The Radiation Chemistry of Acetylene: I. Rare Gas Sensitization. II. Wall Effect in Benzene Formation

In the rare gas-sensitized radiolysis of acetylene, with helium, argon, krypton, and xenon, cyclization to benzene is completely eliminated. In the direct radiolysis the fraction of reacted acetylene going to benzene, which is constant at 0.2 at high pressures, is found to decrease sharply at very low pressures. This is shown to be due to a wall effect. From a consideration of the experimental conditions it is shown that benzene precursors reaching the wall have a lifetime of at least 10/sup -4/ sec. These new observations, together with the earlier kinetic studies, are consistent with a mechanism involving a complete duality of primary process in which the precursors of the polymer and of benzene are distinct. It is suggested that ionization of the acetylene leads to polymerization, whereas cyclization requires electronic excitation of the acetylene, probably to a triplet state. (auth)