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Coupled nuclear and electron dynamics in molecules. from semiclassical to full-quantum
Coupled nuclear and electron dynamics in molecules. from semiclassical to full-quantum
The interaction of light with a molecular system is the fundamental step of various chemical, physical and biological phenomena. Investigating the nuclear and electron dynamics initiated by light-matter interaction is important to understand, optimize and control the underlying processes. In this thesis two theoretical methods describing the coupled nuclear and electron dynamics in molecular systems are addressed. In the presented studies the coupled dynamics induced by photoexcitation, the subsequent relaxation processes and the possibility to control the dynamics in the vicinity of conical intersections (CoIns) are investigated for different molecular systems. In the first part of this work the photorelaxation pathways of a group of molecules commonly used in organic-based optoelectronic devices are characterized with the help of semiclassical ab intio molecular dynamics simulations. The relaxation pathways starting from the first excited singlet state of thiophene and of small oligothiophenes containing up to three rings is characterized by the interplay of internal conversion (IC) and intersystem crossing (ISC). Especially the ISC is mediated by ring-opening via a carbon-sulfur bond cleavage. The resulting entropically favored open-ring structures trap the molecules in a complex equilibrium between singlet and triplet states and a fast ring closure in the ground state is hindered. The extension of the π-system going from the monomer to the trimer weakens and slows down the ring opening process. Consequently the ISC is reduced for longer thiophene chains. The following two chapters are centered around the topics of controlling the molecular dynamics near a CoIn and monitoring the coherent electron dynamics induced by CoIns and laser interactions in the nucleobase uracil and the symmetric molecule NO2. In order to investigate the coherent electron dynamics, the ansatz used in this work allows a full-quantum description of the electron and nuclear motion and is called nuclear and electron dynamics in molecular systems (NEMol). As part of this work NEMol was extended to capture the coupled dynamics in complex high dimensional molecular systems. The observed electron dynamics both in NO2 and uracil reflects coherence, decoherence and reappearance which are all determined by the associated nuclear dynamics. The control of the molecular dynamics at a CoIn is realized with the help of a few-cycle infrared (IR) pulse. The applied control schema utilizes the carrierenvelope phase (CEP) of the pulse and allows to control the population distribution after the CoIn, the nuclear dynamics as well as the coherent electron dynamics. Depending on the chosen laser parameters and the molecular properties around the CoIn given by nature, two different mechanisms enable the control of the system. Both depend on the CEP but one is based on interference, which is generated by the interaction with the CoIn, and the other one is solely due to the few-cycle waveform of the pulse. As demonstrated for NO2 and uracil, the CEP control scheme even works for quite challenging boundary conditions. Therefore, it seems to be a general concept which can be used also in different molecules.
quantum dynamics, mixed quantum-classical dynamics, conical intersections, control
Schnappinger, Thomas Benedikt
2021
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Schnappinger, Thomas Benedikt (2021): Coupled nuclear and electron dynamics in molecules: from semiclassical to full-quantum. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

The interaction of light with a molecular system is the fundamental step of various chemical, physical and biological phenomena. Investigating the nuclear and electron dynamics initiated by light-matter interaction is important to understand, optimize and control the underlying processes. In this thesis two theoretical methods describing the coupled nuclear and electron dynamics in molecular systems are addressed. In the presented studies the coupled dynamics induced by photoexcitation, the subsequent relaxation processes and the possibility to control the dynamics in the vicinity of conical intersections (CoIns) are investigated for different molecular systems. In the first part of this work the photorelaxation pathways of a group of molecules commonly used in organic-based optoelectronic devices are characterized with the help of semiclassical ab intio molecular dynamics simulations. The relaxation pathways starting from the first excited singlet state of thiophene and of small oligothiophenes containing up to three rings is characterized by the interplay of internal conversion (IC) and intersystem crossing (ISC). Especially the ISC is mediated by ring-opening via a carbon-sulfur bond cleavage. The resulting entropically favored open-ring structures trap the molecules in a complex equilibrium between singlet and triplet states and a fast ring closure in the ground state is hindered. The extension of the π-system going from the monomer to the trimer weakens and slows down the ring opening process. Consequently the ISC is reduced for longer thiophene chains. The following two chapters are centered around the topics of controlling the molecular dynamics near a CoIn and monitoring the coherent electron dynamics induced by CoIns and laser interactions in the nucleobase uracil and the symmetric molecule NO2. In order to investigate the coherent electron dynamics, the ansatz used in this work allows a full-quantum description of the electron and nuclear motion and is called nuclear and electron dynamics in molecular systems (NEMol). As part of this work NEMol was extended to capture the coupled dynamics in complex high dimensional molecular systems. The observed electron dynamics both in NO2 and uracil reflects coherence, decoherence and reappearance which are all determined by the associated nuclear dynamics. The control of the molecular dynamics at a CoIn is realized with the help of a few-cycle infrared (IR) pulse. The applied control schema utilizes the carrierenvelope phase (CEP) of the pulse and allows to control the population distribution after the CoIn, the nuclear dynamics as well as the coherent electron dynamics. Depending on the chosen laser parameters and the molecular properties around the CoIn given by nature, two different mechanisms enable the control of the system. Both depend on the CEP but one is based on interference, which is generated by the interaction with the CoIn, and the other one is solely due to the few-cycle waveform of the pulse. As demonstrated for NO2 and uracil, the CEP control scheme even works for quite challenging boundary conditions. Therefore, it seems to be a general concept which can be used also in different molecules.