Multidimensional Multimodal Molecular Motion Viewed With Hard X-rays

Polyatomic photoexcited molecules exhibit many competing modes of atomic motion, such as bending, stretching, and dissociation. The coupling of these modes leads to a wide variety of reaction pathways which are poorly described by present state-of-the-art theory. Time-resolved X-ray scattering (TRXS) can spatially and temporally resolve the complex atomic motion of such photoexcited molecules. Here we study the intramolecular motion of CS₂ following photoexcitation at 200 nm using TRXS with 9.8 keV X-rays at LCLS. CS₂ excited at 200 nm exhibits both symmetric stretch and bending motion followed by dissociation into multiple exit channels. These types of motion are best viewed in the frequency-resolved basis S(Q,ω), where they appear as sparse features with simple interpretations. We employ the Lomb-Scargle periodogram to view our noisy, non-uniformly sampled data in this basis, and observe both vibrational and dissociative motion. Sliding the transform window over a subset of the data enables identification of the onset of vibrations and subsequent coupling to dissociation channels.