“…Such a time resolution can be achieved by optical pump/X-ray probe setups when the dynamics is induced by valence band electrons. Dynamic processes initiated by inner-shell electrons can be studied at FELs with an accelerator-based two-bunch mode (Hara et al, 2013;Lutman et al, 2013;Marinelli et al, 2015), or by means of a split-and-delay unit (SDU) (Roseker et al, 2009;Sorgenfrei et al, 2010;Sobierajski et al, 2013;Wö stmann et al, 2013;Hilbert et al, 2014;Roling et al, 2014a) which may yield elemental specificity to both X-ray pump and the X-ray probe. In the latter case, the pulse will be separated into two partial beams which can be delayed with respect to each other by a variation of their path length.…”