The conversion of light energy into other energy forms in molecules results from a concerted and ultrafast motion of electrons and nuclei, often under the breakdown of the Born-Oppenheimer approximation. This talk is about ultrafast experiments to resolve light-induced ultrafast molecular dynamics of nucleobases with x-ray probe pulses using free-electron lasers.
The molecules in the center of this talk are nucleobases, which encode genetic information in life. Although possessing high UV absorption cross-sections, damaging events by UV absorption are relatively rare. The ultrafast transfer of electronic energy into harmless vibrational energy plays a vital role as an internal photoprotection mechanism. We present experiments probing the very first steps of this process by femtosecond resonant x-ray absorption spectroscopy at the oxygen K-edge. We deduce a less than 100 fs ππ* → nπ* transition, which plays a crucial role in the photoprotection of this nucleobase .
Thiolated nucleobases show an efficient and ultrafast relaxation into long-lived triplet states, contrasting with the ultrafast relaxation to the ground states observed in canonical nucleobases. The triplet channel gives rise to applications as photoinduced-cross linkers but also to problems related to its current use of thionucleobases as medication. We investigate the dynamics of 2-thiouracil via x-ray probing at the sulfur L-edge using the new URSA-PQ instrument for gas-phase spectroscopy at FLASH . We find a direct connection between the charge moving within the molecule and the binding energy shifts observed in the photoelectron spectrum. The mechanism manifests itself into coherently modulated signals due to the oscillating electronic population.
 Probing ultrafast ππ*/nπ* internal conversion in organic chromophores via K-edge resonant absorption, T. J. A. Wolf, R. H. Myhre, J. P. Cryan, S. Coriani, R. J. Squibb, A. Battistoni, N. Berrah, C. Bostedt, P. Bucksbaum, G. Coslovich, R. Feifel, K. J. Gaffney, J. Grilj, T. J. Martinez, S. Miyabe, S. P. Moeller, M. Mucke, A. Natan, R. Obaid, T. Osipov, O. Plekan, S. Wang, H. Koch and M. Gühr, Nature Communications 8, 29 (2017)
 Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy
D. Mayer, F. Lever, D. Picconi, J. Metje, S. Alisauskas, F. Calegari, S. Düsterer, C. Ehlert, R. Feifel, M. Niebuhr, B. Manschwetus, M. Kuhlmann, T. Mazza, M.S. Robinson, R.J. Squibb, A. Trabattoni, M. Wallner, P. Saalfrank, T.J.A. Wolf, M. Gühr, Nature Communications 13, 198 (2022)
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Serguei Molodtsov / Gabriella Mulá-Mathews