We demonstrate that electrons at energies below the threshold for electronic excitation (<3 eV) effectively decompose gas phase uracil generating a mobile hydrogen radical and the corresponding closed shell uracil fragment anion (U-H)(-). The reaction is energetically driven by the large electron affinity of the (U-H) radical. This observation has significant consequences for the molecular picture of radiation damage, i.e., genotoxic effects or damage of living cells due to the secondary component of high energy radiation.

We present results about dissociative electron attachment (DEA) to gas-phase uracil (U) for incident electron energies between 0 and 14 eV using a crossed electron/molecule beam apparatus. The most abundant negative ion formed via DEA is (U-H)-, where the resonance with the highest intensity appears at 1.01 eV. The anion yield of (U-H)- shows a number of peaks, which can be explained in part as being due to the formation of different (U-H)- isomers. Our results are compared with high level ab initio calculations using the G2MP2 method. There was no measurable amount of a parent ion U-. We also report the occurrence of 12 other fragments produced by dissociative electron attachment to uracil but with lower cross sections than (U-H)-. In addition we observed a parasitic contaminating process for conditions where uracil was introduced simultaneously with calibrant gases SF6 and CCl4 that leads to a sharp peak in the (U-H)- cross section close to 0 eV. For (U-H)- and all other fragments we determined rough measures for the absolute partial cross section yielding in the case of (U-H)- a peak value of sigma (at 1.01 eV)=3 x 10(-20) m2.

Electron attachment (EA) and dissociative electron attachment (DEA) to 5-chloro uracil (5-ClU) was studied in the gas phase using a crossed electron/molecule beams technique. Besides production of a parent anion via a zero energy resonance, ion yields of nine different negative ions were observed in the electron energy range from about 0 to 14 eV. In the electron energy range from about zero to 5 eV, the formation of a transient negative ion was induced by electron attachment to the π* resonances located at about 0.24, 1.5, and 3.6 eV leading subsequently by unimolecular decay to various negative fragment ions. Absolute partial cross sections for EA and DEA to 5-ClU were obtained from the measured ion yields using a simple calibrating method. The dominant negative ion observed in the present experiment was (C4H2N2O2)− (corresponding to 5-ClU minus HCl) with a mass to charge ratio of 110, followed by Cl− ion (mass to charge ratios 35 and 37), the partial cross sections being σ(0.23 eV)=5×10−18 m2 and σ(0.23 eV)=3×10−18 m2, respectively. The parent anion produced (5-ClU)− has only a cross section value of σ(0 eV)=3×10−20 m2. The energetic thresholds for the formation of particular negative ions from 5-ClU in the gas phase were calculated at the G2(MP2) level of theory and compared with the experimental results. On the basis of these calculations structure and relative stability of some of the fragment ions was predicted. In the case of the two most abundant ions, their formation was observed well below the calculated electron energy threshold. The formation of these negative ions below the thermodynamic threshold is explained in terms of the high vibrational energy of the 5-ClU prior to the EA process.

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Electron impact ionization cross sections measured close to threshold are reported for the newly discovered greenhouse gas SF5CF3. No stable parent ion is detected but several cations are detected and their appearance energies (AE) have been measured. The AE (CF3+/SF5CF3) = 12.87±0.10 eV is in excellent agreement with a recently reported value derived from a photoelectron-photoion coincidence experiment. Other ion thresholds are: AE (CF+/SF5CF3) = 24.44±0.10 eV, AE (CF2+/SF5CF3) = 17.80±0.50 eV; AE (SF+/SF5CF3) = 12.66±0.20 eV; AE (SF3+/SF5CF3) = 14.48±0.10 eV; AE (SF5+/SF5CF3) = 13.16±0.60 eV. Two thresholds are suggested for AE (SF4+/SF5CF3) = 16.10±0.30 and 12.10±0.30 eV. A tentative assignment of an AE for SF2+/SF5CF3 of 15.6±1.0 is also given. These results confirm that SF5CF3 is unlikely to be photolysed in the lower stratosphere and therefore will have a long lifetime against solar photodissociation.