Imaging Excited State Structures and Ultrafast Dynamics with X-Ray Scattering*

In ultrafast time-resolved x-ray scattering on free molecules in the gas phase optical laser pulses excite the molecules and ultrashort pulses from an x-ray free electron laser probe the transient dynamics. The experiments provide a wealth of information about the structures of molecules in excited states and their dynamic and kinetic time evolution. Complete, excited state structures are obtained by measuring the difference between the excited state and ground state scattering patterns and referencing them to the known ground state structures. Optical excitation leads to a change in the electron density distributions, which gives rise to a distinct signature in the x-ray scattering signal and which is in agreement with predictions from electronic structure calculations. 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 and unstable species are obtained. These topics are discussed using polyatomic molecules such as N-methyl morpholine, 1,3-cyclohexadiene, tri-methyl amine as examples.