Wavelength concept

The effective wavelength concept is useful since it avoids the inclusion of an awkward integral in any absolute method of relating x-ray intensity and chemical composition. It is also of great use in estimating approximate total absorption values using an expression of the form shown in Eq. (2.34). 

The concept of a chromophore is analogous to that of a group vibration, discussed in Section 6.2.1. Just as the wavenumber of a group vibration is treated as transferable from one molecule to another so is the wavenumber, or wavelength, at which an electronic transition occurs in a particular group. Such a group is called a chromophore since it results in a characteristic colour of the compound due to absorption of visible or, broadening the use of the word colour , ultraviolet radiation. 

The hnearity between M and makes the concept of absorbance so usehil that measurements made by sampling methods other than transmission are usually converted to a scale proportional to absorbance. The linearity between M and i is maintained only if the resolution of the spectrometer is adequate to eliminate contributions from wavelengths not absorbed by the species being measured. In addition, the apparent value of a is very dependent on resolution because a is 2l strong function of wavelength (30,31). 

Unlike these light sources, laser technology relies on a concept known as stimulated emission. Wlien an excited atom is stimulated by a photon, light is emitted at precisely the same wavelength and precisely in ph ase with the light wave that stimulated it. 

Modern theories suggest that our concept of space breaks down on a scale of Hr 34 m, so that may constitute a lower bound to electromagnetic radiation wavelength. 

Between 1923 and 1927, the concepts of quantum efficiency (number of photons emitted divided by number of photons absorbed by a sample) and quantum yield (fraction of excited molecules that emit) had been defined and values determined for many compounds by Vavilov (34). The quantum yield indicates the extent that other energy loss mechanisms compete with emission in an excited molecule. Although the quantum yield is influenced by the molecular environment of the emitter, for a given environment it depends on the nature of the emitting compound and is independent of concentration and excitation wavelength, at least at low concentrations (35). Tlius, it serves as another measurable parameter that can be used to identify the compounds in a sample and also, because of its sensitivity to the surroundings of the luminophore, to probe the environment of the emitter. 

Photocatalysis essentially consists in the catalysis exerted by materials (semiconductors) under irradiation of light at an appropriate wavelength. It is therefore an essential part of the sustainable chemistry strategy. Since many good literature reviews are available that explain recent results in understanding the processes involved in photocatalysis [1-5], only the fundamental concepts will be considered here. 

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