Electron and nuclear dynamics following molecular ionisation : computational methods and applications

• Main Author: Vacher, Morgane
• Other Authors: Bearpark, Michael J.
• Published: Imperial College London 2016
• Subjects: 540
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.733087

The emergence of attosecond techniques has opened up the possibility to experimentally probe changes in the electron distribution, that until now have been treated as instantaneous. Photoionisation of molecules with attosecond (broad bandwidth) pulses leads to a non-stationary electronic wave packet. The current state-of-the-art for ab initio theory treats molecular electron dynamics as a purely electronic process, at a single fixed nuclear geometry. The present thesis is concerned with fundamental questions about the physics of non-stationary electronic wave packets and the coupling of this motion to that of the nuclei. To simulate coupled electron and nuclear dynamics, we use the “on-the-fly” mixed quantum-classical Ehrenfest method and the quantum mechanical DD-vMCG method. The results obtained with the two methods are compared. We choose to study electron and nuclear dynamics upon ionisation of benzene, toluene and para- xylene as examples because vertical ionisation takes place at geometries near the conical intersections between ground and first excited states of their cations, leading to a potentially strong coupling between the electronic and nuclear coordinates. One aim is to investigate electron dynamics and how it is affected by the nuclei. We show significant effects of the nuclear motion after a few femtoseconds within the Ehrenfest approximation. We also show how the inherent spatial delocalisation of the nuclei leads to very fast dephasing of electron dynamics, using a Wigner distribution. The DD-vMCG simulations confirm the very fast dephasing of electron dynamics in the molecules studied. A complementary aspect of the dynamics upon ionisation is the nuclear motion induced by an electronic wave packet. We show how the averaged initial nuclear motion (direction and velocity) is controlled by the composition of the electronic wave packet, as predicted by the Ehrenfest method. The DD-vMCG method provides the details of the nuclear dynamics on each electronic state.