How does mercury vapor help ethylene decompose in a photosensitized reaction?

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Underwater view of a glowing chain of tunicates with a scuba diver and boat visible near the water's surface in the background. — photochemical reaction Chain of fluorescent tunicates.

In a photosensitized reaction, mercury vapor acts as an intermediary to facilitate the decomposition of ethylene because the ethylene molecules themselves do not efficiently absorb the light provided by the mercury lamp.1

The process occurs through the following steps:

Energy Absorption: Mercury atoms absorb radiation at specific wavelengths (1849 and 2537 angstroms), which triggers an electronic transition to an excited state.1
Energy Transfer: When these excited mercury atoms collide with ethylene molecules, they transfer their absorbed energy to the ethylene. In this process, the mercury atoms are deactivated and return to their initial energy state.1
Decomposition: The now-excited ethylene molecules possess sufficient energy to undergo decomposition, breaking down into products such as acetylene and hydrogen.1

This technique of photosensitization is essential for driving chemical reactions that require specific light wavelengths that the primary reactants cannot absorb directly.1