Secondary Processes Involving Atoms and Radicals

When the primary photochemical step yields atoms and free radicals, the secondary processes will involve interactions of these species with other molecules. Such processes result either in propagation or termination of the reaction. 

An example of the first type is the process Cl -j- Hg HCl -j- H, next to Clg -r hv — Cl 4- Cl in the photosynthesis of hydrogen chloride from hydrogen and chlorine. A process of the second type is 0 + O3 — 2 Og, following the primary process O3 -f- hv - Og + 0 in ozone photolysis with visible light. 

The nature of termination processes can often be estimated from the experimentally obtained dependence of the photochemical reaction rate on the light intensity. Indeed, when atomic recombination plays no significant part in the balance of atoms, their steady concentration and consequently the reaction rate are proportional to the first power of light intensity. Conversely, when the main termination process is the recombination of atoms and its rate substantially exceeds the summary rate of aU other processes involving atoms, the steady concentration of atoms and the rate of the stationary reaction are proportional to the square root of light intensity. In the general case, the reaction rate is approximately defined by a power dependence with an exponent between 1 and 1/2. 

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