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Investigation of charge migration/transfer in radical cations using Ehrenfest method with fully quantum nuclear motion

Tran, Thierry; (2022) Investigation of charge migration/transfer in radical cations using Ehrenfest method with fully quantum nuclear motion. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

The main focus of this thesis is to investigate the effect of charge migration on molecular dynamics. Upon the creation of a superposition of cationic states by a short ionizing pulse in an attosecond pump-probe experiment, the electronic wavefunction is in a non-stationary state and the initial dynamics are purely electronic, driven by Charge Migration (CM) before the onset of any nuclear motions. The CM can be simulated using a frozen nuclear framework but its importance on long-term dynamics and competition with vibrationally mediated charge motion (i.e. Charge Transfer (CT)) remains unknown. Unravelling the mechanism behind CM and its importance on electron and nuclear coherence can help in designing an initial superposition of electronic states to steer nuclear motions toward a specific product. Further control of the photo-reactivity could be achieved with the use of probe/control laser pulses and open the door for more direct comparison with experimental results. In order to investigate the dynamics upon photoionization with an attosecond pump-pulse, the coupled electron-nuclear dynamics of the system is simulated using nonadiabatic quantum dynamics techniques within the sudden approximation. A single-set approach is adopted for the expansion of the nuclear wavefunction using a linear combination of Gaussian Wavepackets (GWP). The calculation is done using the Quantum-Ehrenfest method (QuEh) and the time-dependent Potential Energy Surfaces (PES) are evaluated with the Complete Active Space Configuration Interatcion (CAS-CI) method. The resulting dynamics are analyzed with adiabatic/diabatic state populations, Normal Mode (NM) displacements and bond lengths averaged over the nuclear wavepacket using Gross Gaussian populations (GGP). To reduce the cost of computation, the algorithm implemented in QUANTICS is parallelized with a Message Passing Interface (MPI). Further, the section of code which interacts with the database that contains previously calculated points on the PES is rewritten using the Structured Query Language (SQL) and the SQLite engine. For the purpose of unravelling the mechanism behind CM, the nonadiabatic dynamics of a model retinal Protonated Schiff Base (rPSB) and benzene are investigated by defining the initial electronic wavefunction in a systematic way. As demonstrated by the results on rPSB, the relaxation mechanism such as single and double bond length alternation and isomerization can controlled by varying the initial composition of electronic states. With the rich symmetry of benzene, the initial nuclear dynamics which are controlled by an initial gradient and electron dynamics can be analyzed using symmetry rules. The initial gradient is a combination of totally symmetric motion and non-symmetric components which correspond to the intra- (eigenstate) and inter-state (couplings) gradients, respectively. The electron dynamics and its associated nuclear motions can be examined by grouping together the localized holes where the CM occurs. With the initial gradient and CM, one can predict the initial nuclear relaxation and possibly control the photo-products formed by designing a specific superposition of electronic eigenstates. To explore the effect of laser pulses on dynamics, an implementation within the dipole approximation using the dipole-electric field dot product is done in the GAUSSIAN program. The dynamics in the presence of an infrared probe pulse is simulated on model systems such as allene and the ethylene cation. The pulse is able to induce change in the electron and nuclear dynamics of the system and some of its effect can be explained using irreducible representations and the alignment of the electric fields. The work presented in this thesis offers an insight into the photocontrol of molecules and opens the door for further investigation of charge-directed dynamics.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Investigation of charge migration/transfer in radical cations using Ehrenfest method with fully quantum nuclear motion
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2022. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Chemistry
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL
URI: https://discovery.ucl.ac.uk/id/eprint/10153026
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