Experimental Evidence Helps to Determine Mechanism Chlorination does not occur at room temperature in the dark. The most effective wavelength of light is blue that is strongly absorbed by Cl2 gas. The light-initiated reaction has a high quantum yield (many molecules of product are formed from each photon of light) C photon (v) :C+C: Experimental Evidence Helps to Determine Mechanism • Chlorination does not occur at room temperature in the dark. • The most effective wavelength of light is blue that is strongly absorbed by Cl2 gas. • The light-initiated reaction has a high quantum yield (many molecules of product are formed from each photon of light)