Why can a photochemical reaction produce more product molecules than photons absorbed?

A photochemical reaction can produce more product molecules than the number of photons absorbed because the initial light-induced event is often followed by a chain reaction.21

While the photochemical equivalence law (or Stark–Einstein law) states that one primary chemical act occurs per photon absorbed, this law applies only to the initial "primary process." In many cases, this primary process generates highly reactive species, such as radicals, which then participate in subsequent "secondary processes."2134

These secondary steps do not require the further absorption of light. Instead, the reactive intermediates collide with other molecules to propagate the reaction repeatedly. In such chain reactions, a single photon can initiate a sequence that yields a large number of product molecules—sometimes reaching a quantum yield on the order of 1,000,000. A classic example of this phenomenon is the synthesis of chloromethane from methane and chlorine.32