Electron attachment to gas-phase uracil

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 σ (at 1.01 eV)=3×10−20 m2.