title: Electronic Transitions in the Non-Orthogonal Group Function Approach: Singlet Hetero Fission and Orbital Tomography
title: Elektronische Übergänge im nichtorthogonalen Gruppenfunktionsansatz: Heteromolekulare Singulettspaltung und Orbitaltomographie
titleType: AlternativeTitle
Creators: name: Giesen, Steffen Maurice
Publisher: name: Philipps-Universität Marburg
Year: 2022
Subjects: Chemistry &amp; allied sciences
Chemie
Nichorthogonal Gruppenfunktionen
Non-orthogonal Group Functions
Quantenchem
Orbital Tomography
Molekülphysik
Theoretische Chemie
Quantum Chemist
Grenzflächen
Orbitaltomographie
Kontinuumsübergänge
Singulett Fission
Electronic Transitions
Elektronische Übergänge
SIngulettspaltung
Interfaces, Continuum Transitions

description: Electronic state transitions in atoms and molecules determine many of the properties of these systems. In turn these properties, and more, can be revealed by analysis of the electronic state transitions. This thesis presents a unified treatment of both bound-bound and bound-continuum transitions between electronic states, based on the non-orthogonal group function (NOGF) approach. An example of a process determined by electronic bound-bound transitions that has received wide attention is singlet exciton fission: A single singlet exciton transforms into two triplet excitons with about half the exciton energy each. This is a charge carrier multiplication effect promising for the development of highly efficient photovoltaic cells. Apart from this application, singlet hetero fission, i.e. intermolecular singlet fission involving different molecular entities, is a interesting, strongly surface sensitive intermolecular correlation effect, highly specific to the structure of the heteromolecular interface. Singlet hetero fission has so far received less attention in the literature but will be one major focus of this thesis. If simultaneous singlet homo fission can be suppressed, a material exhibiting strong singlet hetero fission allows probing interfaces directly, which is normally hindered by the surrounding bulk phases. The present work not only characterizes a range of acenes and perfluoroacenes in regard to their suitability to singlet fission in general, but also identifies a selection of heteromolecular candidate systems that show promise for near-exclusive singlet hetero fission. Furthermore, the strong structural influence of the interface on the singlet fission process is made abundantly clear by a comprehensive study of different intermolecular arrangements and also by variation of intermolecular structural parameters like relative translations and rotations of the molecules. The presented approach models the interfaces involved in the singlet fission process using molecular clusters prepared through the NOGF method, including few-states configuration interaction. By preparing the cluster states in terms of independent molecules, large systems can be described with reduced effort, excited states and specifically charge-transfer states can be localized and prepared in a tailored and customized manner, and wave function quality and optimization can be adjusted for the individual subsystems, including relaxation and correlation effects. These features of the approach are what allow a considerable number of molecular species of substantial size to be compared in this work. This comparison itself is additionally given a clear and manageable representation herein by introducing two scalar descriptors of state population transfer efficiency, as relevant to singlet fission, building on a quantum dynamical assessment of state population transfer. In total, the presented illumination of the connection between intermolecular structure, wave function quality, excitation energies, coupling matrix elements, intermolecular orbital overlap and state population dynamics is used to systematically classify homo- and heteromolecular systems regarding their suitability for singlet fission in general and singlet hetero fission in particular. The results lead to specific recommendations of molecular species and heteromolecular systems for singlet hetero fission and foster the general understanding of the singlet fission process. A fascinating and illuminating example of bound-continuum transitions, specifically the photoelectric effect, is orbital tomography using energy- and angle-resolved measurements of photoelectron emission. Orbital tomography provides a connection between experimental quantities and the molecular wave function, particularly, but not only, in the single-particle approximation. In this thesis, the corresponding theory for differential absorption cross sections and orbital tomography, and all relevant approximations involved, are presented in a top-down manner. This stringent approach clarifies the origin and also the remediability of several artifacts occurring in often used, more approximate approaches. Here, the NOGF method enables a systematic description of the total multi-electron states, even for large molecules, including antisymmetrization and orthogonalization effects between bound and continuum electron wave functions. The results show how the bound-continuum orthogonalization rectifies important shortcomings of the plane wave approximation to the photoelectron wave function, most importantly allowing for photoelectron emission perpendicular to the polarization of the incoming photon, a phenomenom which is also known from experiment. Additionally, in this thesis, the vector potential of the photon is taken into account beyond the dipole approximation, and the effects are shown at high photoelectron energies as well as at increased photon energies. The theory is not only investigated analytically, but also by two- and three-dimensional visualizations that intuitively show the effects of selected approximations. Furthermore, the presented visualizations show the significant influence that excited states, as initial or final bound states, have on the differential absorption cross sections. Lastly, surface symmetry effects are included by partial rotational averaging. The resulting visualizations show how strongly most interesting features of the differential cross sections are washed out after such an averaging, but also that effects of approximations and excited states can nevertheless remain visually identifiable.