Isomerization and dissociation dynamics of 1,2-dichloroethylene probed by time-resolved Coulomb explosion imaging

Coulomb explosion imaging (CEI) is a powerful method to obtain detailed structural information of small quantum systems, such as molecules and clusters, in the gaseous phase. Time-resolved CEI (TRCEI) initiated by ultrafast laser pulses provides a unique opportunity to follow structural and fragmentation dynamics with high temporal resolution. While studying photoinduced isomerization reactions, TRCEI can be used not only to distinguish different isomer geometries but also to track the time evolution during the interconversion between those geometries. Here, we present an application of TRCEI to the cis and trans isomers of 1,2-dichloroethylene (C₂H₂Cl₂) (DCE). After excitation by a 200-nm UV pulse, a high-intensity near-infrared laser pulse is used to induce Coulomb explosion at different time delays between the two pulses. The fragments thus produced are detected in coincidence, and their correlated momenta are used to determine the evolving molecular structure. DCE can follow multiple pathways upon UV excitation, including direct C-Cl bond cleavage, fragment rotation, isomerization, molecular chlorine formation, etc. We discuss the contributions of these different pathways for both the isomers of DCE in the light of different experimental observables. Furthermore, we compare the experimental results with classical Coulomb explosion simulations to guide our interpretations. Our study sheds light on the photoinduced isomerization reaction at the molecular level.