Oxygen

By far the most abundant photochemical reaction in the upper atmosphere is the photolysis of molecular oxygen. Figure 5-1 shows the potential energy curves for the various electronic states of O2 of importance to us; Figure 5-2 is the absorption spectrum in the far-ultraviolet and vacuum ultraviolet regions (note logarithmic absorption cross section vertical axis). It was shown in Section 4.2.1 that the bond dissociation energy of oxygen (5.1 eV, or 492 kj/mol) corresponds to a photon of wavelength 243 nm. Oxygen actually begins to absorb just below this wavelength, at 242.2 nm, in a spectral region known as the Herzberg continuum. Absorption is very weak (a = 10~23 cm2/molecule at 202 nm), but the spectrum is a continuum in this region, indicating that dissociation occurs with ^ = 1. Actually, excitation is to the weakly bound3state (Figure 5-1, curve A) which rapidly dissociates into two ground-state atoms:

3See Figure 10-6.

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FIGURE 5-1 Potential energy curves for molecular oxygen. X = 3E—, a = JAg, b = A = 3E+,B = 3E— . Drawn from data of F. R. Gilmore, J. Quant. Spectroscopy Radiat. Transfer, 5, 369-390 (1965).

Internuclear distance (nm)

FIGURE 5-1 Potential energy curves for molecular oxygen. X = 3E—, a = JAg, b = A = 3E+,B = 3E— . Drawn from data of F. R. Gilmore, J. Quant. Spectroscopy Radiat. Transfer, 5, 369-390 (1965).

120 140 160 180 200

Wavelength (nm)

FIGURE 5-2 Absorption spectrum of molecular oxygen (note logarithmic vertical axis). The dotted line is the Schumann-Runge banded region. Drawn from data of M. Ackerman, in Meso-spheric Models and Related Experiments, G. Fiocco, ed., D. Reidel, Dordrecht, 1971, pp. 149-159.

120 140 160 180 200

Wavelength (nm)

FIGURE 5-2 Absorption spectrum of molecular oxygen (note logarithmic vertical axis). The dotted line is the Schumann-Runge banded region. Drawn from data of M. Ackerman, in Meso-spheric Models and Related Experiments, G. Fiocco, ed., D. Reidel, Dordrecht, 1971, pp. 149-159.

The very weak absorption shown in this case is a good example of the consequence of selection rule violation, the specific one in this case being that excitation from a negative (superscript —) to a positive (superscript +) state is a forbidden transition. The other selection rules given in Section 5.1 for diatomic molecules are obeyed, however.

Below 205 nm the absorption spectrum becomes much stronger and banded (the Schumann-Runge bands). The appearance of the banded spectrum rather than a continuum shows that absorption is now to a different potential energy curve from which dissociation no longer occurs at these energies. The Schumann-Runge bands get closer together as the wavelength decreases, and at 175 nm the bands blend together into a continuum known as the Schumann-Runge continuum which reaches maximum absorption at approximately 147 nm. Excitation in this Schumann-Runge region is to the bound 3£— state, curve B of Figure 5-1:

This is an allowed transition, hence has a large absorption cross section (o-max = 1.5 x 10-17cm2/molecule). The 3X- state dissociates at wavelengths shorter than 175 nm into one ground-state and one excited (*D2) oxygen atom:

The difference between the bond dissociation energy of 02 into two ground-state oxygen atoms (492kJ/mol) and the start of the Schumann-Runge continuum (175 nm, corresponding to a photon energy of 682 kJ/einstein) is 190kJ/mol, which is the energy needed to excite an oxygen atom from its ground (3P2) state to the excited (*D2) state. This is a large excitation energy, making the *D2 atom a very reactive and important species in stratospheric and tropospheric photochemistry.

Below 130 nm there is sufficient energy to produce oxygen atoms in even higher electronic states, and some dissociation to the *S0 state also takes place, hv (< 130 nm) ,1 „ x ,

and below 92.3 nm two excited atoms are produced

However, the ionization potential of 02, 12.15 eV, corresponds to a photon of wavelength X = 102 nm, and therefore a more likely reaction below 102 nm is photoionization hv (< 1°2 nm) + -

followed by dissociative recombination. Reactions (5-17) and (5-18) are two possible examples of this type of process:

with (5-17) found to be the favored reaction.

Two other excited electronic states of O2 are important in atmospheric photochemistry because of specific reactions to be discussed later with reference to their possible roles in the photochemical smog cycle, ozone photolysis, and photooxidation reactions with olefins. These are the 1Ag ("singlet oxygen") and 1Sg states, with energies 94.1 and 157kJ/mol, respectively, above the ground state (Figure 5-1). Direct excitation from the triplet ground state to either of these states is spin forbidden, and therefore oxygen absorbs only very weakly around 760nm (157kJ/mol), giving a banded spectrum called the atmospheric bands. The reverse radiative process is of course also spin forbidden and these states, when produced, are therefore relatively stable in the atmosphere, a factor contributing to their importance; the radiative lifetime of is 12 s, while that of 1Ag is 60min. Furthermore, nonreactive deactivation by collision with other species, called collisional quenching, is also inefficient, the rate constants of deactivation by collision with ground-state O2 being 5 x 10"16 and 2 x 10"18 cm3 molecule"1 s"1 for the and xAg states, respectively.4 In the absence of collisional quenching, the emissions from the normally "forbidden" radiative transitions

turn out to be two of the most intense bands in the atmospheric airglow and in auroral displays.5 The 1Sg state also relaxes to the 1 Ag state by collisional deactivation.

Ground-state oxygen atoms can combine to form ground-state O2 by the three-body recombination reaction

4If deactivation occurred at every collision, the quenching rates would be approximately 4 x 105 and 108 times faster, respectively. See Section 4.3.

5Airglow is the faint glow of the sky produced by solar photochemical processes; it occurs at all latitudes. The aurora is produced by impact of high-energy solar particles; it is more intense than the airglow, but it is irregular and occurs near the poles.

where M can be another oxygen atom or any other atomic or molecular species. If reaction (5-21) is kinetically simple and therefore trimolecular, then r = k[0]2[M] (5-22)

The rate constant k for this reaction is very nearly temperature independent. In fact it has a slightly inverse temperature behavior, the rate decreasing with increasing temperature, implying a negative activation energy, which of course is physically impossible if activation energy is considered to be the height of a potential barrier.6 The rate constant is also of the order of magnitude of the collisional frequency for triple collisions, indicating that the three-body recombination (5-21) occurs essentially at every encounter. Even so, at the very low pressures encountered in the upper atmosphere—for example, 3 x 10"7 bar at 100 km (see Table 2-4)—the rate of reaction (5-21) is small compared to the rate of 02 photodissociation, and therefore above 100 km the primary oxygen species present is atomic oxygen.7

It has been seen that excited singlet (1D2 and 1S0) atoms are formed from the photodissociation of molecular oxygen below 175 nm. These species also relax to lower electronic states with emission of light:

Again, both are "forbidden" transitions and therefore show only weak emission contributing to the airglow and aurora. Below 140 km, however, collisional deactivation to the triplet ground state with any atomic or molecular species M may become the dominant reaction, particularly with 0(*D2). If

6The reason for the negative activation energy is that oxygen atoms form an unstable intermediate complex with M, 0'M, which then reacts with another 0 atom to produce 02. As the temperature is raised, 0'M becomes more unstable so that its concentration is decreased, thereby decreasing the rate at which 0.M reacts with 0, hence decreasing the overall rate of 02 formation represented by reaction (5-21). Since reaction (5-21) is now kinetically complex, as pointed out in Section 4.3.1, the interpretation of the experimental activation energy as the height of a potential energy barrier is not valid. Assuming equilibrium between 0, M, and 0.M leads, however, to the same rate law as that for the trimolecular reaction (5-22). At extremely high pressures, well beyond those in any part of the atmosphere, the order of the reaction would decrease from third order to second order.

It should be noted that radiative recombination processes are possible:

However, these reactions are also governed by spin conservation and atomic radiative selection rules, and they also are negligible in comparison to 02 photodissociation.

M is ground-state O2, electronic energy is transferred and O2 is excited to its state

which decays collisionally to the !Ag state:

An important aspect of this energy transfer or sensitization is that spin is conserved. That is, the oxygen atom undergoes a singlet-triplet transition while the reverse triplet-singlet reaction occurs with molecular oxygen, so that total reactant and product spins are the same. This is a very efficient process under exothermic conditions, probably occurring within an order of magnitude of collisional frequency, and therefore reaction (5-28) is one of the major sources of "singlet oxygen" O2(! Ag) in the atmosphere.

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