  Ground- and excited-state UV photoelectron spectra of thiouracils
(2-thiouracil, 4-thiouracil and 2,4-dithiouracil) have been simulated using
multireference configuration interaction calculations and Dyson norms as
measure for the photoionization intensity. Except for a constant shift, the
calculated spectrum of 2-thiouracil agrees very well with experiment, while no
experimental spectra are available for the two other compounds. For all three
molecules, the photoelectron spectra show distinct bands due to ionization of
the sulphur and oxygen lone pairs and the pyrimidine $\pi$ system. The
excited-state photoelectron spectra of 2-thiouracil show bands at much lower
energies than in the ground state spectrum, allowing to monitor the
excited-state population in time-resolved UV photoelectron spectroscopy (TRPES)
experiments. However, the results also reveal that single-photon ionization
probe schemes alone will not allow monitoring all photodynamic processes
existing in 2-thiouracil. Especially, due to overlapping bands of singlet and
triplet states the clear observation of intersystem crossing will be hampered.
