Electronic and vibrational excitation in gas phase thymine and 5-bromouracil by electron impact

In thymine (T) and 5-bromouracil (BrU) the excitation of the lowest electronic states and vibrational excitation using EEL spectroscopy (0-100 eV) with angular analysis is reported. In the two molecules the singlet electronic states have been found blue-shifted by about 0.3 eV compared to the UV-Vis absorption results. Evidence has been found of triplet states at 3.6 eV for T and 3.35 eV for BrU (+/-0.08 eV). For vibrational excitation both molecules show two resonance regions, around 1-2 and 4-5 eV. The modes excited around 1-2 eV reveal an excitation of the carbonyl, CC double bond and NH stretch modes. At 5 eV, the NH stretch modes are still present but CO and CC double bond stretch modes are much less excited. (C) 2003 Elsevier B.V. All rights reserved.