Picosecond pulse radiolysis studies of the primary processes in hydrocarbon radiation chemistry

Abstract

Picosecond pulse radiolysis studies have been performed on n-pentane, n-hexane and cyclohexane to learn about the primary radiolytic processes occurring in these alkanes. The transient absorption profiles obtained result from the superimposition of several processes: the geminate recombination between positive ions and electrons; the disappearance of highly excited alkali radical-cations (by spontaneous fragmentation of C-H bonds or dimolecular reaction with the solvent); and the exponential decay of the singlet solvent excited states. Fragmentation of excited alkane positive ions appears to be faster when the number of carbon atoms decreases in the aliphatic chain: it becomes so fast in n-pentane that only pentene positive ions are detected after 25 ps pulses. The recombination kinetics between pentene holes and the electrons is similar to that predicted for a geminate ion decay in which the spatial distribution function of ions is exponential. Contrary to linear alkane cations, cyclohexane holes do not exhibit a pronounced tendency to dissociate. The short lifetimes of the n-pentane and n-hexane S 1 states mean that the absorption of the S 1 state is not important; in contrast, the S 1 state of cyclohexane decays more slowly and thus contributes much more to the optical absorption. A non-negligible fraction of cyclohexane S 1 states are generated by geminate recombination.