The ability to observe molecular reactions in real time is expected to aid the exploration of new reaction mechanisms, the development of catalysts, the understanding of biomolecular processes and the control of chemical reactions and material properties on a molecular level. Toward this goal, we have developed a gas-phase X-ray scattering experiment that uses the ultrashort X-ray pulses from the Linac Coherent Light Source (LCLS) to capture atomic motions within molecules.

The time-dependent molecular structures are measured in a pump-probe scheme where the molecules are induced to react by excitation with an ultrafast laser pulse. The X-ray scattering signal is observed as a function of time delay between the pump laser pulse and the X-ray probe pulse. The time-evolving molecular structures are obtained by comparing the time-sequenced scattering signals to computational models.

The method is illustrated on the photochemically induced electrocyclic ring opening reaction of 1,3-cyclohexadiene to form 1,3,5-hexatriene, which serves as a model system for countless chemical applications. Optical excitation prepares 1,3-cyclohexadiene on the excited 1B surface, from where it accelerates past a conical intersection and down the 2A potential energy surface, before opening the ring structure on a 140 fs time scale. A “molecular movie” of the observed dynamics is constructed by comparing ab initio quantum molecular dynamics simulations with the experimental diffraction signal to derive weighted trajectories that provide a representation of the structural dynamics, with the weighted ensemble of trajectories corresponding to the nuclear flux during the chemical reaction.

Direct comparison is made to measurements of the same reaction using a structural spectroscopic method. The X-ray structural data yield reaction pathways for which ionization energies can be calculated at each step. We use ultrafast time-resolved multiphoton-ionization photoelectron spectroscopy to measure the travel time required for the molecule to reach certain photoionization resonance windows involving Rydberg states. This allows for a consistency check of the results from the ultrafast X-ray scattering with observations from ultrafast (structural) spectroscopy. By combining the techniques, it appears that we can make significant progress towards the ultimate goal: a comprehensive understanding of the spatially and temporally resolved photochemical reaction dynamics.

Argonne Physics Division Colloquium Schedule