Multipulse Femtosecond X-ray Spectroscopy in Solution*

The development of commercial femtosecond laser systems has resulted in pump-probe spectroscopy becoming an essential tool to discover and understand complex electronic and structural dynamics in solvated molecular, biological, and material systems. Here we use two femtosecond hard X-ray pulses from LCLS to perform a two-color X-ray pump X-ray probe transient absorption experiment in solution. The X-ray pump creates a localized excitation by removing a 1s electron from an Fe atom in solvated ferro- and ferricyanide complexes. Following the ensuing Auger–Meitner cascade in the complexes, novel electronic excited states with 3p holes are generated. The second X-ray probe pulse interrogates the Fe 1s→3p transitions. The experimental results are compared with theory to extract +2 eV shifts in transition energies per valence 3d hole in the iron complexes. We gain insight into correlated interactions of valence 3d with 3p and deeper-lying electrons.