Photodissociation of ozone

The photochemistry of O3 has attracted widespread interest primarily due to its importance to atmospheric chemistry (Sato, 2001 Matsumi and Kawasaki, 2002 and references cited therein). O3 is bent(l 16.8°) in its ground electronic state and absorbs weakly in the visible and near-IR regions (this accounts for the deep blue/violet colour of O3 in condensed phases). The visible absorption systems are predissociated due to the low dissociation energy of O3, Dq(0 — O2) = 101 2 kJ mol, and the presence of numerous low-lying repulsive electronic states. Photodissociation in the near-IR and visible (the so-called Chappuis band) regions leads to ground electronic state products, i.e. 

It is now well established that photodissociation via the Hartley band produces mainly (90 per cent) electronically excited states of both O2 and the oxygen atom, i.e. 

A second channel is also observed (10 per cent) leading to both O2 and an oxygen atom in their ground electronic states, but with the O2 being highly vibra-tionally excited, i.e. 

Clearly, there is a very large energy release and a bimodal vibrational distribution, with peaks at V = 14 and v = 27 is observed. A simple impulsive model, assuming a geometry close to that of the ground state, explains the main features in the observed energy release. 

Absorption by O3 in the UV not only protects the Earth s surface from harmful UV radiation, it also leads to a warming of the stratosphere, as the oxygen atoms produced in the above processes rapidly combine with O2 to reform O3, i.e. 

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FIdthmann H, Beck C, Schinke R, Woywod C and Domcke W 1997 Photodissociation of ozone in the Chappuis band. II. Time-dependent wave packet calculations and interpretation of diffuse vibrational... 

Table 1. Vibrational populations of 02(1AS) from the photodissociation of ozone with light of wavelength A.

The total fraction of C>2(v > 26) generated from the photodissociation of ozone at a particular wavelength is given by... 

The bimodal velocity distribution of the 0(3Pj) fragments produced via the triplet channel in the UV photodissociation of ozone has also been observed by Syage41,43>46 and Stranges et al.AA at photolysis wavelengths of 226 and 193 nm, respectively. Both authors measured anisotropy parameters for the fast and slow product pathways separately. 

The photodissociation of ozone (2,A,20) in the ultraviolet region is important in atmospheric chemistry, since its main product, 0(1d), reacts further with H2O and N2O to produce OH and NO (2). Recent studies (72-76) have shown that in the Hartley continuum (250-300 nm), 90% of the process is ... 

Fiocco G., Mugnai A. and Forlizzi W., Effects of radiation scattered hy aerosols on the photodissociation of ozone. J. Atmos. Terr. Phys., 40, 949-961 (1978). 

Petroncelli P., Fiocco G. and Mugnai A., Annual variation of the effects of diffuse radiation on the photodissociation of ozone. Pageoph., 118, 20-34 (1980). 

Woywod C., Stengle, M., Domcke, W., Flothmann, H. and Schinke, R. (1997) Photodissociation of ozone in the Chappuis band. I. Electronic structure calculations, J. Chem. Phys. 107, 7282-7295. 

Figure 10-11 and its caption (Crutzen, 1983) depict the most important of the gas phase and photochemical reactions in the atmosphere. Perhaps the single most important interaction involves the hydroxyl free radical, OH . This extremely reactive radical is produced principally from the reactions of electronically excited atomic oxygen, 0( D), with water vapor. Photodissociation of ozone produces 0( D) and also the less reactive 0( P). In the troposphere, O3 is produced largely by photo-... 

Ozone also absorbs in the visible region via the Chappuis bands (Figure 4.35). This spectral regime contributes significantly to the photodissociation of ozone and plays a dominant role in the lower stratosphere and troposphere (z < 25 km). 

Ball, S.M., G. Hancock, and F. Winterbottom, Product channels in the near-UV photodissociation of ozone. Faraday Discuss 100, 215, 1995. 

Takahashi, K., N. Taniguchi, Y. Matsumi, M. Kawasaki, and M.N.R. Ashfold, Wavelength and temperature dependence of the absolute 0(1D) production yield from the 305-329 nm photodissociation of ozone. J Chem Phys 108, 7161, 1998. 

The production of O( S) in the photodissociation of ozone becomes energetically possible at wavelengths less than 237 nm, if a ground-state 02... 

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 irritation of air enriched solely with NO or NO2 did not produce photo-oxidants this only occurred when hydrocarbons were also present in the polluted urban atmosphere. This leads to a build-up of tropospheric ozone, and hence to faster rates of photoinitiation through photodissociation of ozone, and then to a further build-up of ozone, and so on. Besides ozone, which is toxic at low concentrations (0.1-1 ppmv), other intermediates responsible for adverse effects include aldehydes and organic nitrates, such as peroxyacetyl nitrate (PAN). 

Le Quere, F. and Leforestier, C. (1991) Hyperspherical formulation of tlie photodissociation of ozone,, 7. Chern. Phys. 94, 1118-1126. 

As an example of a chemical family, consider photodissociation of ozone to produce 02 and ground-state atomic oxygen ... 

The photodissociation of ozone reverses the reaction that forms it. We thus have a cycle of ozone formation and decomposition, summarized as follows ... 

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