15 research outputs found
Ultrafast Imaging of photochemical dynamics via x-ray scattering: connecting theory and experiments
Although photochemical reactivity has been extensively studied, a clear
picture of the underlying dynamics is largely missing predominantly because of the extremely short reaction times involved in these processes. The
rapid development of X-ray free electron laser (XFEL) facilities in the last
decade has fostered the emergence of new types of experiments that target photochemical dynamics. One of these new prominent techniques is
non-resonant Ultrafast X-ray Scattering (UXS). In a pump-probe fashion,
it enables the direct observation of structural dynamics on a femtosecond
timescale. Due to the extreme brightness of the XFEL, these experiments
can be performed even in gas phase. Because of the unconstrained molecular motion and lack of intermolecular interference, gas-phase UXS is a meeting ground for experimental and theoretical studies of the quantum nature
of photochemical dynamics.
As promising as they are, gas-phase UXS experiments are still in their
early days. A lot of fundamental aspects remain unexplored, and rigorous
theoretical and computational frameworks are not established. This thesis aims to bridge existing gaps between theory and experiments, presenting an account of recent advances in data analysis and interpretation. The
work gives an outline of the theory of time-dependent molecular quantum
mechanics following photoexcitation, as well as X-ray-matter interaction.
Practical aspects of the post-experimental analysis are presented. These include separation of the observed signal into isotropic and anisotropic scattering components, which allows internal and rotational molecular degrees
of freedom to be dealt with independently in the analysis. The process of
extracting useful information about the dynamics of the molecule as the
reaction unfolds requires careful consideration of how to optimally represent the experimental signal and what inversion schemes are feasible given the limitations of the experiment. The data interpretation often relies on
input from computational modelling. This thesis also describes a computational scheme for calculating generalised (elastic, inelastic, total and coherent mixed) isotropic X-ray scattering cross-sections directly from the ab initio wave function of the molecule.
This methodological apparatus is applied in the analysis of a number of
experiments, and the findings are presented. It is shown that X-ray scattering is in principle sensitive even to small rearrangements of the electrons
upon absorption of light. The ability to detect the initially excited electronic
state by means of transition dipole moment alignment is demonstrated in
the case of the excitation of N-methylmorpholene (NMM) by a 200 nm linearly polarised laser. The subsequent dynamics, more specifically the fast
coherent vibrations, are extracted from the experiment creating a âmolecular movieâ with high spatial resolution via high-throughput conformational
sampling guided by computational modelling. Separately, the rate of dissociation of trimethylamine (TMA) after excitation is obtained from the loss
of scattering interference between the fragments over the course of the reaction
Excited Electronic States in Total Isotropic Scattering from Molecules
Ultrafast x-ray scattering experiments are routinely analyzed in terms of the
isotropic scattering component. Here we present an analytical method for
calculating total isotropic scattering directly from ab initio two-electron
densities of ground and excited electronic states. The method is generalized to
compute isotropic elastic, inelastic, and coherent mixed scattering. The
computational results focus on the potential for differentiating between
electronic states and on the composition of the total scattering in terms of
elastic and inelastic scattering. By studying the umbrella motion in the first
excited state of ammonia, we show that the associated electron density
redistribution leaves a comparably constant fingerprint in the total signal
that is similar in magnitude to the contribution from the changes in molecular
geometry