Quantum Yield or Photonic Efficiency Φ
In photochemistry, the quantum yield or photonic efficiency is an important parameter that quantifies the efficiency of a photochemical process. It measures the number of specific events (such as chemical reactions or emitted photons) that occur per photon absorbed by the system.
General Formula for Quantum Yield (Φ):
For a specific photochemical reaction:
Events of interest could be the formation of a product, the dissociation of a molecule, or some other chemical change.
Photons absorbed refers to the amount of light energy absorbed by the chemical system, typically measured in moles of photons (also called einsteins).
Steps to Calculate Photonic Efficiency:
Measure the number of events of interest: This could be the formation of a product or disappearance of a reactant, often measured in moles.
Measure the number of absorbed photons: This is the number of photons that have been absorbed by the system. It can be calculated from the intensity of the light source, its wavelength, and the time of irradiation.
Divide the number of events by the number of absorbed photons to get the quantum yield.
Factors affecting photonic efficiency:
Wavelength of the light: The energy of photons varies with wavelength, which can affect whether a photon has enough energy to induce the reaction.
Absorption spectrum: Not all photons incident on the sample are absorbed; only those matching the absorption spectrum of the reactant will be effective.
Experimental setup: Conditions such as temperature, solvent, and concentration also influence the efficiency of the photochemical reaction.
The quantum yield can vary depending on the reaction mechanism and external factors, and its value can provide insights into the dynamics of the reaction.
In hydrogen evolution photochemistry, the quantum yield (Φ) measures how efficiently absorbed photons are used to produce hydrogen gas (H₂). It quantifies the number of moles of hydrogen produced per mole of photons absorbed.
Formula for Quantum Yield in Hydrogen Evolution:
Steps to Calculate Quantum Yield in Hydrogen Evolution:
1_Calculate the number of moles of hydrogen produced (nₕ₂):
Use the volume of hydrogen gas evolved (in liters) and apply the ideal gas law if needed, or use direct measurements if available.
where 22.414 L/mol is the molar volume of an ideal gas at standard temperature and pressure (STP).
2_Calculate the number of moles of absorbed photons:
Use the intensity of the light source and the irradiation time to calculate the energy of the light absorbed by the system.
where Ephoton is the energy of a photon, given by:
h is Planck's constant (6.626 × 10⁻³⁴ J·s),
c is the speed of light (3.00 × 10⁸ m/s),
λ is the wavelength of light (in meters).
3_Combine the results:
Once you have the number of moles of hydrogen produced and the number of moles of photons absorbed, substitute them into the quantum yield formula:
In some cases, you may also encounter apparent quantum yield (AQY), which is calculated similarly but uses the total incident photons (not just absorbed photons). This differs slightly from the standard quantum yield
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