Research master thesis: Study of the isotope effect on the competition between H and H2 loss in ethylene cation using semi-classical molecular dynamics

Abstract

The engineering of atto second and short femtosecond pulses opened the way to probe ultrafast dynamics during the past 20 years. Being able to follow a mechanism on a sub or few femtoseconds time scale allowed scientists to investigate reaction mechanisms that were still not fully understood. The dissociation of ethylene cation is a multipathway reaction leading to either H loss or H2 loss. Substituting the hydrogen atoms by deuterium provides further understanding on the relaxation and reaction mechanisms and on the competition between the two pathways. The relaxation dynamics of the four lowest electronic states of C2D4+ were computed semi-classically using the Surface Hopping including Arbitrary Couplings (SHARC) software for the first few dozens of fs. After relaxation to the ground state, the longer, picosecond, dynamics was computed using classical Born-Oppenheimer molecular dynamics (BOMD). The relaxation from excited electronic states to the ground state of the cation is ultrafast and occurs during the first 50 fs. Our results demonstrated that the isotope substitution impacts the relaxation dynamics from the excited states. The process is slower due to the mass difference between hydrogen and deuterium. The type of conical intersection visited by the molecule during its relaxation is also impacted by the isotope effect. We compared our results to experimental results for deuterated ethylene cation as well to those obtained for the hydrogenated ethylene in a previous study. We further analyzed the steps of the mechanism for D loss and D2 loss including transition states and intermediates and compared our results to those available in the literature.