Molecular oxygen photodissociation

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 ... 

Nicolet, M., S. Cieslik, and R. Kennes, Aeronomic problems of molecular oxygen photodissociation, V. Predissociation in the Schumann-Runge bands of oxygen. Planet Space Sci 37, 427, 1989. 

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]. 

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...

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

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

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 most important atmospheric photoreagent is, however, molecular oxygen it absorbs UV-C radiation within 100-240nm, with a maximum at 160nm, and undergoes photodissociation to the O atoms in their ground states 0(3P) ... 

Oxidation became an important atmospheric reaction on Earth once molecular oxygen (O2) from photosynthesis had reached sufficiently high levels. This O2 could then photodissociate in the atmosphere to give oxygen atoms, which combine with O2 to form ozone (O3). When O3 absorbs UV light at wavelengths less than 310 nm, it produces excited oxygen atoms (0( D)) which can attack water vapor to produce the hydroxyl free radical (OH). It is the hydroxyl radical that, above all, defines the oxidative capacity of our O2- and... 

Molecular Oxygen, Peroxo, Aquo and Related Complexes. Ab initio calculations have been reported for vibrational wavenumbers for M+(H20)n, where M = Li, Na, K, Rb or Cs n = 1-6.320 vOH mode assignments were proposed from the IR photodissociation spectra of gaseous Mg(H20)4+ and [Mg(H20)4Ar]+ (3000-3450 cm ).321... 

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). 

At this time in the Earth s history the carbon dioxide abundance was higher than its present value since this gas accumulated in the absence of a biosphere. Rutten (1966) speculated that the atmospheric C02 level was 10 times PAL (present atmospheric level) about 3 x 109 years ago. On the other hand the presence of H20 in the atmosphere led, by photochemical dissociation, to the formation of free radicals and molecular oxygen. An estimate of the importance of these reactions is necessary to give some idea of the oxygen level in pre-biospheric times. The photodissociation of water vapour can be represented as follows (Suess, 1966) ... 

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.

Allen, M., and J.E. Frederick, Effective photodissociation cross sections for molecular oxygen and nitric oxide in the Schumann-Runge Bands. J Atmos Sci 39, 2066, 1982. 

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. 

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) ... 

The formation of the stratospheric ozone layer can be understood most simply on the basis of a reaction model composed of a minimum set of four elementary processes (a) the dissociation of oxygen molecules by solar radiation in the wavelength region 180-240 nm (b) the attachment of oxygen atoms to molecular oxygen, leading to the formation of ozone (c) the photodissociation of ozone in the Hartley band between 200 and 300 nm and (d) the destruction of ozone by its reaction with oxygen atoms. The reactions may be written... 

The tacit assumption made in the preceding discussion that ozone is photochemically stable in the troposphere is basically incorrect, and we must finally consider this aspect. It is true that the photodissociation of ozone as far as it leads to 0(3P) atoms causes no losses, because their subsequent attachment to molecular oxygen regenerates ozone. In Section 4.2 it was shown, however, that a part of the O( D) atoms produced in the... 

The photochemical interconversion between NO and N02 was discussed in Section 5-2. During the day N02 undergoes photodissociation, forming NO and O atoms that quickly attach to molecular oxygen, producing ozone. Back-reactions of NO with ozone and with peroxy radicals generated from hydrocarbons establish a steady state between NO and N02. In the absence of local sources, their molar ratio should be given by the steady-state equation... 

---