Pressure Dependent Product Formation in the Photochemically Initiated Allyl+ Allyl Reaction

Photochemically driven reactions involving unsaturated radicals produce a thick global layer of organic haze on Titan, Saturn’s largest moon. The allyl radical self-reaction is an example for this type of chemistry and was examined at room temperature from an experimental and kinetic modelling perspective. The experiments were performed in a static reactor with a volume of 5 L under wall free conditions. The allyl radicals were produced from laser flash photolysis of three different precursors allyl bromide (C₃H₅Br), allyl chloride (C₃H₅Cl), and 1,5-hexadiene (CH₂CH(CH₂)₂CHCH₂) at 193 nm. Stable products were identified by their characteristic vibrational modes and quantified using FTIR spectroscopy. In addition to the (re-) combination pathway C₃H₅+C₃H₅ → C₆H₁₀ we found at low pressures around 1 mbar the highest final product yields for allene and propene for the precursor C₃H₅Br. A kinetic analysis indicates that the end product formation is influenced by specific reaction kinetics of photochemically activated allyl radicals. Above 10 mbar the (re-) combination pathway becomes dominant. These findings exemplify the specificities of reaction kinetics involving chemically activated species, which for certain conditions cannot be simply deduced from combustion kinetics or atmospheric chemistry on Earth.