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FIGURE 4.9 Some measurements of the quantum yields for production of 0('D) in the photolysis of O, at 298 K.

production in the region from approximately 306 to 324 nm. As expected, if this is the case, the quantum yield in this region decreases with temperature, since the Boltzmann population of the absorbing, vibrationally excited state of 03 decreases; for example, at 320 nm and 298 K, </>[0('D)] = 0.15, but decreases to 0.06 at 203 K (Talukdar et al., 1998).

The second process generating small amounts of O('D) is thought to be the spin-forbidden reaction

This spin-forbidden process appears to be the major source of O('D) beyond 325 nm, particularly at low temperatures where the contribution of vibrationally excited 03 is minimized. Thus, in contrast to the decreasing yields of O('D) with temperature in the 306-to 324-nm region, yields in the 325- to 329-nm region are relatively constant with temperature; at lower temperatures, the quantum yield in this region approaches 0.06, which has therefore been assigned by Talukdar et al. (1998) as the upper limit for the production of O('D) in this region by reaction (8).

Table 4.6 summarizes the wavelength and temperature dependence for O('D) production recommended by Talukdar et al. (1998). Beyond 329 nm, the quantum yield from the spin-forbidden process appears to be ~ 0.05-0.06 (Silvente et al., 1997; Talukdar et al., 1998). The absorption cross sections of 03 become sufficiently small beyond 360 nm that O('D) production beyond this wavelength is not expected to be significant for atmospheric applications.

Ball et al. (1993) and Ball and Hancock (1995) measured relative yields of 02('A ), the other product expected in the spin-allowed reaction (5), and found that it too was produced at significant yields. For example, Fig. 4.10 shows the measured yields of 02(*A ) normalized to 0[O('D)] = 0.95 in the 290- to 300-nm region. The yields are similar in magnitude to those for O('D). If reaction (8) is responsible for 0( D) production in the region beyond 325 nm, the yield of 02('Ag) might also be expected to fall to zero. However, 02(*A ) may also be generated by other paths, for example, that giving 0(3P) + 02('A ), which requires light of 612 nm or shorter wavelengths (Table 4.5).

This is consistent with studies of absorption in the Huggins bands, in which the products appear to be O^'A ) and ground-state oxygen atoms (Okabe, 1978). Thus, the spin-forbidden process (9) to give electronically excited 02 and ground-state oxygen atoms must be occurring:

03 + hv -ยป 02('Ag and/or3S+) + 0(3P). (9) In the Chappuis region (440-850 nm), the products

TABLE 4.6 Parameterization of Quantum Yields for 0( D) Production from 03 Photolysis in the 306- to 329-nm Region at Various Temperatures"

Wavelength (nm)

0 0

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