Radiation near ultraviolet

Both CI2 and CINO2 absorb visible and near ultraviolet radiation, so that they can readily photolyze even with the faint amount of sunlight... 

The last-mentioned compound has been chosen in preference to benzoyl peroxide in many of the more recent kinetic investigations on account primarily of the freedom of its decomposition from side reactions such as radical-induced decomposition (see p. 113). It may also serve as a photoinitiator under the influence of near ultraviolet radiation, which... 

The most stable resin for many of our purposes has proven to be a copolymer of ethyl methacrylate and methyl acrylate. This comes as little surprise the Rohm and Haas Company has for years sold a durable resin based on these two monomers, Acryloid B-72 (6,28). We have also prepared polymers of similar physical properties based on methyl methacrylate and ethyl acrylate and have found that their behavior is practically the same - the methyl and ethyl groups apparently do not become seriously involved in crosslinking. As reported elsewhere( 23), rather than crosslink, Acryloid B-72 tends to chain break under visible and near-ultraviolet radiation, although at a very slow rate. Polyvinylacetate is another polymer used in the care of museum objects that tends more to chain break than crosslink under these conditions(23), but it is not our purpose to discuss its properties at this time. 

Diazo A family of reprographic processes (including Diazotype), based on the coupling of diazonium compounds with dye couplers to form colored compounds. Exposure of the dia-zonium compounds to near-ultraviolet radiation destroys them, so illuminated areas do not develop color. 

Jellinek, H. H. G., and A. K. Chaudhuri. Inhibited degradation of nylon 66 in presence of nitrogen dioxide, ozone, air, and near-ultraviolet radiation. J. Polymer Sci. Part A-1. 10 1773-1778, 1972. 

Certain compounds, when excited in solution by visible or near ultraviolet radiation, re-emit all or part of this energy as radiation. According to Stokes law, the maximum of the spectral emission band is located at a higher wavelength than that of either the incident radiation or the excitation band maximum (see Figs I2.l and 12.2). After excitation, the intensity of the emitted light decreases (decays) exponentially according to equation (l2.l), which relates the instantaneous intensity to time ... 

An example of transfer of electron from nucleophile to substrate is seen in the formation of the radical anions (observable by ESR) of 5-halo-2A/,3W-benzotf>]thiophene-2,3-diones on treatment with nucleophiles.20 It has been proposed in some cases, that this single-electron transfer step takes place through a charge transfer complex between the nucleophile and the aromatic substrate.21-22 Some reactions occur spontaneously, i.e. without any catalysts or reagents other than the substrate and the nucleophile, but the initiation process is usually, although not invariably, photostimulated (near-ultraviolet radiation, 300-... 

Damaging Effects of Visible and Near-Ultraviolet Radiation on Paper... 

These preliminary studies to evaluate the potentially damaging effects of the direct exposure to test sheets to visible and near-ultraviolet radiation employed exposures tens of times greater than those customarily used in "light bleaching" procedures. Moreover, the test sheets have been exposed directly to the radiation in contrast to the customary exposure of the papers in an aqueous bath to achieve bleaching. Experiments to be reported elsewhere show that the damage to the paper is very much reduced when the sheets are immersed at less than a centimeter... 

Work functions of alkali metal surfaces are only a few electronvolts1 so that the energy of near ultraviolet radiation is sufficient to produce ionization. 

While it may be true that many materials will become severely embrittled or discolored before Stage III is far advanced, some special situations exist in which a polymeric system can be regarded as being in Stage IV. For example, in the life history of polymers that tend to crosslink when exposed to near ultraviolet radiation, the conservator may have to remove them or at least have a sound understanding of their swelling properties in solvents under conditions in which the films are in Stage IV with respect to the formation of insoluble matter (27). In a similar sense,... 

In investigations carried out in our laboratory some years ago, but as yet unpublished, neither poly (isoamyl methacrylate) or the more commonly encountered poly(n-butyl methacrylate) were found to exhibit a similar tendency to chain break at lower temperatures. Analysis of the data by Charlesbys method indicated that these two polymers still tended to undergo cross-linking almost exclusively even when exposed to visible and near ultraviolet radiation at temperatures as low as 25°-35°C. 

Photochemistry concerns itself with the study of the effect of visible and near ultraviolet radiation on chemical reactions, the rates and mechanisms by which reactions initiated by light proceed. 

The significance of vibrational optical activity becomes apparent when it is compared with conventional electronic optical activity in the form of optical rotatory dispersion (ORD) and circular dichroism (CD) of visible and near-ultraviolet radiation. These conventional techniques have proved most valuable in stereochemistry, but since the electronic transition frequencies of most structural units in a molecule occur in inaccessible regions of the far-ultraviolet, they are restricted to probing chromophores and their immediate intramolecular environments. On the other hand, a vibrational spectrum contains bands from most parts of a molecule, so the measurement of vibrational optical activity should provide much more information. 

D.P. Arfsten, D.J. Schaeffer, D.C. Mulveny (1996). The effects of near ultraviolet radiation on the toxic effects of polycyclic aromatic hydrocarbons in animals and plants a review. Ecotoxicol. Environ. Saf., 33,1-24. 

Johnston, H.S., and G. Selwyn, New cross sections for the absorption of near ultraviolet radiation by nitrous oxide (N2O). Geophys Res Lett 2, 549, 1975. 

Giblin, F.J. (2000) Glutathione a vital lens antioxidant,./. Ocul. Pharmacol. Ther., 16,121-135. Ham, W.T., Mueller, H.A., Ruffolo, J.J., Guerry, D., and Guerry, R.K. (1982) Action spectrum for retinal injury from near ultraviolet radiation in the aphakic monkey, Am. J. Ophthal., 93, 299-305. 

Coexposure of D. magna to nitroaromatic compounds (TNT, 2-A-4,6-DNT, or 4-A-2,6-DNT) and near-ultraviolet radiation (near-UV 312-354 nm), after an initial 12-h exposure to the chemicals in the dark, promoted a significant toxicity increase (four- to fivefold) of the three chemicals [64], Similar experiments with the planarian Dugesia dorotocephala only exhibited increased toxicity of 2-A-4,6-DNT (tenfold) with near-UV coexposure, but not of the other two chemicals [64], In the absence of organisms, TNT was phototransformed, that is, transformed into breakdown products, by near-UV exposure (from 336 to 400 nm), whereas 2-A-4,6-DNT and 4-A-2,6-DNT remained stable. Therefore, photoactivation would be the likely cause of the enhanced effect of the ADNTs, whereas phototransformation products may have contributed to the increase in toxicity exhibited by simultaneous exposure of the organisms to TNT and near-UV. Alternatively, it is possible that the phototransformation products were photoactivated. Coexposure to near-UV for 30 min, but not for 15 min, also enhanced toxicity of these chemicals to the bacterium V. fisc fieri (Microtox test) [38], It is not clear if this is a result of the formation of a new, more toxic TNT metabolite in 30 min, but not 15 min, or if 30 min would be the minimum time for a sufficient amount of the chemicals to be bioaccumulated to enable the occurrence of the photoactivation process within the cells. 

Convection ceases at the tropopause level, and the temperature in the stratosphere and mesosphere is determined strictly by radiation balance. At altitudes above 20 km the absorption of solar ultraviolet radiation becomes increasingly important. The temperature peak at the stratopause has its origin in the absorption of near-ultraviolet radiation by stratospheric ozone. In fact, the existence of the ozone layer is in itself a consequence of the ultraviolet (UV) irradiation of the atmosphere. The enormous temperature increase in the thermosphere is due to the absorption of extremely shortwaved and thus energetic radiation coupled with the tenuity of the atmosphere, which prevents an effective removal of heat by thermal radiation. Instead, the heat must be carried downward by conduction toward denser layers of the atmosphere, where H20 and C02 are sufficiently abundant to permit the excess energy to be radiated into space. 

Another mechanism for the removal of surface films was discovered by Wheeler (1972), who found that fatty acid films on seawater collapsed to form particles when exposed to near-ultraviolet radiation. The results indicated that there was an introduction of hydroperoxide groups into the parent fatty acid molecule with resultant polymerization of the products. Instead of polymerization, Timmons (1962) found that the constituents of plankton oil films were converted to smaller and more soluble fragments when exposed to artificial sunlight. Photochemically initiated solubilization appears to be a process common to some constituents of crude oil and fuel oil films as well, with low molecular weight acids, sulfoxides, and peroxides comprising some of the product soluble fraction (Burwood and Speers, 1974 Hansen, 1975 Larson et al., 1977). The rate of photo-oxidation of films of various fractions of crude oil spread on water was greatly increased... 

The radiation source, ultraviolet (UV) radiation or more specifically near-ultraviolet radiation, is a very important ingredient of the photocatalytic process. Ultraviolet radiation refers to electromagnetic radiation in the 200-400 nm wavelength range. UVA covers from 315 to 400 nm, UVB from 280 to 315 nm and UVC from 200 to 280 nm. 

Finally, the monochromatic accumulation rate Is Integrated over the solar spectrum to obtain the total accumulation rate. The results of this procedure are Illustrated In Figure 1, which shows the variation with depth of the photochemical rate of production of 2 2 midday at 40 N for summer and winter In a freshwater sample. A rapid decrease of Che rate with depth Is apparent, particularly In this dark colored freshwater sample which attenuates most of Che near-ultraviolet radiation In Che upper 20 cm. Figure 2 shows how Che calculated rate of photochemical production varies with time of day and extent of cloud cover at a chosen latitude and season. 

The thermal decomposition of benzoyl peroxide, which takes place between 60 and 90°C, involves the homolytic cleavage of the 0-0 bond to yield benzoyl free radicals that may react to yield phenyl radicals and carbon dioxide. An example of photochemically induced free-radical formation is the decomposition of azo-bisisobutyronitrile by short-wavelength visible light or near-ultraviolet radiation at temperatures as low as 0°C, where no thermal initiation occurs. 

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