The Mechanism of Excited State Formation in the Radiolysis of Hydrocarbon Liquids.

A variety of saturated hydrocarbon liquids were chosen as model systems to investigate the primary processes that occur upon excitation with ('85)Kr beta particles. Radiolytic yields (G values) were measured for the production of the lowest excited singlet states of cyclohexane, methylcyclohexane, 2,3-dimethylbutane, dodecane, hexadecane, cis-decalin, and bicyclohexyl. The measurement is based on a direct fluorescence technique utilizing the emission properties of these liquids and directly compared to neat benzene. The results obtained in this manner with those obtained by a solute sensitization technique indicate that the solute technique may strongly underestimate the true G value. The failure of the solute sensitization technique was further investigated via the measured energy transfer efficiencies of a large number of donor plus acceptor systems. From the concentration dependence of perfluorocarbon electron scavengers on the fluorescence intensity of a typical saturated hydrocarbon liquid (i.e. bicyclohexyl) when excited with beta particles and 184.9 nm light, two relevant questions pertaining to the geminate ion recombination process were answered. That is, (i) What contribution does the geminate ion recombination process make to the total yield of the lowest excited singlet and triplet states?, and (ii) What is the probability (averaged over the spur size distribution) that a geminate ion pair will generate an excited singlet state on recombination? This latter value is compared to theoretical values and it is concluded that in bicyclohexyl the geminate electron does not spin relax prior to recombination. Preliminary results for p-xlene are also presented and compared to bicyclohexyl.