Photolysis of cis-Hexene−2 and cis-Butene−2 at 2026 and 2062 Å

cis‐Hexene−2 and cis‐butene−2 were photolyzed directly using 2026‐ and 2062‐Å radiation at varying methane pressures up to 52 cm and olefin pressures of 2 cm. Collisional stabilization of the trans isomer in each case competes with the first‐order decomposition reactions: $$\begin{array}{c}{\mathrm{C}}_6{\mathrm{H}}_{12}^{*}\to {\displaystyle {lim}_{\text{(1)}}}{\mathrm{C}}_4{\mathrm{H}}_7+{\mathrm{C}}_2{\mathrm{H}}_5,\\ {\mathrm{C}}_4{\mathrm{H}}_8^{*}\to {\displaystyle {lim}_{\text{(2)}}}{\mathrm{C}}_3{\mathrm{H}}_5+{\mathrm{CH}}_3,\\ \to {\displaystyle {lim}_{\text{(3)}}}{\mathrm{C}}_4{\mathrm{H}}_7+\mathrm{H}.\end{array}$$ From the observed pressure dependencies, the values of the first‐order rate constants are k₁=8.2×10⁷ sec⁻¹, k₂=5.0×10⁸ sec⁻¹, k₃=3.5×10⁸ sec⁻¹. Quantum yields for the total of trans olefin and cleavage Reaction (2) are of the order of unity for both butene−2 and hexene−2 photolyses. It is concluded that the cleavage reactions occur after rapid internal conversion to the ground electronic states. The cis—trans isomerization and an isomerization to the 1‐olefin (5% of total reaction) occur in a time short compared to the time between deactivating collisions at the highest pressures.