Scattering and. Absorption

Radiation traveling through a medium, in this case the atmosphere, can be absorbed, scattered, or transmitted without interruption. The energy of absorbed radiation usually results in either heating or the breaking of a chemical bond in the absorbing molecule for example the oxygen molecule in the stratosphere 

If a parallel beam of monochromatic radiation travels at an incident angle 6 through a uniform, plan-parallel layer (thickness h) of simple absorbing gas, the transmitted radiation is given by the Beer-Lambert law 

For the atmosphere the angle 0 is the solar zenith angle, for direct beam radiation. As the atmosphere is spherical and not plan-parallel, the path length of the radiation through the atmosphere has to be described by the relative atmospheric mass m, which can be approximated for zenith angles less than about 85° by 

The larger the angle 0, the longer the distance that the radiation has to pass through the atmosphere, or one of its layers, and the greater the chance of attenuation. 

The angular redistribution of light during a scattering event is described by the scattering phase function, P(0). This gives the probability that a photon from 

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Bohren C F and Hoffman D R 1983 Absorption and Scattering of Light by Smaii Particies (New York Wiley)... 

All forms of spectroscopy require a source of energy. In absorption and scattering spectroscopy this energy is supplied by photons. Emission and luminescence spectroscopy use thermal, radiant (photon), or chemical energy to promote the analyte to a less stable, higher energy state. 

Background correction is used to compensate for background absorption and scattering due to interferents in the sample. Such interferences are most severe for analytes, such as Zn, that absorb at wavelengths of less than 300 nm. 

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. 

Attenuation. The exceptional transparency, or low attenuation, of siUca-based glass fibers has made them the predominant choice for optical transmission because of the low level of absorption and scattering of light as it traverses the material. Together these comprise optical attenuation, or loss, measured in dB where... 

Lasing occurs whenever the gain arising from stimulated emission exceeds the cavity losses. Internal losses, a, result from absorption and scattering of light. The reflectivity, R, of the mirror facet must be <1 and this contributes a loss term of (1/L)ln(l/E), where Eis the cavity length. At threshold, the gain, is equal to losses and... 

Although the usual absorption and scattering spectroscopies caimot distinguish enantiomers, certain techniques are sensitive to optical activity in chiral molecules. These include optical rotatory dispersion (ORD), the rotation by the sample of the plane of linearly polari2ed light, used in simple polarimeters and circular dichroism (CD), the differential absorption of circularly polari2ed light. 

When both absorption and scattering are present, the Beer-Lambert law must be replaced by the Kubelka-Munk equation employing the absorption and scattering coefficients iC and S, respectively. This gives the redectivity... 

Figures 2b and 3 demonstrate that X rays are produced over a range of depth into the sample. The X rays must propagate along a finite path through the specimen to reach the detector, and are subject to photoelectric absorption and scattering, which follows an exponential relation ...

C. F. Bohren and D. R- Huffman. Absorption and Scattering of Light by Small Particles. John Wiley Sons, New York, 1983. Parts 1 and 2 describe the theory of the scattering problem in some detail. Part 3 compares theory with experiment. 

The atmospheric transmissivity, t, greatly affects the radiation transmission by absorption and scattering by the separating atmosphere. Absorption may be as high as 20-40%. Pietersen and Huerta (1985) give a correlation that accounts for humidity (equation 9.1-31), where t = atmospheric transmissivity, = water partial pressure (Pascals), X = distance from flame surface to target (m). 

Lambert-Beer law The mathematical description of the attenuation of a light beam by absorption and scattering by dust particles in the airstream. 

The atmospheric attenuation factor takes into account the influence of absorption and scattering by water vapor, carbon dioxide, dust, and aerosol particles. One can assume, as a conservative position, a clear, dry atmosphere for which = 1. 

Transmissivity The fraction of radiant energy transmitted from a radiating object through the atmosphere to a target after reduction by atmospheric absorption and scattering. 

Attenuation—A process by which a beam from a source of radiation is reduced in intensity by absorption and scattering when passing through some material. 

Most previous attempts to obtain X-ray diffraction data at very low temperatures (< 80 K) have used custom built systems with closed cycle helium refrigerators mounted on large, robust four circle diffractometers. In order to remove the inherent disadvantages of these systems - cost, single application, absorption and scattering of the windows - we have built an open flow system from mainly off-the-shelf components which uses liquid helium as the cryogen. This is not the first open flow helium system [19, 20] but is the first that is mainly off-the-shelf and is mountable on any diffractometer. It is based on an ADP Helitran ESR cryostat with modifications to the nozzle assembly and to the direction of the gas flow. The lowest temperature is estimated to be <30K. At the current price for liquid helium in... 

Similar models using a multi-flux analysis of particle absorption and scattering are reported by Radauer, Glatzer and Linzer (1996). 

The development of new oximeters is also in progress, with the application of time- and frequency-domain techniques which are, in principle, capable of discriminating between the absorption and scattering contributions coming from human tissue, thus making possible the detection of tissue oxygenation37 39. 

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