Formation of negative ions from gas phase halo-uracils by low-energy (0–18 eV) electron impact

We report on low-energy electron-induced processes on gas phase halo-uracils (5-XU, X=F, Cl, Br, and I). The dominant dissociative electron attachment (DEA) channel is formation of X − +Uyl (uracil-yl radical) via a pronounced feature near 0 eV at estimated absolute cross-sections of 3×10 −14  cm −2 (Cl − ), 4×10 −14  cm −2 (Br − ), and 9×10 −14  cm −2 (I − ). At that energy the complementary channel, with respect to the negative charge, namely formation of the closed shell uracil-yl anion (Uyl − ) plus neutral X is also operative at weaker intensity. An exception is fluoro-uracil (5-FU) where both the formation of F − and Uyl − are absent. In addition 5-ClU, generates (Uyl−H) − +HCl rather than Uyl − +Cl as reported very recently [J. Chem. Phys. 118 (2003) 4170]. In competition to dissociation, all halo-uracils additionally form a long-lived parent anion (XU − ) near 0 eV. From the energy balance of the DEA reaction, we estimate the electron affinity of the uracil-yl radical (Uyl ) to be about 2.9–3.2 eV. At higher electron energies we observe various anion ring fragments with 68 and 57 amu, as well as OCN − and CN − . The two first anions are tentatively ascribed to H 2 C 3 NO − and CN 2 OH − , respectively. The present experiments are directly related to reveal the fundamental molecular mechanisms how the radiosensitizers 5-XU operate in cancer therapy. In radiotherapy, secondary electrons which are created in exceeding amounts along the radiation track are believed to play a crucial role.