Photochemical Chain Reactions in Amorphous Solids

In this article, we review recent work in the authors' laboratory on the subject of free radical chain reactions of chlorine with small hydrocarbons in amorphous solids. The solids are formed as thin films by spray deposition of the gaseous reagents onto a cryogenic window. Reactions are initiated by excimer laser photolysis at 308 nm, which dissociates a small fraction of the chlorine molecules to atoms. Product yields and branching ratios are determined by infrared absorption spectroscopy. Reactions of chlorine with cyclopropane or cyclobutane proceed by true chain reactions, as evidenced by high product quantum yields (number of product molecules formed per laser photon absorbed by the sample). Measurements of the dependence of the product yield on the relative concentrations of chlorine and hydrocarbon provide clues to the reaction mechanism in the solid state. The cyclobutane reaction appears to involve H atom transfer from cyclobutane to cyclobutyl radical as an intermediate step in the overall reaction. Reaction of chlorine with propane, n-butane, or isobutane does not appear to involve chain propagation and is dominated by radical recombination processes which result in low quantum yields. All of these results are discussed in terms of reactions which occur in a solid state environment where molecular motion is severely restricted.