Actinic radiation

Although the solar spectrum is very broad, extending from cosmic rays to radio waves (see Figure 2.1), the photochemical reactions are driven only by visible and ultraviolet (UV) radiation (actinic light - Figure 9.2). 

FIGURE 1 Photodissociation of ozone in the near ultraviolet spectral region. Overlap of solar radiation actinic intensity, ozone absorption cross section, and 0( D) quantum yield to derive the 0( D) production frequency as a function of wavelength. [From Zellner, R., ed. (1999). Global Aspects of Atmospheric Chemistry, Steinkopff/Springer, Darmstadt, Germany.]... 

UV absorbers are used to protect the plastic material as well as the foodstuff packaging from the actinic action of UV radiation. Actinic effects may cause discoloration of both the plastic material and the foodstuff, and may also occasion changes in taste and loss of vitamins in the food. 

The prime driver of the chemical system of the earth s atmosphere is photochemical reactions caused by solar radiation. The atmosphere of the earth is divided into levels called the troposphere, stratosphere, mesosphere and thermosphere from nearest the ground to farthest according to the characteristics of the temperature gradient as shown in Fig. 3.1. The cause of the temperature inversion in the stratosphere, which characterizes the earth s atmosphere, is the formation of an ozone layer by the photolysis of oxygen, one of the major components of the atmosphere. In this chapter, the spectrum of solar radiation, actinic flux, and so on, that is necessary to calculate the photolysis rate of atmospheric molecules are explained. 

In the case of some types of polyesters, the dried treated sheet is very tacky, and must be interleaved with a plastic release film to prevent blocking in the roU prior to use. Certain polyesters and acryhcs can be converted to a gelled state by exposure to actinic radiation. 

Complexation of the initiator and/or modification with cocatalysts or activators affords greater polymerization activity (11). Many of the patented processes for commercially available polymers such as poly(MVE) employ BE etherate (12), although vinyl ethers can be polymerized with a variety of acidic compounds, even those unable to initiate other cationic polymerizations of less reactive monomers such as isobutene. Examples are protonic acids (13), Ziegler-Natta catalysts (14), and actinic radiation (15,16). 

Cross-linked PVP can also be obtained by cross-linking the preformed polymer chemically (with persulfates, hydrazine, or peroxides) or with actinic radiation (63). This approach requires a source of free radicals capable of hydrogen abstraction from one or another of the labile hydrogens attached alpha to the pyrrohdone carbonyl or lactam nitrogen. The subsequently formed PVP radical can combine with another such radical to form a cross-link or undergo side reactions such as scission or cyclization (64,65), thus ... 

Causes of actinic keratosis include prolonged exposure to sun, ultraviolet light from artificial sources and x-radiations. 

Photochemical activation of [ESE, TNM] complexes. Colored solutions of the ESE derived from the relatively unreactive cycloalkanones and acyclic ketones in Table 2 and TNM persist for prolonged periods, and they can thus be separately subjected to filtered actinic radiation with /exc > 420 nm. (Note that under these conditions, neither TNM nor ESE alone is excited since they absorb only below 400 nm.) The photonitrations of ESE with TNM are carried out at low temperatures (—40°C) at which the thermal reactions are too slow to... 

A number of a-aryl-A-alkyl nitrones and contrast enhancement compositions, which can be used to make contrast enhancement layer photoresist composites (230, 231), and inhibitors of free radical polymerization of monomers in nonexposed regions of the photoresist layer at selective actinic radiation (232). Histidine was used as a catalyst in the synthesis of a, A-diaryl nitrones in situ (233). To study diphenylborate chelates with mono- and bidentate ligands, a series of hydroxyl-containing nitrones have been synthesized (Fig. 2.7) (234-237). 

A major source of OH in both clean and polluted air is the photodissociation of 03 by actinic UV radiation in sunlight to produce an electronically excited oxygen atom, O( D),... 

Sources of H02 include the reactions of 02 with hydrogen atoms and formyl radicals, both of which are produced, for example, by the photodissociation of gaseous formaldehyde following absorption of solar actinic UV radiation. 

Gas-phase nitrous acid Gaseous HONO plays an important role in the chemistry of irradiated mixtures of VOC and NOx in air, whether in smog chambers or in ambient atmospheres. Thus, it strongly absorbs actinic UV radiation and, at sunrise (Fig. 1.5), decomposes into OH radicals and NO with a high quantum efficiency ... 

Because of the presence of these absorbing species in the upper atmosphere, only light of A > 290 nm is available for photochemical reactions in the troposphere. It is often expressed as the integrated radiation coming from all directions to a sphere and is referred to as actinic radiation, although in the strictest sense,... 

The light available to a molecule in air for absorption and photodissociation includes both direct and scattered and reflected radiation coming from all directions as described earlier and depicted in Fig. 3.16. The term actinic flux or spherically integrated actinic flux, denoted by F( A), is used to describe the total intensity of this light and is the quantity of interest in calculating kp. 

Determination of OH relative rate constants for compounds that photolyze significantly in actinic radiation requires a nonphotolytic source of OH. Three such OH sources are H202-N02-C0 mixtures (Campbell et al., 1975, 1979 Audley et al., 1982), the thermal decomposition of H02N02 in the presence of NO (Barnes et al., 1982), and 03-hydrazine reactions (Tuazon et al., 1983) or 03 alkane reactions in the dark (Finlayson-Pitts et al., 1993). However, in these cases, the reactant must not react with 03, HOz, or H202, and care must be taken in interpreting the data since these systems have the potential of being rather complex. Indeed, the rate constants derived have not always agreed well with literature values. Until the... 

The importance of OH radicals in atmospheric chemistry is the basis of another reactivity scale for organics that do not photolyze in actinic radiation (Darnall et al., 1976 Wu et al., 1976). This scale is based on the fact that, for most hydrocarbons, attack by OH is responsible for the majority of the hydrocarbon consumption, and this process leads to the free radicals (H02, R02) that oxidize NO to N02, which then leads to 03 formation. Even for alkenes, which react with 03 at significant rates, consumption by OH still predominates in the early portion of the irradiation before 03 has formed. It has therefore been suggested that the rate constant for reaction between OH and the... 

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