Fraction of radiation absorbed

The fraction of radiation absorbed by H202, fPER, approaches 1 at relatively small concentrations of H202. 

The numerator of Equation (7.64) represents the production rate of hydroxyl radicals by hydrogen peroxide photolysis. When several UV absorbers are present, the fraction of radiation absorbed by peroxide to produce the hydroxyl radicals will be smaller. The denominator of Equation (7.64) represents the consumption rate of hydroxyl radicals by key scavengers. The value of /M and fPER denotes the relative importance of the destruction... 

Equation 5 corresponds to the Beer-Lambert law, which implies that the fraction of radiation absorbed is independent of the intensity of the radiation employed, and that the amount of radiation absorbed by the system is proportional to the number of molecules absorbing the radiation. 

The multiple reflections increase r compared to f and reduce t compared to Tj. The fraction of radiation absorbed in the body can be calculated from (5.93). The spectral absorptivity is obtained as ... 

No ordinary monochromator is capable of yielding a band of radiation as naiTOW as the width of an atomic absorption line (0.002 to 0.005 nm). As a result, the use of radiation that has been isolated from a continuum source by a monochromator inevitably causes instrumental departures from Beer s law (see the discussion of instrument deviations from Beer s law in Section 24C-3). In addition, since the fraction of radiation absorbed from such a beam is small, the detector receives a signal that is less attenuated (that is, P —> Pq) nd the sensitivity of the measurement is reduced. This effect is illustrated by the lower curve in Figure 24-17 (page 733). 

Figure 8JL0 Fractions of radiation absorbed, reflected, and transmitted.

The fraction of radiation absorbed by a solution of an absorbing analyte can be quantitatively related to its concentration. Here, we present calculations for single species and for mixtures of absorbing species. 

E Fraction of radiation absorbed per unit pathlength in the sample... 

Since radiation arriving at a black surface is completely absorbed, no problems arise from multiple reflections. Radiation is emitted from a diffuse surface in all directions and therefore only a proportion of the radiation leaving a surface arrives at any other given surface. This proportion depends on the relative geometry of the surfaces and this may be taken into account by the view factor, shape factor or configuration F, which is normally written as F, for radiation arriving at surface j from surface i. In this way, F,y, which is, of course, completely independent of the surface temperature, is the fraction of radiation leaving i which is directly intercepted by j. 

Many compounds are capable of absorbing uv radiation and subsequently emitting radiation of a longer wavelength, either instantly (fluorescence) or after a time delay (phosphorescence). Usually, the fraction of the absorbed energy that is re-emitted is quite low, but for a few compounds values of 0.1-1 are obtained, and such compounds... 

The value of s (the absorbance of a 1 M solution in a 1 cm cell) depends upon the nature of the absorbing species and on the wavelength of the incident radiation. Absorbance is thus seen to be directly proportional both to the concentration of the absorbing species and to the thickness of the absorbing medium. It is related to transmittance T defined as I/I0 (the fraction of radiation transmitted) by the equation... 

The total amount of radiation absorbed in a system with n + 1 absorbing components (cf. Eq. 3) being /0, only fraction IA is absorbed by compound A (cf. Eq. 4). For the rate of disappearance of the compound A we obtain therefore... 

The probability or rate of absorption is given by the Lambert-Beer Law. The Lambert law states that the fraction of incident radiation absorbed by a transparent medium is independent of the intensity of incident radiation and that each successive layer of the medium absorbs an equal fraction of incident radiation. The Beer law states that the amount of radiation absorbed is proportional to the number of molecules absorbing the radiation, that is the concentration C of the absorbing species. The two are combined and expressed as... 

In practice a sample is irradiated with monochromatic radiation of wavelength Ar during a time t. The concentrations of reactants and products are determined by usual analytical methods. The second problem concerns the measurement of the number of photons absorbed. This is calculated from the number of photons incident on the sample, multiplied by the fraction of light absorbed x9... 

The intensity of radiation absorbed, 4bs> is. of course, /(l - ft, so that the fraction absorbed is given by... 

Figure 1 The ratio of infrared excess/stellar luminosity is a measure of the fraction of starlight absorbed by circumstellar dust and re-radiated in the infrared. The plot from Spangler et al. (2001) shows the temporal decline of dust around Vega-like stars (points) and stars in clusters with measured ages (circles). At least for the longer ages, the dust is most probably generated by comets.

When radiation strikes a surface, pari of it is absorbed, part of it is reflected, and llie remaining part, if any, is iransmiticd, as illustrated in Fig. 12-31. The fraction of irradiation absorbed by the surface is called ihe absorptivity a, the fraction reflected by the surface is called the reflectivfly p, and the fraction transmitted is called the transmissivity r. That is. 

The sum of the absorbed, reflected, and transmilled fractions of radiation energy must be equal to unity,... 

The view factor represents tlie fraction of radiation leaving surface 1 that strikes surface 2 directly, and F21 represents the fraction of radiation leaving surface 2 that strikes surface 1 directly. Note that the radiation that strikes a surface does not need to be absorbed by that surface. Also, radiation that strikes a surface after being reflected by other surfaces is not considered in the evaluation of view factors. 

The view factor has proven to be very useful in radiation analysis because it allows us to express the fraction of radiation leaving a surface that strikes another surface in terms of Ihe orientation of these two surfaces relative to each other. The underlying assumption in this process is that the radiation a surface receives from a source is directly proportional to the angle the surface subtends when viewed from the source. This would be the case only if the radiation coming off the source is uniform in all directions throughout its surface and the medium between the surfaces does not absorb, emit, or scatter radiation. That is, it is the case when Ihe surfaces are isothermal and diffuse emitters and reflectors and the surfaces are separated by a iioiiparticipating medium such as a vacuum or air. 

Note that - I when no radiation is absorbed and thus radiation intensity remains constant. Also, the spectral transmissivity of a medium represents the fraction of radiation transmitted by the medium at a given wavelength. 

It has also been shown above that, at a given moment, a certain fraction of the absorbed energy, generally more important in a semiconductor than in an insulator, is stored in excited electronic states. This fraction, which cannot be evaluated in the case of fission fragment irradiation, represents for all types of radiations, except in the case of neutron irradiation, the major part of the incident energy. In the case of fast neutron irradiation, this fraction is very small for heavy element targets it becomes even more... 

The model from R.E. Bird and R.L. Hulstrom [5.42], see also [5.34], is once again mentioned for the calculation of the diffuse fraction of radiation in (5.126). The source of the scattered radiation is the non-absorbed direct solar radiation with the irradiance... 

During irradiation of the actinometer, the solutions should be stirred with a magnetic bar. After irradiation, pipette an aliquot (2 ml) of the solution into a 20 ml calibrated flask. Add a volume of buffer (b) equal to half the volume of photolyte taken (1 ml) and 2 ml of the phenanthroline solution (c). Make up to the mark with water, mix and allow to stand for at least 0.5 h. Measure the absorbance at 510 nm and repeat with the same volume of unexposed actinometer solution. Convert the absorbance difference of ferrous iron using the calibrated slope. Convert the quantity of ferrous ion formed in the total volume of the irradiated solution to a radiation dose (see Equation 2.24) using the recommended quantum yield given in Table 3.2. If necessary, allow for the fraction of light absorbed (Equation 3.17). 

Linear attenuation coefficient (p). The fraction of radiation energy absorbed and scattered per unit thickness of absorber. 

Now since we are interested in the probability of absorption by an isolated molecule, let us define 0 as the fraction of energy absorbed per molecule encountered in the path of a parallel beam of radiation of unit cross section. Evidently the diminution in intensity as the light traverses a differential path is given by... 

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