Grand Séminaire d'Institut

Mardi 31 Octobre 2023 à 11h00.

Dissociation and Isomerization Following Ionization of Ethylene: a Theoretical Perspective

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Photoionized and electronically excited ethylene cation can dissociate via H- and H2-loss, where the latter has been hypothesized to be preceded by isomerization to the ethylidene form. Recent experiments with few-femtosecond XUV pulses and complementary theoretical studies have shed light on the photo-dynamics of ethylene cation. However, the details of the mechanisms of H-loss, H2-loss, and ethylene-ethylidene isomerization, and the factors that govern the competition between these photochemical channels, are still not well understood.

Here, we simulate the dissociation- and isomerization dynamics of ethylene following photoionization and electronic excitation to its four lowest-energy cationic states using the mixed-quantum classical non-adiabatic dynamics method surface hopping. The electronic structure is described at the CASSCF level, with an active space comprising all valence electrons. The large active space allows us to, in contrast to previous dynamics studies, observe a significant number of dissociation and isomerization events and reproduce the experimentally observed pathways (H-loss, H2-loss, and ethylene-ethylidene isomerization). The electronic relaxation is ultrafast; regardless of the initial electronic state, more than half of the population decays within 50 femtoseconds to the cationic ground state, where most dissociation and isomerization events occur. Several conical intersections characterized by planar and twisted geometries act as funnels between the low-lying cationic states, and their effects on the photochemical outcome have previously been discussed. The time scale of ethylene-ethylidene isomerization has previously been inferred in experimental studies employing XUV-pump NIR-probe schemes. We find a large discrepancy between the experimentally inferred time scale and that predicted by the simulations and suggest an alternate explanation for this difference.

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