Ultrafast X-ray imaging of coherently controlled molecular dynamics in real-space and time

Since the advent of femtochemistry, researchers have endeavored to both manipulate and monitor chemical dynamics at the latent length and timescales of atoms and their bonds. Separately, these goals have been experimentally realized through advances in non-linear laser spectroscopy, and time-resolved scattering measurements of high-energy radiation. In this contribution, we consolidate, for the first time, coherent control and ultrafast X-ray scattering approaches to directly visualize optically steered wavepackets in a benchmark molecular system. Through a Tannor-Kosloff-Rice excitation scheme, we deploy a 520 nm ‘pump’ to photoexcite diatomic iodine vapor from the X state to the B state, and a time-delayed 800 nm ‘dump’ to stimulate population transfer back to the X state. We track the excited charge density at angstrom and femtosecond scales using ultrashort 9 keV X-ray ‘probe’ pulses enabled by the LCLS free electron laser. Legendre decomposition of the high-order anisotropic scattering components allow us to retrieve information on the efficiency of the multi-photon two-color Raman transitions, and their resultant dynamics, at different pump-dump delays. By cross-refencing these observations against numerical solutions of the time-dependent Schrödinger equation, we parameterize the specific Franck-Condon windows that govern light-driven intramolecular dephasing, relaxation and dissociation channels in I₂. Finally, we implement Fourier-based inversion of the scattering signals from reciprocal to real space to create spatiotemporal movies of wavepacket evolution along each of these pathways. In addition to providing fresh perspectives on intensely explored photochemical phenomena, the methods herein represent a new class of techniques for directing and detecting quantum dynamics in molecular and condensed media.