Anion Fragment Formation in 5-Halouracil Films Induced by 1−20 eV Electron Impact

Abstract

We present measurements of electron-induced dissociation of 5-halouracils (5-XU, U = uracil, X = fluorine, chlorine, bromine, and iodine) in the condensed phase for incident electron energies between 1 and 20 eV. We find that, for all 5-XU, dissociative electron attachment (DEA) to the molecules leads to molecular fragmentation into the anion fragments H - , X - , CN - , and OCN - (plus associated neutral radical products) at energies as low as 2-3 eV. We also observe two dissociation channels for X - production via DEA at 7.2 and 12.1 eV, and at 7.2 and 8.7 eV, for 5-FU and 5-ClU, respectively, and one dissociation channel at 6.4 and 5.7 eV for 5-BrU and 5-IU, respectively. Furthermore, for all molecules studied, the fragments desorbing with the highest yields are always H - and X - . The formation and desorption of H - occurs with a somewhat higher intensity than the formation and desorption of X - , with ratios of the H - to X - yield peak heights of about 16, 1, 2, and 10 for 5-FU, 5-CIU, 5-BrU, and 5-IU, respectively. For the same range of incident electron energies, DEA to 5-halouracils also leads to small yields of CN - and OCN - , which desorb only upon film charging. Although the sum of these latter anion fragment desorption yields is smaller by at least one order of magnitude than the yields of H - , they are of much importance since the cyano-containing anion moieties indicate aromatic ring dissociation.