Dr. Thomas Wolf will present his topic, “Investigating Ultrafast Photochemistry with Diffraction and X-ray Spectroscopic Methods,” in-person.

Photoinduced processes are essential to nature’s ability to convert and store solar energy. Key steps in these processes occur on ultrafast timescales, involving nonadiabatic dynamics across multiple electronic states, which are poorly understood. The latter are connected through conical intersections, where correlated motion of a molecule’s electrons and nuclei takes place and the Born-Oppenheimer approximation is invalid. Therefore, they provide ongoing challenges both to their experimental investigation and their theoretical description and are still poorly understood. To gain mechanistic insights into ultrafast photochemical reactions mechanisms, we follow a bottom-up approach by investigating comparably small, isolated organic model systems, enabling comparisons of experimental results with high-level quantum chemical simulations. We combine experimental observables with targeted sensitivity to electronic or nuclear degrees of freedom to get complementary information about the coupled electronic and nuclear dynamics. We have developed corresponding experimental methods using the unique user facilities at SLAC National Accelerator Laboratory: the X-ray free electron laser LCLS and the megaelectronvolt ultrafast electron diffraction facility.
In the first part of my talk, I will focus on a series of investigations of electrocyclic ring-opening reactions of 1,3-cyclohexadiene (CHD) and derivatives using ultrafast electron diffraction (UED) with specific sensitivity to the nuclear degrees of freedom. Our study of the electrocyclic ring-opening in CHD, showcases the detailed information about the involved structural dynamics accessible through UED. In follow-on studies on derivatives of CHD, we demonstrate that the stereospecificity of electrocyclic reactions, an important property for their synthetic application, can be followed in real-time with diffraction observables. Moreover, we gain detailed insight into the dynamical origins of their stereospecificity.
In the second part of my talk, I will present results from complementary investigations of the evolving electronic structure during photochemical reaction dynamics, focusing on an investigation of excited state intramolecular proton transfer in acetylacetone using time-resolved soft X-ray absorption spectroscopy. We find direct spectroscopic evidence for the proton transfer reaction and the involved nonadiabatic population dynamics.