Strong-field induced ionization and dissociation of <i>cis</i> - and <i>trans</i> -1,2-dichloroethylene: Cl ⁺ and HCl ⁺ fragments

Strong-field induced ionization of polyatomic molecules using femtosecond laser pulses leads to complex fragmentation dynamics like bond-breaking, intramolecular proton migration, roaming, bond formation, and Coulomb explosion. In this work, we report the strong-field induced ionization and dissociation dynamics of <i>cis</i>- and <i>trans</i>-1,2-dichloroethylene (DCE) employing ion momentum spectroscopy with multi-ion coincidence detection of fragment ions. Two-body dissociation from the doubly ionized molecular ion involving Cl⁺ ejection, <i>i.e.</i>, the Cl⁺ + C₂H₂Cl⁺ channel, and HCl⁺ ejection, <i>i.e.</i>, the HCl⁺ + C₂HCl⁺ channel, is studied in detail as a function of laser pulse duration. A lower kinetic energy release (KER) is observed for <i>cis</i>-DCE as compared to <i>trans</i>-DCE for both fragmentation channels, which is attributed to the different conformations of the isomers. Two distinct peaks are observed in the KER spectra for both Cl⁺ and HCl⁺ ejection channels, indicating that two different excited states/distinct reaction pathways are involved in the dissociation process. Quantum chemical calculations reveal that the Cl⁺ ejection channel follows three different pathways, one from direct C-Cl bond break-up, and the other two pathways are from intermediate states. On the other hand, HCl⁺ formation is possible <i>via</i> two pathways with two intermediate states. Dissociation lifetimes of the parent ions are estimated from the angular distribution of fragments. A longer lifetime is observed for <i>cis</i>-DCE as compared to <i>trans</i>-DCE for both the fragmentation channels.