Ultrafast X-ray Scattering: Excited State Structures, Chemical Dynamics and Molecular Movies
Peter M. Weber
The determination of excited state molecular structures and the observation of chemical dynamics challenges X-ray scattering in two fundamental ways: First, in order to watch molecular structures during a chemical reaction one needs to achieve a time resolution approaching 10 fs (10-14 s). Secondly, molecules undergoing chemical reactions are far from equilibrium, so that conventional methods to analyze gas phase scattering patterns are no longer satisfactory. The development of X-ray free electron lasers (XFEL) has enabled substantial advances on both fronts.
In ultrafast time-resolved X-ray scattering on free molecules in the gas phase, optical laser pulses excite the molecules to reacting states and ultrashort pulses from an XFEL probe the transient dynamics. The scattering experiments provide a wealth of information about the structures of molecules in excited states and their dynamic and kinetic time evolution. Measuring the differences between the excited state and ground state scattering patterns and referencing them to the known ground state structures yields complete, excited state structures. The quantitative analysis of time-evolving signal intensities results in detailed information about chemical reaction kinetics including sequential and branching reactions. In addition to the kinetic time constants, structures of transient radicals and unstable species are obtained. These topics are discussed using polyatomic molecules such as N-methyl morpholine, 1,3-cyclohexadiene and N,N′-dimethylpiperazine as examples.