Ultraviolet radiation characteristics

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. 

FIGURE 1.15 When a metal is illuminated with ultraviolet radiation, electrons are ejected, provided the frequency is above a threshold frequency that is characteristic of the metal. 

For nineteenth-century scientists, the obvious way to account for the laws of black-body radiation was to use classical physics to derive its characteristics. However, much to their dismay, they found that the characteristics they deduced did not match their observations. Worst of all was the ultraviolet catastrophe classical physics predicted that any hot body should emit intense ultraviolet radiation and even x-rays and y-rays According to classical physics, a hot object would devastate the countryside with high-frequency radiation. Even a human body at 37°C would glow in the dark. There would, in fact, be no darkness. 

The spectral characteristics of EDL are of a general interest in microwave photochemistry. The right choice of filling material can provide a desirable ultraviolet radiation. Atomic fills usually furnish line emission spectra (e.g. that of an Hg-EDL is... 

Electromagnetic radiation can be absorbed or emitted. The absorption of ultraviolet radiation by our skin may cause sunburn. When we cook food in a microwave oven, the absorption of microwave radiation by the water in the food causes the water molecules to vibrate, generating heat that cooks the food. However, when electromagnetic radiation is absorbed or emitted by matter, it behaves more like a stream of particles than as a wave motion. These particles are called photons and so electromagnetic radiation can be considered both as a stream of photons and as waves with characteristic properties, such as wavelength (1) and frequency (/). Therefore we say that electromagnetic radiation has a dual nature wave motion and streams of photons. 

Dave, J. V., Development of Programs for Computing Characteristics of Ultraviolet Radiation, Final Report under Contract NAS 5-21680, NASA Report CR-139134, National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, MD, NTIS No. N75-10746/6SL, 1972. 

The use of ultraviolet lamps has been practiced for a number of years in some hospitals, schools, and factories to check the spread of respiratory infections, but dieir effectiveness is inconclusive Possibly the most effective use of the characteristics of ultraviolet radiation is in optical and instrumentation applications. See. also Ultraviolet Spectrometers. 

The necessary starting point for any study of the chemistry of a planetary atmosphere is the dissociation of molecules, which results from the absorption of solar ultraviolet radiation. This atmospheric chemistry must take into account not only the general characteristics of the atmosphere (constitution), but also its particular chemical constituents (composition). The absorption of solar radiation can be attributed to carbon dioxide (C02) for Mars and Venus, to molecular oxygen (02) for the Earth, and to methane (CH4) and ammonia (NH3) for Jupiter and the outer planets. 

So far as the terrestrial atmosphere is concerned, the dissociation of molecular oxygen by ultraviolet radiation leads to the formation of ozone (03) in dry air, which consists essentially of 78% nitrogen, 21% oxygen, and 1% argon. It follows that the characteristic element in the chemistry of the terrestrial atmosphere is oxygen in its three forms 02, O, and 03, since nitrogen is not dissociated by the radiations absorbed by oxygen and ozone. 

Exposure of a tetrahydrofuran solution of W(CO)6 and the 1,2-B9C2H,, 2 ion to ultraviolet radiation produced immediate carbon monoxide evolution. Ultimately the air-sensitive (1,2-B9C2H,, )W(CO)32 ion was obtained as the tetramethylammonium salt (18). The corresponding chromium and molybdenum complexes have been obtained in the same manner (17, 18). These dianions undergo nucleophilic reactions characteristic of the analogous 77-C5H5Mo(CO)3 ion... 

Absorption Characteristics of Polypropylene. In discussions of the degradative effects of ultraviolet radiation, and photostabilization of polypropylene, absorption characteristics of the polymer as well as the energies involved should be considered. 

Red and yellow iron oxides have excellent weathering characteristics both absorb ultraviolet radiation. They offer good opacity and excellent value in use. Their chroma and saturation, however, are limited. 

Fig. 3.9 Phenomenological subdivision of ultraviolet radiation (according to DIN 5031-7) into four sub-bands and their characteristic effects.

Absorption of electromagnetic radiation in the visible and ultraviolet regions of the spectrum by atoms resulting in changes in electronic structure. Observed by passing radiation characteristic of a particular element through an atomic vapour of the sample. Sample vaporized by aspiration of solution into a flame or evaporation from electrically heated surface. 

Both the hydrated and anhydrous forms of the chelate give a red fluorescence (characteristic of the Eu " ion) when exposed to ultraviolet radiation. Fluorescence is also shown in solution. Interest in the fluorescence behavior of europium chelates is prompted by the use of these compounds in optical maser (laser) devices. Optical maser action has been demonstrated for europium chelates derived from benzoylacetone (though not for the tris chelate reported here ). 

Due to these characteristics, these pigments are useful for the photostabilization of some white compounds, e.g. corticosteroids, whose photodegradation is only caused by absorption of ultraviolet radiation. As shown in Figure 25, the addition of 0.5% of titanium dioxide or zinc oxide leads to the effective photoprotection of... 

The interaction of the atoms and electrons within a specific molecule determines the impact of the compound at the molecular level. The contribution of the physical-chemical characteristics of a compound to the observed toxicity is called quantitative structure-activity relationship (QSAR). QSAR has the potential of enabling environmental toxicologists to predict the environmental consequences of toxicants using only structure as a guide. The response of a chemical to ultraviolet radiation and its reactivity with the abiotic constituents of the environment determine the fate of a compound. 

---