Ultrafast X-ray Spectroscopy for Probing a Nuclear Wavepacket in Photoexcited Molecular Complexes

Revealing details of coherent nuclear motion during photochemical reactions on the femtosecond timescale is indispensable for understanding the reaction mechanism in the excited state manifold. Owing to the improvement in time resolution and data quality associated with the advent of X-ray free-electron lasers (XFELs), this can now be achieved with atomic structural sensitivity using time-resolved X-ray absorption near edge structure (TR-XANES) spectroscopy. In this chapter, we describe how vibrational motions are observed and interpreted with this X-ray technique. Femtosecond vibrational modes are studied on two prototypical transition metal complexes, [Fe(bpy)3]2+ (bpy = 2,2′-bipyridine) and [Cu(dmphen)2]+ (dmphen = 2,9-dimethyl-1,10-phenanthroline), and future perspectives exploiting ultrafast X-ray spectroscopies with high repetition rate XFEL machines are presented.