Mazza, T.Ilchen, M.Kiselev, M. D.Gryzlova, E. V.Baumann, T. M.Boll, R.De Fanis, A.Grychtol, P.Montaño, J.Music, V.Ovcharenko, Y.Rennhack, N.Rivas, D. E.Schmidt, Ph.Wagner, R.Ziolkowski, P.Berrah, N.Erk, B.Johnsson, P.Küstner-Wetekam, C.Marder, L.Martins, M.Ott, C.Pathak, S.Pfeifer, T.Rolles, D.Zatsarinny, O.Grum-Grzhimailo, A. N.Meyer, M.2023-12-072023-12-072020-12-18https://hdl.handle.net/2097/44077The nature of transient electronic states created by photoabsorption critically determines the dynamics of the subsequently evolving system. Here, we investigate K-shell photoionized atomic neon by absorbing a second photon within the Auger-decay lifetime of 2.4 fs using the European XFEL, a unique high-repetition-rate, wavelength-tunable x-ray free-electron laser. By high-resolution electron spectroscopy, we map out the transient Rydberg resonances unraveling the details of the subsequent decay of the hollow atom. So far, ultra-short-lived electronic transients, which are often inaccessible by experiments, were mainly inferred from theory but are now addressed by nonlinear x-ray absorption. The successful characterization of these resonances with femtosecond lifetimes provides the basis for a novel class of site-specific, nonlinear, and time-resolved studies with strong impact for a wide range of topics in physics and chemistry.Creative Commons Attribution 4.0 International licensehttps://creativecommons.org/licenses/by/4.0/Text