Journal of Physical Chemistry B, Vol.108, No.44, 17251-17260, 2004
Anion fragment formation in 5-halouracil films induced by 1-20 eV electron impact
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-ClU, 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.