There are many reactions, where in the reactants undergo transformation not by direct absorption of photons but via the process of photosensitization. In such reactions, a photosensitive substance is added which actually absorbs the photon. The absorbed energy is then passed on to one of the reactants. The photosensitiser does not undergo any chemical transformation, it only initiates a chemical reaction.
Formation of formaldehyde from hydrogen and carbon monoxide in the presence of trace amounts of mercury vapor
Example 21:
The reactions that occur in these photosensitization reactions are as follows.
In the reaction, some glyoxal HCO.HCO is formed by dimerization of formyl radicals. In this reaction mercury acts as the photosensitizer.
