Direct observation of $ππ^*$ to $nπ^*$ transition in 2-thiouracil via time resolved NEXAFS spectroscopy

The photophysics of nucleobases has been the object of both theoretical and experimental studies over the past decades, due to the challenges posed by the non-BOA (Born-Oppenheimer Approximation) dynamics in their relaxation pathway. In this context, the ultrafast dynamics of 2-thiouracil has been investigated with a time-resolved NEXAFS study at the Free Electron Laser FLASH.We show that x-ray absorption spectroscopy can be used to observe electronic transitions in ultrafast molecular relaxation. We use a uv-pump / x-ray probe scheme to measure the changes induced by photoexcitation to the x-ray absorption at the sulfur 2s and 2p edges, identifying absorption features corresponding to the S2 (ππ*) and S1 (nπ*) electronic states. We observe a delay of 102 ± 11 fs in the population of the nπ* state with respect to the initial optical excitation. We furthermore identify the presence of coherent oscillations in the absorption signal that match a similar observation in our previous x-ray photoelectron spectroscopy study.