Why can quantum yield be very high in a chain reaction?

In a photochemical reaction, the quantum yield represents the number of product molecules formed per photon absorbed. While the Stark–Einstein law suggests an ideal yield of unity, the value can be significantly higher—sometimes reaching the order of 1,000,000—when the primary product initiates a chain reaction.1

A chain reaction achieves this high yield because the initial absorption of a photon creates a reactive species, such as a free radical, which then participates in a series of repeating propagation steps. For example, in the synthesis of chloromethane, a chlorine atom produced by ultraviolet irradiation reacts with methane to form a methyl radical, which subsequently reacts with a chlorine molecule to produce the final product and regenerate a chlorine atom. Because this regenerated atom can continue the cycle for many additional steps before a termination step occurs, a single absorbed photon can result in the formation of a vast number of product molecules.21