Photodissociation of molecular oxygen

Ozone is formed by the photodissociation of molecular oxygen, 02, with UV light of very short wavelength (around 200 nm)... 

This reaction not only restores the hydroxyl, but it also results in the formation of nitrogen dioxide which, because of its almost immediate photodissociation, leads to the liberation of an oxygen atom and to the formation of ozone. This last reaction plays an important role in the lower stratosphere where the photodissociation of molecular oxygen is no longer important. 

Above the mesopause, Tg increases rapidly. In this region, termed the thermosphere (Fig. 2), absorption of short wavelength solar radiation is occurring (Fig. 3) which results in the efficient photodissociation of molecular oxygen, and the photoionization of the O atoms so produced and of the 02 and N2 molecules. Thus, Tg increases beyond 1000 K, approaching 2000 K at times. Whereas below 100 km the neutral gas particles, the ions and the electrons in the plasma all possess the same kinetic temperature, above 100 km, due to the lower pressure and the subsequent reduced electron/heavy particle collision frequency and the large amount of energy imparted to the photoelectrons, the electron temperature, Te increases above Tg (and Tj the ion temperature, which is Tg, see Fig. 2). 

The theoretical framework just presented was first suggested by Chapman (1931). This theory provides an explanation for the behavior of the layers of ionization in the thermosphere or of photodissociation in the middle atmosphere. The production of ozone through photodissociation of molecular oxygen exhibits a maximum near 45 km dependent on the insolation. The rate of heating through absorption of ultraviolet radiation by ozone similarly leads to a maximum near the stratopause. Numerous examples of such layers can be found in the neutral and ionized atmosphere. 

Figure 4-34- Contribution of each spectral region to the photodissociation of molecular oxygen as a function of altitude.

In the middle atmosphere, molecular nitrogen is particularly stable since it cannot be photodissociated below the mesopause. On the other hand, the photodissociation of molecular oxygen can occur at altitudes as low as 20 km. Where transport processes can replace the photodissociated molecules, their abundances remain constant, but as photodissociation rates increase at higher altitudes, mixing ratios begin to decline. Oxygen photolysis initiates a series of reactions which determine the chemistry of the oxygen atmosphere these will be the subject of the next section. 

The photodissociation of molecular oxygen by ultraviolet radiation at wavelengths less than 242.4 nm produces atomic oxygen (Chapman, 1930) ... 

Figure 13.13 Schematic of potential energy curves of the photodissociation of molecular oxygen, showing some of the electronic states of oxygen. (Reprinted with permission from University Science Books. )...

Table 3.3 summarizes important light-absorbing molecules in atmospheric chemistry. Photodissociation of molecular oxygen is key to stratospheric chemistry and will be ad-... 

It has long been assumed that the main reaction which balanced the production of odd oxygen particles by photodissociation of molecular oxygen was that between atomic oxygen and ozone. In recent years it has become clear, however, that this reaction is not sufficiently fast (Schiff 1969). In a search for other destruction mechanisms reactions between OH, HO2 and O3 have been proposed (Hunt 1966 Hampson 1965). It has, however, been indicated in a previous study (Crutzen 1969) that this hypothesis does not succeed in explaining the ozone observations between 30 and 35 km. 

Figure 4.34 Photoelimination reactions of nitrogen, (a) Formation of a carbene through triplet state sensitization, and addition of molecular oxygen, (b) Formation of nitrenes through photodissociation of azo compounds and azides...

During the thermally driven differentiation of the Earth into core-mantle-crust, numerous reactions would have produced oxidized forms of iron, sulfur and carbon. These would have contributed to the redox chemistry in the early planet development. Volcanic and hydrothermal emission of sulfur dioxide, SO2, delivered oxidants to the oceans and atmosphere. Photodissociation of water vapor in the atmosphere have undoubtedly provided a small but significant source of molecular oxygen. Furthermore, UV-driven ferrous iron oxidation could have been coupled to the reduction of a variety of reactants, for instance, CO2 (Figure 16). 

Hudson, R.D., and S.H. Mahle, Photodissociation rates of molecular oxygen in the mesosphere and lower thermosphere. J Geophys Res 77, 2902, 1972. 

Kockarts, G., Absorption and photodissociation in the Schumann-Runge bands of molecular oxygen in the terrestrial atmosphere. Planet Space Set 21, 589, 1976. 

Nicolet, M., and R. Kennes, Aeronomic problems of molecular oxygen photodissociar tion, IV. Photodissociation frequency and transmittance in the spectral range of the Schumann-Runge bands. Planet Space Sci 37, 459, 1989. 

Table 5.4. Photodissociation reactions of molecular oxygen and ozone [22, 24]... 

The presence of ozone in higher atmospheric layers is a results of the photodissociation reactions of molecular oxygen. 

The first step in the production of ozone, the photolysis of molecular oxygen [reaction (1)], is rate limiting. While ozone production is slow, there are chemical reactions that can rapidly destroy it. One of the major species that is efficient in the removal of ozone is chlorine. The role of chlorine species in the depletion of ozone has been investigated actively since 1974, when Rowland and Molina [2] drew attention to the potential impact of human-made materials known as chlorofluorocarbons (CFCs) on ozone produced in the stratosphere. Chlorofluorocarbons are widely used in our daily life as refrigerants, aerosol propellants, cleaning solvents, and in fire-extinguishing applications. CFCs are stable, chemically inert, and have low toxicity. These properties make CFCs ideal for many applications and account for their wide use. However, the release of chlorine from the photodissociation of chlorofluorocarbons poses a central threat to ozone produced in the stratosphere ... 

Reactions with molecular species above the arrow e.g. RIO) involve subsequent reactions with these species to produce the indicated products. In most cases the reactants shown to the left of the arrow participate in the slowest or rate-determining step]. The CH3O radical formed in Rll then follows reaction R7. The H02 radical formed in RIO is the other member of the family and is linked with HO in a variety of chain reactions. These radicals are produced following HO attack on hydrocarbons or by photodissociation of oxygenated hydrocarbons such as formaldehyde (RIO) and acetaldehyde ... 

Evidence for adiabatic photolytic cycloreversions at room temperature has been obtained more frequently in recent years [121,122], The adiabatic generation of singlet oxygen by photochemical cycloreversion of the anthracene and 9,10-dimethylanthracene endoperoxides 105 and 106 proceeds with wavelength-dependent quantum yields of 0.22 and 0.35, respectively, and involves the second excited singlet state of the endoperoxides [123]. Photodissociation of the 1,4-endoperoxide from l,4-dimethyl-9,10-diphenylanthracene was found to yield both fragments, i.e., molecular oxygen and l,4-dimethyl-9,10-diphenylanthracene, in their electronically excited state [124]. 

Molecular oxygen photodissociation is feeding reaction (1) with atomic oxygen in the stratosphere, the part of the atmosphere extending from above the troposphere to about 50 km. In the troposphere, the lowest part of the atmosphere extended up to 7-16 km, 02 photolysis is not significant. Nitrogen dioxide (N02) photolysis provides the required 03P for 03 production ... 

The radiation of wavelengths less than 242 nm is absorbed by molecular oxygen and leads to its photodissociation ... 

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