Advanced Topic 461 of 1102 Kinetics & Equilibrium
Quantum Yield
The number of molecules reacting per photon absorbed.
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In-depth explanation of Quantum Yield
The quantum yield, written as the Greek letter phi, is the number of molecules that react for each photon absorbed by the system. It is defined as phi = (number of molecules reacted) divided by (number of photons absorbed), and it measures the efficiency of the photochemical process. A quantum yield greater than one indicates that a chain reaction is taking place, because a single photon starts a sequence of many reactions. A quantum yield below one indicates that excited molecules lose their energy by other processes, such as fluorescence or collisional deactivation, without reacting.
Going deeper: Quantum Yield
Key ideas: Compute photons absorbed as energy absorbed divided by hc/lambda, and moles of photons from Avogadro, where one einstein is 6.022 x 10^23 photons. Phi = rate of reaction / rate of light absorption. Primary quantum yield counts only the first step, while overall phi includes secondary dark reactions. Values below 1 reflect fluorescence, internal conversion and quenching.
Common mistakes and connections: Students use the energy incident rather than absorbed, or forget to convert units from joules to photons. Another mistake is claiming phi cannot exceed one since a photon is only one unit; chain reactions make phi large. Phi near 2 may signal two molecules reacting via a secondary step. Connects to photochemical reactions and chain reactions.
Visualization of Quantum Yield
Quantum yield is molecules reacted per photon: below 1 means energy is lost as heat or light, above 1 means a chain (H2 + Cl2 can reach about a million). The interactive lesson shows this as a live animation or chart.
Worked example and use case
The photochemical reaction between hydrogen and chlorine can have a quantum yield of around 10^6, because each photon starts a long radical chain.
Frequently asked questions about Quantum Yield
What is Quantum Yield?
The number of molecules reacting per photon absorbed.
Is Quantum Yield a beginner, intermediate or advanced topic?
Quantum Yield is a advanced topic in Kinetics & Equilibrium. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Quantum Yield?
Study these earlier topics in Kinetics & Equilibrium first: Chain Reactions, Oscillating Reactions, Photochemical Reactions.
What comes after Quantum Yield?
The next topics in Kinetics & Equilibrium are Reversible Reactions and Dynamic Equilibrium.
Can I practise Quantum Yield with quizzes?
Yes. The interactive lesson includes a self-check question on Quantum Yield, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Quantum Yield fits in Kinetics & Equilibrium
Quantum Yield is topic 42 of 91 in Kinetics & Equilibrium.