S Ocio

Figure 4.38 shows the absorption spectrum of OCIO at 204 K (Wahner et al., 1987; DeMore et al., 1997). The photochemistry, which is complex, is reviewed by DeMore et al. (1997), with the recommendation that a quantum yield of f be adopted for reaction (35):

The absorption spectrum for the analogous bromine compound, OBrO, is also highly structured, extending from ~ 400 to 650 nm (e.g., Miller et al., 1997).

Figure 4.39 shows the absorption spectrum of HOC1 and Table 4.33 summarizes the absorption cross sections, for which most measurements are in relatively good agreement. The major products are OH + CI, with a quantum yield of 1 above 290 nm (e.g., Vogt and Schindler, 1992; Schindler et al., 1997):

Barnes et al. (1998) have measured the yield of OH from HOC1 photolysis and find, in addition to the strong absorption shown in Fig. 4.39, a weak absorption feature at 380 nm due to excitation to the lowest triplet state. Although the absorption cross section of this weak absorption is only 4 X 10~21 cm2 molecule-1, its contribution lowers the calculated stratospheric lifetime of HOC1 by ~ 10-20%.

Figure 4.40 shows measurements of the HOBr absorption cross sections (Benter et al., 1995; Orlando and Burkholder, 1995; Barnes et al., 1996; Rattigan et al., 1996; Deters et ai, 1996; Ingham et al., 1998). As is evident from this figure, there is significant disagreement in the absorption cross sections, particularly for the weak absorption band at wavelengths beyond 400 nm. However, this appears to be a real feature of the HOBr absorption. Thus, Sinha and co-workers (Barnes et al., 1996) monitored the production of OH as a function of photolysis wavelength in a mixture containing HOBr. Production of OH in the 440- to 540-nm region was observed, indicative of a weak absorption band here, possibly due to a transition from the ground state to a dissociative triplet state. Although it is a

FIGURE 4.35 Absorption spectrum of CIO (adapted from DeMore et al., 1997).

FIGURE 4.35 Absorption spectrum of CIO (adapted from DeMore et al., 1997).

much weaker absorption than at the shorter wavelengths, in this region the solar flux is increasing significantly (see Chapter 3.C.1). Barnes et al. (1996) and Ingham et al. (1998) calculate that at large solar zenith angles, this weak absorption could decrease the lifetime with respect to photolysis by almost a factor of two.

Because of the uncertainties in the absolute cross sections for HOBr, both the DeMore et al. (1997) recommendation and the more recent values of Ingham et al. (1998) are given in Table 4.34.

The process again appears to be primarily dissociation to OH + Br, with a quantum yield for bromine atoms of >0.95 at 363 nm (Benter et al., 1995).

FIGURE 4.36 Absorption spectrum of BrO at room temperature (adapted from Wahner et al., 1988).

FIGURE 4.36 Absorption spectrum of BrO at room temperature (adapted from Wahner et al., 1988).

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