quantum yield
/ KWON-tum yeeld /
Quantum yield asks a simple bookkeeping question: out of all the photons a molecule swallows, what fraction does it pay back as light? Picture a vending machine that takes a hundred coins; if it dispenses ninety snacks and pockets ten, its yield is ninety percent. A fluorophore that re-emits ninety of every hundred photons it absorbs has a quantum yield of 0.9.
More precisely, the fluorescence quantum yield is the number of photons emitted divided by the number absorbed — a value between 0 and 1. The energy that does not come back out as light is lost through other routes, mainly turning into heat or being passed off to neighboring molecules.
It matters because the brightness of a fluorescent signal depends directly on quantum yield: a high-yield dye glows brightly and is easy to detect, while a low-yield one stays dim no matter how much light you pour in. The honest caveat is that quantum yield is not fixed — temperature, solvent, pH, and nearby quenchers can all push it up or down.
Fluorescein in mild alkaline solution has a quantum yield near 0.9 — nearly every absorbed photon comes back as light. Dissolve oxygen into the same solution and the yield drops, because oxygen quenches the excited molecules before they can glow.
Quantum yield sets how brightly a fluorophore pays back the light it absorbs.
Quantum yield measures the fraction of absorbed light re-emitted; it says nothing about how much light a molecule absorbs in the first place. Overall brightness is roughly the molar absorptivity multiplied by the quantum yield.