Absorption of/? radiation

The visible spectrum extends approximately from 4,000 to 8,000 A. The human eye is not sensitive to light of wave lengths much shorter or longer than these. The near infrared region extends from 8,000 up to perhaps 200,000 A, and the far infared region extends still further toward the long waves from meters to thousands of meters, which are familiar to anyone who turns a radio dial. On the shorter side of the visible spectrum, we have ultraviolet light 

It will be remembered that most chemical reactions which proceed with measurable velocities require from approximately 20,000 to 60,000 calories of energy. It is clear that visible radiation can supply this energy and that ultraviolet light has sufficient energy to produce almost any chemical reaction. 

The different types of absorption spectra can be combined together as is illustrated in Fig. 24, for the different kinds of absorption can operate simultaneously. There are only single lines 

A more complete tabulation of the wave lengths and energies in different parts of the spectrum is given in Table I. 

Description Wave length A Wave number cm-1 Fre- quency sec-1 Ergs per quantum Cals. per einstein Elec- tron volts 

The properties of electromagnetic radiation are discussed elsewhere [3-5]. Absorption of such radiation is essential to start the photoreaction. Ac- 

V = 1/A is called the wavenumber which is given in cm (old textbooks use Kaiser), v and v are directly proportional to the energy of the photon. Taking a mole of photons (sometimes photochemists call this an Einstein) the energy of the photon is given in kJ/mol as 

A/l is Avogadro s or the Loschmidt number. A comparison between the energies in different units is given in Table 1.1. 

The radiation falling on a body can be partially reflected at its surface, whilst the portion that is not reflected penetrates the body. Here the radiative energy is absorbed and then converted into internal energy or part of it may be allowed to pass through the body. The absorbed portion is very important in terms of heat transfer. It is covered by the four absorptivities described in the following. These four belong to the four groups of physical radiation quantities introduced in 5.1.1. 

Just as in 5.1.3 we will consider a radiation flow d3 lin according to (5.24), coming from a solid angle element dec, that hits a surface element d/1 and only 

Using a x allows the absorbed portions of the integrated radiation flows introduced in 5.1.3 to be calculated. By integrating (5.33) over all solid angles in the hemisphere, the absorbed part of the hemispherical irradiation of the surface element d 4 in the wavelength interval dA is obtained 

however, (5.33) is integrated over all wavelengths then the absorbed portion of the total radiative power from a solid angle element dw is obtained. This gives 

Several approaches with different degrees of complexity are reported in the technical literature to estimate the radiation distribution and consequently the light absorption in sluiTy reactors. 

In summary, the current methods for the measurement of radiation absorption lead to either simple approaches that tend to overestimate the radiation absorption based on broad assumptions or more complex approaches, that tend to underestimate the absorption of radiation while giving local infonnation about the energy absorbed. 

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Chromophore (Section 13 21) The structural unit of a mole cule principally responsible for absorption of radiation of a particular frequency a term usually applied to ultraviolet visible spectroscopy... 

An important question to consider when using a flame as an atomization source, is how to correct for the absorption of radiation by the flame. The products of combustion consist of molecular species that may exhibit broad-band absorption, as well as particulate material that may scatter radiation from the source. If this spectral interference is not corrected, then the intensity of the transmitted radiation decreases. The result is an apparent increase in the sam-... 

Some of the target molecules gain so much excess internal energy in a short space of time that they lose an electron and become ions. These are the molecular cation-radicals found in mass spectrometry by the direct absorption of radiation. However, these initial ions may react with accompanying neutral molecules, as in chemical ionization, to produce protonated molecules. 

The electromagnetic spectrum measures the absorption of radiation energy as a function of the frequency of the radiation. The loss spectrum measures the absorption of mechanical energy as a function of the frequency of the stress-strain oscillation. 

Chemistry students are familiar with spectrophotometry, the qualitative and quantitative uses of which are widespread in contemporary chemistry. The various features of absorption spectra are due to the absorption of radiation to promote a particle from one quantized energy state to another. The scattering phenomena we discuss in this chapter are of totally different origin classical not quantum physics. However, because of the relatively greater familiarity of absorption spectra, a comparison between absorption and scattering is an appropriate place to begin our discussion. 

The next step is to consider tire cross-sections of the absorption of radiation by the diatomic halogen molecules in order to decide if the relative effects result from the efficiency of the radiation photon-molecule interactions. These are reflected in the dissociation cross-sections of tlrese interactions. 

The effective absorption of radiation by a gas can be increased by the addition of a sensitizer. For example, when mercuty vapour is added to silane gas, the mercury vapour absorbs radiation to form an excited atom Hg which collides with a silane molecule to lead to partial dissociation... 

Figure 14-9 also shows a flowchart for analysis of wet and dry precipitation. The process involves weight determinations, followed by pH and conductivity measurements, and finally chemical analysis for anions and cations. The pH measurements are made with a well-calibrated pH meter, with extreme care taken to avoid contaminating the sample. The metal ions Ca, Mg, Na, and are determined by flame photometry, which involves absorption of radiation by metal ions in a hot flame. Ammorda and the anions Cl, S04 , NO3 , and P04 are measured by automated colorimetric techniques. 

Atmospheric attenuation is the consequence of absorption of radiation by the medium present between emitter and receiver. For thermal radiation, atmospheric absorption is primarily due to water vapor and, to a lesser extent, to carbon dioxide. Absorption also depends on radiation wavelength, and consequently, on hie temperature. Duiser approximates transmissivity as... 

The emissive power of a fireball, however, will depend on the actual distribution of flame temperatures, partial pressure of combustion products, geometry of the combustion zone, and absorption of radiation in the fireball itself. The emissive power ( ) is therefore lower than the maximum emissive power (E ) of the black body radiation ... 

This is a steady-state competitive method, applicable when a solute is capable of fluorescing. We consider the simplest case. The solute A undergoes excitation to the excited singlet state A upon absorption of radiation of frequency... 

Atomic absorption spectroscopy involves atomising the specimen, often by spraying a solution of the sample into a flame, and then studying the absorption of radiation from an electric lamp producing the spectrum of the element to be determined. 

To investigate the absorption of radiation by a given solution, the solution must be placed in a suitable container called a cell (or cuvette) which can be accurately located in the beam of radiation. The instrument is provided with a cell-carrier which serves to site the cells correctly. Standard cells are of rectangular form with a 1 cm light path, but larger cells are available when solutions of low... 

Since an atom of a given element gives rise to a definite, characteristic line spectrum, it follows that there are different excitation states associated with different elements. The consequent emission spectra involve not only transitions from excited states to the ground state, e.g. E3 to E0, E2 to E0 (indicated by the full lines in Fig. 21.2), but also transisions such as E3 to E2, E3 to 1( etc. (indicated by the broken lines). Thus it follows that the emission spectrum of a given element may be quite complex. In theory it is also possible for absorption of radiation by already excited states to occur, e.g. E, to 2, E2 to E3, etc., but in practice the ratio of excited to ground state atoms is extremely small,... 

In practice, the emission line is split into three peaks by the magnetic field. The polariser is then used to isolate the central line which measures the absorption Ax, which also includes absorption of radiation by the analyte. The polariser is then rotated and the absorption of the background Aa is measured. The analyte absorption is given by An — Aa. A detailed discussion of the application of the Zeeman effect in atomic absorption is given in Ref. 51. 

The absorption of radiation produces unstable species. Flash photolysis does so by interaction of light with a solute. The transient may be a photoexcited state or a molecular fragment. Pulse radiolysis starts with highly reactive entities formed by dissociation of the solvent (e.g., H, eaq, and HO from H20) and consists of a study of their reactions or of reactive transients derived from them. In either case one monitors the ensuing reactions by luminescence (for excited states), light absorption, or conductivity changes. 

The infrared region of the electromagnetic spectrum lies between the wavelengths 1000 and 15000 nm (Kemp AVellacdo, 1980). Absorption of radiation in this region by organic compounds has been known since 1866, when Tyndall first conducted experiments on the interaction of radiation with compounds such as chloroform, methyl and ethyl iodides, benzene. 

Total Ionization—The total electric charge of one sign on the ions produced by radiation in the process of losing its kinetic energy. For a given gas, the total ionization is closely proportional to the initial ionization and is nearly independent of the nature of the ionizing radiation. It is frequently used as a measure of absorption of radiation energy. 

Self-Absorption—Absorption of radiation (emitted by radioactive atoms) by the material in which the atoms are located in particular, the absorption of radiation within a sample being assayed. 

The most important situation occurs when a film of different optical properties is formed at the electrode surface. In this case, theory predicts that the R value can be changed, even for non-absorbing films, as a result of existence of a third phase with different refractive index interspaced between the electrode and electrolyte. Therefore, the entire observed decrease in reflectivity R is not necessarily caused by the absorption of radiation in the film. This approximation, is, however, reasonably acceptable when the film is supported by a highly reflective phase, such as smooth metal electrode. 

The radiation emitted by a real surface is less than the radiation emitted by a blackbody, and the absorption of radiation by a real surface is incomplete. Many surfaces are excellent approximations to a blackbody, but some are not. Of the radiation incident upon a real surface, I(k, 9), a portion is reflected, some is... 

Absorption of radiation by solutes as a function of concentration, c, is described by the Beer-Lambert law ... 

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