Quantum Yield (Φ)
Definition
Photons emitted divided by photons absorbed. A unit-less efficiency between 0 and 1. Quinine sulphate in 0.5 N sulphuric acid sits at Φ ≈ 0.55 and is the canonical reference standard.
- Symbol
- Phi
- Range
- 0 to 1 (unit-less)
- Reference standard
- Quinine sulphate in 0.5N sulphuric acid
- Reference value
- Phi approx. 0.55
Common questions
Why is quinine sulphate used as the reference standard for quantum yield?+
Its quantum yield in 0.5N sulphuric acid is well characterised and reproducible, so unknown fluorophores are measured against it by comparing integrated emission intensity and absorbance under matched conditions.
What causes a fluorophore's quantum yield to fall below 1?+
Any process that returns the excited molecule to the ground state without emitting a photon, such as internal conversion, intersystem crossing, or collisional quenching, competes with emission and lowers the yield.
Related terms
- Fluorescence
- Light emission following absorption of light at a shorter (higher-energy) wavelength. Fluorescein absorbs blue-green excitation light and emits yellow-green light. The emitted...
- Jablonski Diagram
- An energy-level diagram showing the ground state, singlet excited states, and triplet state of a molecule, with arrows indicating absorption, internal conversion,...
- Phosphorescence
- Light emission from the triplet excited state (T1) via a spin-forbidden radiative transition; characterised by emission persisting after the excitation source is...
- Phosphoroscope
- A rotating chopper that alternately blocks the excitation beam and the detector. Fluorescence dies during the blocked window, phosphorescence persists, so only...
- Stokes Shift
- The wavelength difference between the excitation peak and the emission peak of a fluorophore. A positive Stokes shift (emission at longer wavelength...