This thesis describes the investigation of time-resolved phenomena using X-ray
techniques, and in particular the new possibilities and challenges arising from the
application of these techniques on the femtosecond time-scale.
The thesis will review the processes following laser excitation of molecular species
in solution, describing the interplay between electronic and structural dynamics, as
well as the role of the solvent. This will be followed by an introduction of the three
X-ray techniques used in this work, and it will be shown how the application of these
techniques in a laser pump / X-ray probe framework can help elucidate the excited
state dynamics. The theoretical information content of the X-ray techniques will be
discussed, as well as the practical experimental considerations for their implementation.
This is done in order to demonstrate the potential increase in information
content yielded by (and the practical challenges connected to) their simultaneous implementation
in a single experiment. Finally, the experimental results of a signicant
set of laser pump / X-ray probe experiments will be presented and discussed in order
to gauge the applicability of these techniques as tools for characterizing excited states
and excited state dynamics.
A central component of the work has been related to the transition from synchrotron
based X-ray sources to X-ray free-electron laser (XFEL) sources. The timeresolution
and
ux of the XFEL source is roughly three orders of magnitude better
than the standardized laser pump / X-ray probe experiment at synchrotron sources.
This has necessitated both a systematic experimental investigation, and the formulation
of a theoretical framework within which to describe the very inhomogeneous
and highly energetic states of the molecular system under investigation and their local
solvent environment on the sub picosecond time-scale from an X-ray perspective.