Excitation of lowest electronic states of thymine by slow electrons

Excitation of lowest electronic states of the thymine molecules in the gas phase is studied by elec- tron energy loss spectroscopy. In addition to dipole-allowed transitions to singlet states, transitions to the lowest triplet states were observed. The low-energy features of the spectrum at 3.66 and 4.61 eV are identified with the excitation of the first triplet states 1(3) A' (pi -> pi*) and 1(3) AaEuro(3) (n -> pi*). The higher-lying features at 4.96, 5.75, 6.17, and 7.35 eV are assigned mainly to the excitation of the pi -> pi* transitions to the singlet states of the molecule. The excitation dynamics of the lowest states is studied. It is found that the first triplet state 1(3) A'(pi -> pi*) is most efficiently excited at a residual energy close to zero, while the singlet 2(1) A'(pi -> pi*) state is excited with almost identical efficiency at different residual energies.