformation, i.e., whether the nucleic acid is single- or double-stranded (see-Sect. 5.2.1). 

FIGURE 11.1 Absorption Spectrum of [CulHjOlg]. Introduction to Ligand Fields, p. 221. Used by permission from Brian Figgis) 

It is not always possible to make a simple prediction of color directly from the absorption spectrum, in large part because many coordination compounds contain two or more absorption bands of different energies and intensities. The net color observed is the color predominating after the various absorptions are removed from white light. 

For reference, the approximate wavelengths and complementary colors to the principal colors of the visible spectrum are given in Table 11.1. 

By how many kilojoules per mole is the energy of 02 increased when it absorbs ultraviolet radiation with a wavelength of 147 nm How much is the eneigy of C02 increased when it absorbs infrared radiation with a wavenumber of 2 300 cm-1  

This is enough energy to break the 0=0 bond in oxygen. For C02, the energy increase is 

Transmittance, T, is defined as the fraction of the original light that passes through the 

Therefore, T has the range 0 to 1. The percent transmittance is simply lOOT and ranges between 0 and 100%. Absorbance is defined as 

When no light is absorbed, P = Pa and A = 0. If 90% of the light is absorbed, 10% is transmitted and P = fy 10. This ratio gives A = 1. If only 1% of the light is transmitted. A = 2. Absorbance is sometimes called optical density. 

In the first case, corpuscular description, the energy of the photons is 

the energy of electromagnetic waves is directly proportional to the reciprocal wavelength. In particular in vibrational spectroscopy, the reciprocal wavelength is used and denoted as wavenumber k  

Handbook of Spectroscopy, Volume 1. Edited by Gunter Gauglitz and Tuan Vo-Dinh 

Copyright 2003 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-29782-0 

In the second case, describing light as an electromagnetic wave, its propagation may be written as 

Transmittance lies in the range 0 to 1. If no light is absorbed by the sample, the transmittance is 1. If all light is absorbed, the transmittance is 0. Percent transmittance (1007) ranges from 0% to 100%. A transmittance of 30% means that 70% of the light does not pass through the sample. 

The most useful quantity for chemical analysis is absorbance. A, defined as 

The higher the absorbance, the less light is transmitted through a sample. 

Test Yourself What absorbance corresponds to 1 % transmittance To 50% transmittance (Answer 2.0, 0.30) 

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While a laser beam can be used for traditional absorption spectroscopy by measuring / and 7q, the strength of laser spectroscopy lies in more specialized experiments which often do not lend themselves to such measurements. Other techniques are connnonly used to detect the absorption of light from the laser beam. A coimnon one is to observe fluorescence excited by the laser. The total fluorescence produced is nonnally proportional to the amount of light absorbed. It can be used as a measurement of concentration to detect species present in extremely small amounts. Or a measurement of the fluorescence intensity as the laser frequency is scaimed can give an absorption spectrum. This may allow much higher resolution than is easily obtained with a traditional absorption spectrometer. In other experiments the fluorescence may be dispersed and its spectrum detennined with a traditional spectrometer. In suitable cases this could be the emission from a single electronic-vibrational-rotational level of a molecule and the experimenter can study how the spectrum varies with level. 

A technique is any chemical or physical principle that can be used to study an analyte. Many techniques have been used to determine lead levels. For example, in graphite furnace atomic absorption spectroscopy lead is atomized, and the ability of the free atoms to absorb light is measured thus, both a chemical principle (atomization) and a physical principle (absorption of light) are used in this technique. Chapters 8-13 of this text cover techniques commonly used to analyze samples. 

Photovoltaic Devices. For many inorganic semiconductors, absorption of light can be used to create free electrons and holes. In an organic semiconducting soHd, however, absorption of a photon leads to the formation of a bound electron—hole pair. Separation of this pair in an electric field can... 

Because of the narrow line width, absorption of laser energy can excite one specific state in an atom or molecule. The laser is tuned so that its wavelength matches an absorption corresponding to the desired state, which may be an electronic state or vibrational state. Absorption of laser energy can lead to excitation of specified states much more effectively than absorption of light from conventional light sources. 

Concurrent with requirements for low levels of mercurials in discharge water is the problem of their deterrnination. The older methods of wet chemistry are inadequate, and total rehance is placed on instmmental methods. The most popular is atomic absorption spectrophotometry, which rehes on the absorption of light by mercury vapor (4). Solutions of mercury compounds not stabilized with an excess of acid tend to hydrolyze to form yeUow-to-orange basic hydrates. These frequendy absorb onto the walls of containers and may interfere with analytical results when low levels (ppm) of mercury are determined. 

A number of electronic and photochemical processes occur following band gap excitation of a semiconductor. Figure 5 illustrates a sequence of photochemical and photophysical events and the possible redox reactions which might occur at the surface of the SC particle in contact with a solution. Absorption of light energy greater than or equal to the band gap of the semiconductor results in a shift of electrons from the valence band (VB) to... 

Dyes, on the other hand, ate colored substances which ate soluble or go into solution during the appHcation process and impart color by selective absorption of light. In contrast to dyes, whose coloristic properties ate almost exclusively defined by their chemical stmcture, the properties of pigments also depend on the physical characteristics of its particles. 

Cartiers can also be generated in a semiconductor by the absorption of light or injected into the semiconductor from ap—n or Schottky junction. In either case, as soon as the source is removed the density of those excess carriers begins to decrease exponentially with time. The time it takes for the density to be reduced to 1/ of the original value is defined as the carrier lifetime, T. For siUcon, T is typically in the microsecond range. 

The molecular extinction coefficients (at various wavelengths) of the four main components of the irradiation are shown in Table 5. The absorption of light above 300 nm is favored by tachysterol. A yield of 83% of the previtamin at 95% conversion of 7-dehydrocholesterol can be obtained by irradiation first at 254 nm, followed by reirradiation at 350 nm with a yttrium aluminum garnet (YAG) laser to convert tachysterol to previtamin D. A similar approach with laser irradiation at 248 nm (KrF) and 337 nm (N2) has also been described (76). 

Photochromism Based on Triplet Formation. Upon absorption of light, many polycycHc aromatic hydrocarbons and their heterocycHc analogues undergo transitions to their triplet state which has an absorption spectmm different from that of the ground state (24). In rigid glasses and some plastics, the triplet state, which may absorb in the visible, has a lifetime of up to 20 seconds. 

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