Negative ions in thymine and 5-bromouracil produced by low energy electrons

We have compared the negative ion production processes in thymine (T) and 5-bromouracil (BrU) after impact of low energy electrons (0-2 eV). In agreement with earlier studies we have observed in BrU a long lived parent ion BrU- and fragment ions Br- and Uyl(-) with cross sections in the 10(-14) cm(2) range. In contrast with other observations in thymine, the most intense ions are found to be (T-H)(-) fragment ions and not the parent ion T-. For the fragment ions Br-/BrU, Uyl(-)/BrU and (T-H)(-)/T the shape of the cross section vs. electron energy reveals a complex nature. Close to zero energy, several peaks separated by a few hundreds of meV appear in these cross sections, instead of only one peak reported in earlier works. Our observations are understood as vibrational structures due to predissociation of rather long lived anion states, possibly dipole bound states, by valence Pi* resonant states, close to zero energy. (C) 2003 Elsevier Science B.V. All rights reserved.