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How can one photon trigger the formation of many hydrogen chloride molecules?

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A single photon can trigger the formation of many hydrogen chloride (HCl) molecules through a process known as a chain reaction. This process is initiated by the absorption of light, which provides the energy necessary to dissociate a chlorine molecule ( $C l_{2}$ ) into two highly reactive chlorine atoms ( $\cdot C l$ ).12

Once these chlorine atoms are generated, they participate in a two-step propagation cycle:

A chlorine atom reacts with a hydrogen-containing molecule (such as $H_{2}$ or a hydrocarbon) to produce hydrogen chloride and a new radical species.12
This radical then reacts with another chlorine molecule ( $C l_{2}$ ), which produces the desired product and regenerates a chlorine atom.12

Because the chlorine atom is regenerated in the second step, it is free to initiate the cycle again. A single photon-induced chlorine atom can thus catalyze the production of many molecules of hydrogen chloride before the chain is eventually terminated by the combination of radicals. This mechanism is a fundamental example of chemical kinetics where a transient intermediate is continually recycled to sustain the reaction.12