Rottke, HorstEngel, Robin Y.Schick, DanielSchunck, Jan O.Miedema, Piter S.Borchert, Martin C.Kuhlmann, MarionEkanayake, NagithaDziarzhytski, SiarheiBrenner, GünterEichmann, UlrichKorff Schmising, Clemens vonBeye, MartinEisebitt, Stefan2022-06-202022-06-202022-05-20https://depositonce.tu-berlin.de/handle/11303/17132http://dx.doi.org/10.14279/depositonce-15911Extending nonlinear spectroscopic techniques into the x-ray domain promises unique insight into photoexcited charge dynamics, which are of fundamental and applied interest. We report on the observation of a third-order nonlinear process in lithium fluoride (LiF) at a free-electron laser. Exploring the yield of four-wave mixing (FWM) in resonance with transitions to strongly localized core exciton states versus delocalized Bloch states, we find resonant FWM to be a sensitive probe for the degree of charge localization: Substantial sum- and difference-frequency generation is observed exclusively when in a one- or three-photon resonance with a LiF core exciton, with a dipole forbidden transition affecting details of the nonlinear response. Our reflective geometry–based approach to detect FWM signals enables the study of a wide variety of condensed matter sample systems, provides atomic selectivity via resonant transitions, and can be easily scaled to shorter wavelengths at free-electron x-ray lasers.en530 Physikhole colocalizationspectroscopyextreme ultravioletprobing electronnonlinear spectroscopic techniquesArticle2375-2548