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The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a textbook example of electrocyclic reactions in organic chemistry and a model for photobiological reactions in vitamin D synthesis. Here, we present direct and unambiguous observation of the ring-opening reaction path on

The coherence and dephasing of vibrational motions of molecules constitute an integral part of chemical dynamics, influence material properties, and underpin schemes to control chemical reactions. In the present study, we measure coherent structural dynamics in optically excited N-methyl morpholine by scattering with ultrashort X-ray pulses from the Linac Coherent Light Source. The scattering signals are corrected for the different electron density in the excited electronic state of the molecule compared to the ground state. The experiment maps the evolution of the molecular geometry with femtosecond resolution, showing coherent motion that survives electronic relaxation and appears to persist for longer than previously seen using other methods.

All stages of the electrocyclic ring-opening of 1,3-cyclohexadiene (CHD) were observed by time-resolved photoionization-photoelectron spectroscopy. Spectra of the 1B state, previously unobserved using time-resolved methods, were obtained upon optical excitation using ultrashort laser pulses at 4.60 or 4.65 eV, followed by ionization with pulses at 3.81, 3.85, and 4.10 eV, revealing a 1B lifetime of 30 fs. In an experiment using 3.07 eV probe photons and a 4.69 eV pump, we observed a time-sequenced progression of Rydberg states that includes s, p, and d states of the series n = 3 to 6. The sequentiality of the Rydberg signals points to an ionization mechanism that captures the molecule on different points along the reaction path in 2A. A dynamic fit of the Rydberg signals, coupled with MS-CASPT2 calculations, reveals that as the wavepacket moves down the potential energy surface it acquires kinetic energy at a rate of 28 eV/ps before reaching the conical intersection to the 1A ground state. During the reaction, the terminal carbon atoms separate at a speed of 16 Å/ps. A deconvolution of the Rydberg signals from a broad feature assigned to structurally disperse 1,3,5-hexatriene (HT) shows the formation of the open-chain hexatriene structure with an onset 142 fs after the initial absorption of a pump photon. The experimental observations are discussed in the context of recent ultrafast X-ray scattering experiments and theoretical quantum dynamics simulations.

We discuss the application of ab initio X-ray diffraction (AIXRD) to the interpretation of time-resolved and static X-ray diffraction. In our approach, elastic X-ray scattering is calculated directly from the ab initio multiconfigurational wave function via a Fourier transform of the electron density, using the first Born approximation for elastic scattering. Significant gains in efficiency can be obtained by performing the required Fourier transforms analytically, making it possible to combine the calculation of ab initio X-ray diffraction with expensive quantum dynamics simulations. We show that time-resolved X-ray diffraction can detect not only changes in molecular geometry but also changes in the electronic state of a molecule. Calculations for cis-, trans-, and cyclo-butadiene, as well as benzene and 1,3-cyclohexadiene are included.