Probing nucleobase photoprotection with soft x-rays

Abstract

Abstract. Nucleobases absorb strongly in the ultraviolet region, leading to molecular excitation into reactive states. The molecules avoid the photoreactions by funnelling the electronic energy into less reactive states on an ultrafast timescale via non-BornOppenheimer dynamics. Current theory on the nucleobase thymine discusses two conflicting pathways for the photoprotective dynamics. We present our first results of our free electron laser based UV-pump soft x-ray-probe study of the photoprotection mechanism of thymine. We use the high spatial sensitivity of the Auger electrons emitted after the soft x-ray pulse induced core ionization. Our transient spetra show two timescales on the order of 200 fs and 5 ps, in agreement with previous (all UV) ultrafast experiments. The timescales appear at different Auger kinetic energies which will help us to decipher the molecular dynamics.Although nucleobases absorb strongly in the near ultraviolet region transmitted by the Earth's atmosphere, the UV excitation surprisingly does not lead to photoinduced chemistry or damage of the base molecules. Theory and previous experiments indicate that the photoprotection of the nucleobases proceeds via fast (femtoseconds to picoseconds) non-adiabatic transitions [1,2]. Even for isolated nucleobases the understanding of the non-Born-Oppenheimer approximation (non-BOA) dynamics is currently controversial. Past experiments rely heavily on simulations to interpret the observed timescales and depending on the level of ab-initio approximations, different timescales are predicted. The UV light excites an electron from the  orbital (see Fig. 1) to the initially unoccupied * orbital. This * (one electron in  one in *) state is highly reactive. To avoid photoinduced reactions, nonBorn-Oppenheimer dynamics funnels the electronic population down to the ground state on a fast timescale. The first step is a radiationless transition in which the hole in the  orbital is filled by the electron from the n orbital, leading to the n* state. For thymine, theoretical models predict either a This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.