Absorption of ultraviolet and visible light

Absorption of ultraviolet and visible light by molecules results in electronic transitions in which changes in both electronic and vibrational states occur. Such transitions are called vibronic transitions. 

The measurement of the absorption of ultraviolet and visible light as a function of wavelength. Ultraviolet light consists of wavelengths from about 100 to 400 nm. Visible light is from about 400 nm (violet) to 750 nm (red), (p. 696)... 

The mechanism of the absorption of ultraviolet and visible light was discussed in Chapter 7. In this chapter, two processes that involve absorption of radiation as the first step will be discussed these are fluorescence and phosphorescence. 

Absorption of Ultraviolet and Visible light due to electrons moving to higher energy levels... 

Figure 3.11 Absorption of ultraviolet and visible light of (A) HDPE/CB and (B) HDPE/ HDPE-g-CB composites with the content of 1.0 and 5.0 wt%, respectively.

Phenomenologically, we can distinguish the following photochemical reactions photoisomerization, photocyclization (photoaddition), photocleavage, hydrogen abstraction, photo-concerted reaction, etc. For a photochemical reaction to occur, efficient absorption of ultraviolet or visible light is necessary, thus the photoreactive molecules should contain in their structure one of the bond types listed in Table 1.13. The characteristics of the typical photochemical reactions of C = C and C = O groups are summarized as follows. 

A typical photochemical isomerization of the azobenzene amphiphile was found in an ethanol solution. A trans isomer converted to a cis isomer with ultraviolet irradiation. Back reaction from cis to trans was accelerated when a weak n-n absorption band of the cis isomer at ca.450nm was excited (Figure 21a). An alternative irradiation of uv and visible light to the ethanol solution gave reversible changes of the ji-ji transition between 355nm and 325nm attributed to the trans and cis isomers, respectively. 

This photograph shows a simple spectrophotometer, which measures the amount of visible light that is ahsorhed hy a coloured solution. More sophisticated devices can measure the absorption of ultraviolet and infrared radiation. 

Photochemical or photoinitiated polymerizations occur when radicals are produced by ultraviolet and visible light irradiation of a reaction system [Oster and Yang, 1968 Pappas, 1988]. In general, light absorption results in radical production by either of two pathways ... 

It has been mentioned by Baly279 that, although carbohydrates do not selectively absorb ultraviolet and visible light, they do show selective absorption of infrared radiation in the wavelength range of 1.5-6 /im. This observation prompted Sem-mens280-281 to examine the action of polarized infrared radiation upon starch. She realized that mild hydrolysis takes place in starch subjected to such treatment, as well as to irradiation by nonpolarized infrared radiation. The effects of such radiation are observed in samples both in vitro and in vivo (namely in plants). 

This is the simplest type of transfer. The efficiency depends only on the extent of overlap of the donor emission spectrum with the acceptor absorption spectrum. Ultraviolet or visible light is emitted by the excited donor molecule and absorbed by the acceptor molecule in a two-step process. Increasing the acceptor concentration results in a decrease of fluorescence yield of the donor but the decay time of the donor fluorescence remains unaffected. The range of energy transfer by this mechanism depends on the undisturbed path of the emitted light and falls off with distance as R. ... 

Ultraviolet and visible light (UV-VIS) analysis is the most used identification technique for anthraquinones. The UV-VIS absorption spectrum of an anthraquinone is a combination of the absorptions arising from partial acetophenone- and benzoquinone chromophores (Fig. (7)) [1,20,62]. 

Fig. 20.4 Electronic and vibrational energy levels of an isolated molecule. This diagram is a more detailed version of Fig. 20.3. The transitions labelled a and c involve the absorption and emission (respectively) of ultraviolet or visible light. The transitions labelled b and d involve the absorption and emission (respectively) of infrared light.

This technique comprises a group of quantitative instrumental analytical methods based on the capacity of free atoms of both emitting and absorbing radiation at a specific wavelength. The radiation lies within the range for ultraviolet and visible light. A distinction is made between atomic emission spectrometry (AES), atomic absorption spectrometry (AAS), and atomic fluorescence spectrometry. The most commonly applied techniques are flame-AAS, graphite furnace-AAS, and ICP-AES. With ICP, excitation takes place in a plasma at a temperature of 7000 K. 

Ultraviolet and visible light absorption spectroscopy can be used to identify chromophores (e.g. benzene rings and carbonyl groups) and to determine the lengths of sequences of conjugated multiple bonds in polymers. It also can be used to analyse polymers for the presence of additives such as antioxidants or for detection of residual monomer(s). Additionally, fluorescence and phosphorescence techniques are important in studies of polymer photophysics. 

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