Molecular Oxygen

The absorption of light by both molecular oxygen and ozone is a strong determinant of the intensity and wavelength distribution that reaches the troposphere and stratosphere and hence is available to cause photochemical reactions. 02 absorbs light particularly strongly in the ultraviolet at wavelengths below ~ 200 nm, giving rise to the term "vacuum ultraviolet" for this region; experimental studies involving light in this wavelength range require an optical path from which air has been removed.

1. Absorption Spectra

The potential energy curves for the ground state and for the first four electronically excited states of 02 are shown in Fig. 4.1. The ground state, X3X~, is unusual in that it is a triplet; as a result, only transitions to upper triplet states are spin-allowed. The transition from the ground state to the A3£ + state is also theoretically forbidden because it involves a ( + )-»( — ) transition (see Chapter 3.A.1); however, this X32~-> A3S„ transition does occur weakly, resulting in weak absorption bands (cr < 10"23 cm2 molecule-1) known as the Herzberg continuum at wavelengths between ~ 190 and 300 nm. Further details on the spectroscopy can be found in Slanger and Cosby (1988).

The absorption cross sections between 205 and 240 nm recommended by the NASA evaluation (DeMore et al., 1997) are shown in Table 4.1.

The X3X~—> B3X~ transition is allowed and as seen in Figs. 4.2 and 4.3 results in an absorption in the 130-to 200-nm region known as the Schumann-Runge system. The banded structure from about 175 to 200 nm corresponds to transitions from v" = 0 as well as v" =

1 (i.e., hot bands) of the ground X J" state to different vibrational levels of the upper state.

The upper 3X„ state is crossed by the repulsive 311U state (Fig. 4.f) at ~ v' = 4, providing a mechanism for the production of two ground-state 0(3P) atoms from the B3XU state. The absorption spectrum becomes continuous at ~ 175 nm, with a strong absorption

FIGURE 4.1 Potential energy curves for ground and first four excited states of 02. S-R = Schumann-Runge system, H = Herzberg continuum, A-A = atmospheric bands (adapted from Gaydon, 1968).

down to ~ 130 nm. This continuum is believed to be due to dissociation of the B3X~ state to 0(3P) + O('D). Below 130 nm, a banded absorption again appears, as seen in Fig. 4.4; at wavelengths below 133.2 nm, there is sufficient energy to produce O('S) atoms. There is a minimum in the absorption at 121.6 nm,

TABLE 4.1 Absorption Cross Sections (Base e) for 02 between 205 and 240 nm"

Wavelength

1024<T

Wavelength

1024<t

(nm)

(cm2 molecule

') (nm)

(cm2 molecule ])

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