Molecular Detachment Processes in the Vacuum uv Photolysis of Gaseous Hydrocarbons. I. Ethylene. II. Butane

Abstract

The photolysis of ethylene and of butane has been studied at room temperature with light of 1470 A. The results of isotopic studies, along with a detailed examination of the products of reaction, establish conclusively that molecular detachment processes are of major importance in the primary decomposition of the photoexcited states formed. In the photolysis of ethylene at 1470 A the primary processes are $$\begin{array}{cc}{\mathrm{C}}_2{\mathrm{H}}_4^{*}={\mathrm{C}}_2{\mathrm{H}}_2+{\mathrm{H}}_2& {\phi }_1\\ {\mathrm{C}}_2{\mathrm{H}}_4^{\mathrm{**}}={\mathrm{C}}_2{\mathrm{H}}_2\text{+2}\mathrm{H}& {\phi }_2\end{array}$$ with φ₁≅φ₂. Rupture of only a single carbon‐hydrogen bond is not an important primary process. The foregoing reactions, along with the subsequent interactions of ethyl radicals, formed by hydrogen atom addition to ethylene, furnish a unique description of almost all the photochemistry observed under the conditions of the experiments. In the photolysis of butane at 1470 A, molecular detachment of hydrogen: $${\mathrm{C}}_4{\mathrm{H}}_{10}^{*}={\mathrm{C}}_4{\mathrm{H}}_8+{\mathrm{H}}_2$$ is a major primary process. The detailed nature of any other primary reactions has not been established.