Femtosecond X-ray Scattering

Researchers in California have used an X-ray scattering method to monitor step-by-step femtosecond structural rearrangements of a molecule undergoing a ring-opening reaction (Phys. Rev. Lett. 2015, DOI: 10.1103/physrevlett.114.255501). The study describes a new ultrafast pump-probe method that deepens understanding of chemical reaction dynamics and lays the groundwork for recording X-ray movies of chemical reactions in real time. To trigger the reaction, Michael P. Minitti of SLAC National Accelerator Laboratory and colleagues irradiated 1,3-cyclohexadiene vapor with ultraviolet laser light, which initiates ring opening and forms several conformers of 1,3,5-hexatriene. Then the team probed the evolution of the diene to the triene by scattering intense 30-femtosecond X-ray pulses from the unfolding molecules. By varying the delay between the UV and X-ray pulses, the team recorded a series of femtosecond X-ray diffraction snapshots, which they analyzed with quantum mechanical methods. The analysis shows that the reaction rapidly expands cyclohexadiene’s carbon bonds and that within one or two oscillations of the carbon skeleton, the C1–C6 bond breaks. The analysis also shows that the reaction’s stereochemical fate is sealed as early as 30 femtoseconds after the UV pulse.