Chapter 6 - Probing reaction dynamics with Rydberg states: The ring opening reaction of 1,3-cyclohexadiene

Abstract

Electrocyclic reactions involving polyenes are important in many natural processes, such as photosynthesis and the natural synthesis of vitamin D. A detailed understanding of the intramolecular energy flow upon excitation is essential to understand the photochemistry of such molecules. This chapter investigates the energy flow associated with the ring-opening reaction of 1, 3-cyclohexadiene (CHD), which often serves as a model system for electrocyclic reactions. It applies a newly developed technique that uses photoionization via Rydberg states as a probe. The decay kinetics of the intermediate electronic states involved in the photo-induced electrocyclic ring-opening reaction of 1,3-cyclohexadiene to form 1,3,5- hexatriene are determined. The initially populated 1B2 state decays rapidly to the 21A1 state within the duration of the laser pulse. The 21A1 state crosses to the ground electronic state with a decay time of 93 fs. The experiments show that photoionization via a highly excited valence state, and a set of Rydberg levels, provides a useful probe of the molecular dynamics taking place on electronic surfaces of lower energy. Since all neutral molecules have Rydberg states, it can be presumed that the technique is widely applicable. The internal conversion dynamics to the Rydberg levels is very fast, providing for an accurate representation of the molecular dynamics. Finally, the Rydberg states are long lived, which makes the Rydberg spectra well defined and straightforward to interpret.