A photochemical reaction's quantum yield:
The following relationship illustrates how effective a photochemical reaction is:
Φ= Number of molecules decomposed/ Number of quanta absorbed.
Quantum efficiency is the name given to this ratio, which has a very wide range of variation. While the quantum efficiency of the discoloration of methylene blue by light barely exceeds 10 to the power of minus four, the quantum yield of the chain reaction of chlorine with hydrogen reaches 10 to the power of 5, meaning that the absorption of a luminous quantum results in the appearance of 100,000 HCl molecules.
- Because the substance does not absorb all of the photons efficiently, the quantum yield is frequently less than 1. But in the case of a chain reaction, where a single photon can start a lengthy series of transformations, the quantum yield can be greater than 1.
Some factors that affect affect the quantum yield:
-the composition, position, and dosage of the reactive chromophores.
A conjugated chromophore- retinal- Source: wikipedia
-the environment's solvent type.
-the incident light's wavelength.
-Temperature (The quantum yield will decrease at higher temperatures).
- The ions and radicals produced during the primary act are the most active participants in secondary reactions. We will only mention the most recognizable photochemical reactions among the many that are known to occur.
When atoms in a monoatomic gas or vapor absorb light radiation, they gain energy and re-emit it as fluorescence. However, some of this energy can be utilized in a chemical reaction when excited atoms collide with molecules. A wavelength of 2537 angstroms is absorbed by mercury vapor. Exceeding energy of 112.6 Kcal is present in one mole of excited mercury atoms. It is possible to start reactions that use less energy per mole by using this energy accumulation.
Thus, since the dissociation of hydrogen molecules into atoms only needs an energy of 103 Kcal/mole, mercury vapor excited by radiation of the above-mentioned wavelength is capable of doing so.
Sensitization:
Mercury vapor heated to around 200°C, via interaction with propane molecules facilitates the loss of hydrogen atoms and the formation of corresponding radicals:
Hg+ hv=== Hg*
Hg*+ C3H8=== Hg+ C3H7+ H
Mercury vapor causes hydrogen or hydrocarbons to become sensitive to the action of light; this phenomenon is known as sensitization.
References:
[Smail Meziane: Livre Chimie générale- Structure de la matiére. Berti edition, Alger, 2006]
[Principles & Applications of Photochemistry, Brian Wardle,Wiley, ISBN 0470014938]
[R. OUAHES et B. DEVALLEZ- Chimie générale- Office des publacations universitaires- Alger]
[A. Schirrmacher, Experimenting theory : the proofs of Kirchhoff's radiation law before and after Planck, Münchner Zentrum für Wissenschafts und Technikgeschichte, 2001]
D. M. Jameson, Introduction to Fluorescence. 1st ed. CRC Press; 2014
Quantum yield variation across the three pathways of photosynthesis: not yet out of the dark