Quantum yield is the ratio of the number of molecules reacted or formed to the number of photons of light absorbed in a photochemical reaction. It measures the efficiency of the reaction.
Alternatively, in molar terms:
Note: One mole of photons is also known as one Einstein.
To determine the quantum yield (φ) experimentally, two measurements are needed in the same amount of time:
Light energy is absorbed in the form of photons.
Energy of one photon (E) is given by:
Where:
h = Planck’s constant
c = Speed of light
λ = Wavelength
N = Avogadro's number (6.02 × 10²³)
Energy of one mole of photons (one Einstein, E):
Dimensionless (no unit).
Because it is a ratio: molecules / molecules or moles / moles.
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Quantum Yield: Definition, Formula & Experimental Determination is a fundamental concept in physical chemistry. Understanding the mechanisms, reaction conditions, and stereo-chemical outcomes is crucial for mastering organic chemistry. Our curated resources provide step-by-step visualizations to help you excel.
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What is Quantum Yield (Φ)?
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A quantum yield (Φ) greater than 1 indicates a chain reaction. A single absorbed photon initiates a primary process that produces highly reactive species (like free radicals), which then go on to cause many secondary reactions.
This happens when the activated molecules lose their energy through other processes (like fluorescence, phosphorescence, or collisions with other molecules) before they have a chance to react and form the product.
The Stark-Einstein law states that each molecule taking part in a primary photochemical process absorbs exactly one quantum of light (one photon) to become activated.