We investigated theoretically the relaxation dynamics of ethylene upon ionisation. Photo-ionised and electronically excited ethylene C2H4+ can undergo H-loss, H2-loss, and ethylene−ethylidene isomerisation, where the latter entails a hydrogen migration. Recent pioneering experiments with few-femtosecond extreme ultraviolet pulses and complementary theoretical studies have shed light on the photodynamics of this prototypical organic cation. However, no theoretical investigation based on dynamics simulations reported to date had described the mechanisms and time scales of dissociation and isomerization. We simulated the coupled electron−nuclear dynamics of ethylene following vertical ionisation and electronic excitation to its four lowest-lying cationic states. The electronic structure was treated at the CASSCF level with an active space large enough to describe bond breaking and formation. The simulations indicated that dissociation and isomerization take place mainly on the cationic ground state and allow the probing of previous hypotheses concerning the correlation between the photochemical outcome and the traversed conical intersections. The results, moreover, support the long-standing view that H2-loss may occur from the ethylidene form. However, the ethylene−ethylidene isomerisation time predicted by the simulations is considerably longer than those previously inferred from indirect experimental measurements.
We also performed a methodological project regarding the simulation of attochemical processes in molecules. Such molecular processes can, in principle, be simulated with various nonadiabatic dynamics methods, yet the impact of the approximations underlying the methods was rarely assessed. We evaluated the performances of widely used mixed quantum-classical approaches, Tully surface hopping and classical Ehrenfest methods, against the high-accuracy DD-vMCG quantum dynamics. This comparison was conducted for the valence ionisation of fluorobenzene. Analysing the nuclear motion induced in the branching space of the nearby conical intersection, the results showed that the mixed quantum-classical methods reproduce quantitatively the average motion of a quantum wavepacket when initiated on a single electronic state. However, they fail to properly capture the nuclear motion induced by an electronic wavepacket along the derivative coupling, the latter originating from the quantum electronic coherence property, key to attochemistry. Our simulations further validate the control over the molecular motion in the branching space achieved by tuning the initial electronic wave packet composition. They also unravel both interstate and intrastate quantum interferences that leave clear signatures of attochemistry and charge-directed dynamics in the shape of the autocorrelation function. The latter is accessible experimentally via high-harmonic spectroscopy (HHS). We collaborated with the experimental team of Prof. Jon Marangos (Imperial College London). The predicted autocorrelation functions are in very good agreement with experimentally measured dynamics via HHS in both benzenes and fluoro-benzene, highlighting the sensitivity of autocorrelation functions to the composition of the initial electronic wavepacket. These results are direct evidence of attochemical control.
