Photochemical Quantum Yield
The number of molecules reacting for each photon of light absorbed by the system.
Formula / equation
Φ = molecules reacted / photons absorbed
Unit
Dimensionless
In depth
A quantum yield near one indicates a simple photochemical process, while values far above one prove that a chain mechanism is operating. Values well below one show that most excited molecules lose their energy harmlessly by fluorescence or collision before reacting.
Examples in the real world
The H₂ + Cl₂ chain reaction reaches quantum yields around 10⁶; many photodissociations give values near 1.