Low energy electron and O− reactions in films of O2 coadsorbed with benzene or toluene

Abstract

A detailed understanding of nascent reactive events leading to DNA damage is required to describe ionizing radiation effects on living cells. These early, sub-picosecond events involve mainly low energy (E < 20 eV) secondary electrons (SE), and low energy (E < 5 eV) secondary ion (and neutral) fragments; the latter are created either by the primary radiation, or by SE via dissociative electron attachment (DEA). While recent work has shown that SE initiate DNA strand break formation via DEA, the subsequent damage induced by the DEA ion fragments in DNA, or its basic components is unknown. Here, we report 0–20 eV electron impact measurements of anion desorption from condensed films containing O₂ and either benzene (C₆H₆), or toluene (C₆H₅CH₃); these molecules represent the most fundamental structural analogs of pyrimidine bases. Our experiments show that all of the observed OH⁻ yields are the result of reactive scattering of 1–5 eV O⁻ fragments produced initially by DEA to O₂. These O⁻ reactions involve hydrogen abstraction from benzene or toluene, and result in the formation of benzyl radicals, or toluene radicals centered on either the ring or exocyclic methyl group. O⁻ scatters over nm distances comparable to DNA dimensions, and reactions involve a transient anion collision complex. Anion desorption is found to depend on both, the temperature of hydrocarbon film formation (morphology), and the order of overlayer adsorption, e.g. O₂ on benzene, or benzene on O₂. Our measurements support the notion that in irradiated DNA similar secondary-ion reactions can be initiated by the abundant secondary electrons, and may lead to clustered damage.