Quantum Yield -

It is also called quantum yield or quantum efficiency. It is the number of chemical changes that occur per quantum of light absorbed in a system, i.e., the number of molecules that disappear or are created. If a molecule that absorbs a quantum of light always completes one reaction, the quantum yield will be 1. If a chain reaction is involved, the quantum yield will be greater than 1. For example,

In this case, the quantum yield of HCl production reaches 10 ⁵ .

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

量子収率または量子効率ともいう．系に吸収された光量子1個当たりに生じた化学変化数，すなわち消滅した分子数，または生成した分子数をいう．光量子を吸収した分子が，つねに一つの反応を完結すれば量子収量は1になる．連鎖反応が含まれていれば量子収量は1以上になる．たとえば，

の場合，HCl生成の量子収量は 10⁵ にも達する．

出典　森北出版「化学辞典（第2版）」化学辞典 第2版について　情報