Time-resolved X-ray Scattering of Excited State Structure and Dynamics

Abstract

Ultrafast gas-phase X-ray scattering using X-ray free-electron lasers (XFELs) has enabled the measurement of molecular structures and dynamics in excited states with atomic spatial and femtosecond time resolution. This chapter reviews important recent advances in the X-ray scattering of excited states. In particular, details will be given of additional observables such as the orientation of optical transition dipoles within the molecular frame, the excited state molecular electron densities, and rate constants for chemical kinetics. In excited states, the structures of medium sized molecules (up to eight non-hydrogenic atoms) have been determined with high precision in the non-hydrogenic atom–atom distances. Effects arising from the redistribution of electron density upon optical excitation are observed and confirmed by high-level calculations, allowing for the observation of ultrafast excited state charge transfer reactions. Fragmentation of molecules is readily observed, and is identifiable from a characteristic decrease in the X-ray scattering signal at small scattering angles. Furthermore, the structures of transient radical fragments have been measured.