The structure of short-lived excited states of molecular complexes by time-resolved X-ray diffraction

Experimental and computational methods for time-resolved (TR) diffraction now allow the determination of geometry changes on molecular excitation. The first results indicate significant changes in the interatomic distances and molecular shape on photo-excitation, but also a dependence of the induced changes on the molecular environment. Though the use of high-brightness synchrotron sources is essential, it limits the time resolution to the width of the synchrotron pulse which is currently 70 - 100 ps. The experiments discussed fall into two categories: ( i) picosecond powder diffraction experiments on the molecular excitation to a singlet state, and ( ii) microsecond experiments on the excited states of inorganic complexes. Both involve reversible processes for which a stroboscopic technique can be applied.