Ultrafast X-ray Molecular Dynamics*

Transient absorption in the extreme ultraviolet and X-ray spectral regimes is used to probe molecular dynamics on both femtosecond and attosecond timescales. Laser-produced high-order harmonics are generated to use as the probe, while ultraviolet or visible pulses excite the molecular system by well-defined transitions. The method is used to investigate dissociating molecules, ring opening, passage through curve crossings and conical intersections, singlet-to-triplet transitions, and electronic and vibrational coherences. By extending the probe photon energies to 300 eV in the soft X-ray, carbon species are investigated to reveal atom-specific electronic orbitals during electronic transformations of organic molecules. Time domain measurements provide an assessment of multiple excited state products and key points along the reaction trajectories. Passage through curve crossings and conical intersections reveal distinct changes in electronic state character at the crossings, and shifts in core level spectroscopic features provide information on the changes in orbital energetics and occupancies, such as when bonds break. X-ray spectroscopic transitions of transient intermediate states are directly characterized. On ultrashort timescales, electronic and vibrational wave packet coherent dynamics are also observed. The results form the basis of an X-ray spectroscopic revolution in molecular dynamics, sensitive to electron state character.