A measure of how efficient an emission process is. The quantum yield is a ratio of absorbed photons and photons used up in the chemical process. Quantum yield is usually dependent on temperature, pressure, and wavelength. [Chemosphere; v51; 175-179; 2003.] [Atmospheric Environment; v37; 1835-1851; 2003.] [Science; v299; 1566-1568; 2003.]