| Summary: | Physical, biological, and chemical transformations are initiated by changes in the
electronic configuration of the species involved. These electronic changes occur on the
timescales of attoseconds (10−18 s) to femtoseconds (10−15 s) and
drive all subsequent electronic reorganization as the system moves to a new equilibrium or
quasi-equilibrium state. The ability to detect the dynamics of these electronic changes is
crucial for understanding the potential energy surfaces upon which chemical and biological
reactions take place. Here, we report on the determination of the electronic structure of matter
using a single self-seeded femtosecond x-ray pulse from the
Linac Coherent Light Source hard x-ray free electron
laser. By measuring the high energy resolution off-resonant spectrum (HEROS), we
were able to obtain information about the electronic density of states with a single
femtosecond x-ray pulse. We show
that the unoccupied electronic states of the scattering atom may be determined on a shot-to-shot basis
and that the measured spectral shape is independent of the large intensity fluctuations of
the incoming x-ray beam. Moreover,
we demonstrate the chemical sensitivity and single-shot capability and limitations of
HEROS, which enables the technique to track the electronic structural dynamics in matter
on femtosecond time scales, making it an ideal probe technique for time-resolved
X-ray experiments.
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