Wolf, T. J. A.Myhre, R. H.Cryan, J. P.Coriani, S.Squibb, R. J.Battistoni, A.Berrah, N.Bostedt, C.Bucksbaum, P.Coslovich, G.Feifel, R.Gaffney, K. J.Grilj, J.Martinez, T. J.Miyabe, S.Moeller, S. P.Mucke, M.Natan, A.Obaid, R.Osipov, T.Plekan, O.Wang, S.Koch, H.Guehr, M.2017-07-102017-07-102017-07-10201710.1038/s41467-017-00069-7https://infoscience.epfl.ch/handle/20.500.14299/138963WOS:000403876000002Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed pi pi* to a dark n pi* state. The pi pi*/n pi* internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the n orbital during pi pi*/n pi* internal conversion, the absorption spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that pi pi*/n pi* internal conversion takes place within (60 +/- 30) fs. High-level-coupled cluster calculations confirm the method's impressive electronic structure sensitivity for excited-state investigations.text::journal::journal article::research article