The mechanisms responsible for the main products in liquid benzene radiolysis (biphenyl, molecular hydrogen, and phenyl radical) are probed with protons, helium ions, and carbon ions of a few to 30 MeV energy. Phenyl radical yields have been examined using iodine scavenging techniques. The results are combined with similar data for γ-rays and suggest that phenyl radicals mainly react with benzene to give a long-lived adduct, which leads to polymer formation. Iodine can react with this adduct to give enhanced yields of biphenyl. Biphenyl is the predominant single hydrocarbon product in the radiolysis of neat benzene with a yield of 0.075 molecule/100 eV. Its yield is nearly independent of radiation type and energy suggesting that its formation in neat benzene is due to a fast ion−molecule process and not due to phenyl radicals. The total yield of 0.7 radicals/100 eV is almost entirely due to phenyl radicals and H atoms. A reexamination of the fluorescence from the singlet excited state of benzene suggests that this state is the precursor for molecular hydrogen and acetylene, whereas the triplet excited state decays to phenyl radicals and H atoms. Most of the excited states formed in the γ-radiolysis of benzene seem to decay to ground without formation of any product.

A combined experimental and theoretical approach has been used to probe the radiolytic decomposition of liquid pyridine. The major single condensed phase product in the gamma-radiolysis of pyridine is dipyridyl with a yield of 1.25 molecules/100 eV total energy absorbed. Scavenging studies suggest that most, if not all, dipyridyl has a radical precursor, but only about 10% of that is due to the pyridyl radical. The remainder of the dipyridyl may be due to reaction of the parent radical cation with pyridine. Iodine scavenging and quantum chemical calculations both show that the ortho-pyridyl radical (2-pyridyl) is far more stable than the other two isomers.

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The radiation chemical yields of the main products produced in liquid pyridine radiolysis (molecular hydrogen and dipyridyl) have been examined as a function of particle linear energy transfer (LET) with protons, helium ions, and carbon ions of a few to 30 MeV and compared to gamma-radiolysis published in a previous work (J. Phys. Chem. A 2005, 109, 461). Anthracene and biphenyl scavenging techniques have been used to clarify the role of the triplet excited state. An increase in triplet scavenger concentration leads to a decrease in pyridine triplet excited state with a concurrent decrease in dipyridyl, but formation of the latter does not primarily involve pyridyl radicals expected to be produced in the decomposition of the triplet excited state. A decrease in the yield of dipyridyl and an increase in molecular hydrogen are observed with increasing track average LET. The dipyridyl yield with 10 MeV carbon ions is 0.20 molecules/100 eV, which is only 16% of that of observed with gamma-rays. The low yield of dipyridyl with carbon ions is attributed to intratrack triplet-triplet (T-T) annihilation processes due to the increase in local triplet excited-state concentrations with increasing LET. An increasing yield of molecular hydrogen with increasing LET is probably due to an increase in the formation and subsequent decay of singlet excited states produced by the T-T annihilation. A complete mechanism for the radiolysis of liquid pyridine is presented.