What defines quantum yield (Φ)?
Quantum efficiency quantifies how effectively absorbed photons convert to fluorescence. Defined as ratio of photons emitted to photons absorbed, symbol Φ, ranging zero to one. It equals radiative rate divided by sum of radiative plus all non-radiative decay rates, including internal conversion, collisional quenching and intersystem crossing. Bright fluorophores exhibit high yield approaching unity, dim ones lose energy via heat. This parameter distinct from molar absorptivity or absorbance logarithm. Determination uses reference dyes of known yield, crucial for selecting probes for quantitative imaging.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.