This thesis comprises a detailed study of the electronic properties of the polycyclic hydrocarbons tetracene, perylene and coronene. The at room temperature grown organic layers consisted of a up to several hundred nanometer thickness, and have been studied with the experimental techniques soft x-ray emission spectroscopy (SXES) and NEXAFS-spectroscopy by means of synchrotron radiation. In particular, resonant inelastic scattering (RIXS) was employed in order to obtain possible band structure or MO-symmetry information of the studied systems.
The studied materials consist of large organic molecules, evaporated under high vacuum on silicon wafers. The goal was to figure out, whether inside these organic layers the hydrocarbon molecules polymerize and form a band structure or if the hydrocarbon molecules do not interact with each other and retain their original molecular like symmetry and behavior. The discrete peaks in the hydrocarbon NEXAFS-spectra could be assigned by comparison with the literature to chemical shifted C1s→ π∗ transitions, caused by different chemical surrounded C-atoms. Hence, due to excitation on a certain π∗-resonance only C1s-electrons of equal C-atoms can be excited, which leads to a site selective excitation.
The fluorescent decay of the created hole, which is as well localized on these former excited C-atoms was recorded energy dispersed in the SXE-spectrometer. This method is usually referred to as SXE. Therefore, the local MO-density was recorded. Hartree-Fock based ground state MO-calculations have been performed for each molecule, in order to simulate the SXE-spectra. Good agreement between experiment and simulation was shown, under the strict retainment of symmetry selection rules along the transitions. This result states, that the different hydrocarbon molecules conserve their symmetry and structure in an up to several hundred nanometer thick organic layer. Only in the region of the valence band maximum (π-states) weak indications for a band dispersion could be observed.
Contents
1 Introduction
2 Soft X-Ray Spectroscopy
2.1 Interaction of Electromagnetic Radiation with Electrons
2.1.1 Two Step Model
2.1.2 Electric Dipole Selection Rule
2.1.3 Parity Selection Rule
2.1.4 One Step Model
2.2 Resonant Inelastic X-Ray Scattering “RIXS”
2.3 Vibronic Coupling
3 MO- and RIXS-Calculations
3.1 Hartree-Fock (HF) Self Consistent Field Method
3.1.1 HF-Calculation Results
3.2 Simulation of Molecular RIXS-Spectra
3.2.1 Site and Symmetry Selection in Molecule RIXS
3.2.2 RIXS-Simulation
4 Experimentals
4.1 Soft X-Ray Emission Spectrometer Design
4.1.1 Piezo-Slit Mechanism and Calibration
4.1.2 Diffraction Gratings
4.1.3 Multi Channel Plate (MCP) Detector
4.1.4 Conclusion
4.2 Synchrotron Radiation
4.3 Organic Semiconductors
4.3.1 Sample Preparation
4.4 NEXAFS Spectroscopy
5 Results
5.1 NEXAFS of Aromatic Hydrocarbons
5.1.1 Tetracene NEXAFS
5.1.2 Perylene NEXAFS
5.1.3 Coronene NEXAFS
5.1.4 Excitonic Character of the Hydrocarbon NEXAFS-Spectra
5.1.5 Conclusion
5.2 Hydrocarbon Soft X-Ray Emission Spectra
5.2.1 RIXS-Spectra Corrections
5.2.2 First Inspection of Hydrocarbon SXE
5.2.3 Nonresonant SXE versus Valence Band Photoemission
5.2.4 Site and Symmetry Selection in Hydrocarbon RIXS-Spectra
5.2.5 Band Dispersion-Effects in the RIXS-Spectra
5.2.6 Vibronic Coupling
5.2.7 Spectator Shifts and Screening
6 Summary
Research Objectives & Topics
This thesis investigates the electronic properties of the polycyclic hydrocarbons tetracene, perylene, and coronene, focusing on whether these organic layers form a band structure or maintain their molecular characteristics. By employing soft X-ray emission spectroscopy (SXES) and resonant inelastic X-ray scattering (RIXS), the research aims to resolve the site and symmetry-selective density of occupied molecular orbitals.
- Electronic structure of polycyclic aromatic hydrocarbons (tetracene, perylene, coronene).
- Application of Soft X-ray Emission Spectroscopy (SXES) and NEXAFS spectroscopy using synchrotron radiation.
- Investigation of band structure formation vs. molecular preservation in organic thin films.
- Development and construction of a high-resolution soft X-ray emission spectrometer.
- Theoretical interpretation using Hartree-Fock based MO-calculations and RIXS simulation models.
Excerpt from the Book
Interaction of Electromagnetic Radiation with Electrons
If x-rays penetrate a solid, the electromagnetic field interacts with the electrons of the the atoms. The hamiltonian in (2.4) describes the perturbation, induced by the electromagnetic field.
H = \sum_{i} [-\frac{e}{2mc}(\mathbf{p}_{i} \cdot \mathbf{A}_{i} + \mathbf{A}_{i} \cdot \mathbf{p}_{i}) + \frac{e^2}{2mc^2} \mathbf{A}_{i} \cdot \mathbf{A}_{i}]. (2.4)
A_{i} is the vector potential of the magnetic field at the location of the ith electron and p_{i} is the momentum of the ith electron. The summation is over all atomic electrons which participate in the interaction. Due to the transversal condition \nabla \cdot A = 0, it is legitimate to replace p_{i} \cdot A_{i} by A_{i} \cdot p_{i}. This term describes transitions, where the total number of photons has changed by ±1. The second term considers, in contrast, transitions with an entire photon change of 0, ±2. For the moment we neglect this squared term of A, as well as magnetic interactions between A and the spin of the atomic electrons.
Summary of Chapters
1 Introduction: Provides an overview of Soft X-ray Spectroscopy (SXS) as a technique for studying electronic structures and defines the goals of analyzing polycyclic hydrocarbons.
2 Soft X-Ray Spectroscopy: Details the theoretical principles of SXS, including the interaction of radiation with electrons, selection rules, and the Kramers-Heisenberg formula for scattering.
3 MO- and RIXS-Calculations: Describes the Hartree-Fock methods used to compute molecular orbital energies and the formalisms required to simulate RIXS spectra.
4 Experimentals: Documents the construction and design of the new high-resolution SXE spectrometer, beamline setup, and sample preparation methods.
5 Results: Presents and interprets the experimental NEXAFS and RIXS spectra of tetracene, perylene, and coronene, comparing them with theoretical MO calculations.
6 Summary: Concludes the thesis by summarizing the findings regarding the electronic properties and structural integrity of the studied hydrocarbon films.
Keywords
Soft X-ray Spectroscopy, SXS, RIXS, NEXAFS, Polycyclic Hydrocarbons, Tetracene, Perylene, Coronene, Electronic Structure, Hartree-Fock, Synchrotron Radiation, Band Structure, Vibronic Coupling, Molecular Orbitals
Frequently Asked Questions
What is the primary objective of this research?
The work aims to investigate the electronic properties of organic films made of tetracene, perylene, and coronene to determine if they form a band structure or retain their original molecular symmetry.
What are the main research themes?
Key themes include Soft X-ray emission spectroscopy, resonant inelastic X-ray scattering, the study of molecular electronic structure, and the influence of vibronic coupling on spectral transitions.
Which scientific methodology is employed?
The research uses synchrotron-based NEXAFS and RIXS spectroscopy, supported by Hartree-Fock ground state MO calculations and custom-built soft X-ray emission instrumentation.
What does the main body of the work cover?
It covers the theoretical framework of X-ray spectroscopy, the technical design of the spectrometer, the experimental measurements of three aromatic hydrocarbons, and the simulation of their spectra to interpret electronic states.
What are the characterizing keywords of this study?
The study is characterized by terms such as RIXS, NEXAFS, polycyclic hydrocarbons, Hartree-Fock calculations, and organic semiconductors.
How is the spectrometer design improved?
The new spectrometer design improves mechanical stiffness and incorporates a Piezo-slit and refined rotary system, ensuring higher reproducibility of focus positions compared to previous versions.
How is the site selectivity in NEXAFS achieved?
Site selectivity is achieved by tuning the excitation energy to correspond to the specific C1s-electron binding energies of chemically distinct carbon atoms within the hydrocarbon molecules.
How is the coherent fraction of the RIXS spectrum extracted?
The coherent fraction is isolated by subtracting the non-resonant background (representing incoherent scattering) from the resonant emission spectrum under controlled normalization conditions.
What role does vibronic coupling play?
Vibronic coupling describes the intermixing of electronic and vibrational states, leading to symmetry-forbidden transitions that broaden spectral features and influence the spectral shape.
- Quote paper
- Dr. Ricardo Scherer (Author), 2002, Soft X-ray emission and resonant inelastic scattering study of polycyclic hydrocarbons, Munich, GRIN Verlag, https://www.grin.com/document/494337
