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

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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.