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' pulse to photoexcite diatomic iodine vapor from the X state to the B state, and a time-delayed 800 nm 'control' pulse to stimulate population transfer back to the X state or higher-lying dissociative states. We track the excited charge density at angstrom and femtosecond scales using ultrashort 9 keV hard X-ray 'probe' pulses enabled by the LCLS free electron laser. These results are supported by numerical solutions of the time-dependent Schrödinger equation, creating spatiotemporal movies of wavepacket evolution, providing new insights into light-driven quantum dynamics such as wavepacket splitting and delayed dissociation.