This thesis deals with the study of the electronic structure of substances relevant to molecular materials technology. Detailed investigations were carried out by means of core and valence photoionization spectroscopy supported by Density Functional Theory (DFT) calculations. An approach similar to the “bottom-up” strategy was adopted, starting from the study of the simpler molecular building blocks as model systems and increasing complexity to larger molecular systems and deposited films. In the first part, three simple organic system where characterized in detail:oligothiohenes, biphenylene and pyridines. A systematic study of oligothiophenes was performed as a function of the chain length. The evolution of the spectral features is analyzed as a function of the number of thiophene rings; a tendency to a stabilization for increasing chain length is found. Theoretical computation assisted in the assignment the spectral features to the different carbon sites. For biphenylene, the characters of the non-equivalent carbons were separated in gas phase core hole spectra. In a film, the adsorbed biphenylene is characterized by important intermolecular interactions but maintains, substantially, a molecular character. The molecules choose an ordered arrangement and switch their orientation from lying to standing on the surface for increasing coverage. In the study of pyridines and fluorinated derivatives the contributions of molecular vibrations was been added in the analysis and simulation of the experiments. This study is particularly appealing in view of the sufficient reliability and low computational cost of our computational protocol. The second part is dedicated to Transition Metal Phthalocyanines (Pc), which are systems of higher complexity than the previous ones. They are widely studied for the possibility to deposit them in molecular films suitable for a variety of technological applications. Fe and Mn Pc’s were chosen since the different metal in the molecular center gives rise to Highest Occupied Molecular orbitals (HOMO) with different atomic characters. Our analysis reveals that the electronic structure of the Pc molecules rises from the combination of orbitals of all the atoms in the molecule - carbon, nitrogen and the metal in the molecular center - and that the HOMO and HOMO-1 features depend on the hybridization between the metal atom and mostly C 2p and N 2p orbitals. In the third part, a parallel research activity towards the development of novel light sources in view of their application for time resolved photoionization studies of energy transfer processes in novel materials. CITIUS, a new light source for ultrafast science is presented. CITIUS provides tunable, intense, femtosecond pulses in the spectral range from infrared to extreme ultraviolet (XUV). The generated pulses are produced by laser induced high order harmonic generation (HHG) in gas and then monochromatized by a time preserving monochromator. We present the results of two pump-probe experiments: one to characterize the temporal duration of harmonic pulses; the second one to demonstrate the capability for selective investigation of the ultrafast dynamics in a magnetic compound. Secondarily, the Low Density Matter (LDM) beamline of the FERMI free electron laser (FEL) is described. LDM was designed for experiments with supersonic beams of atoms, molecules or clusters to explore nonlinear multiple ionization processes and energy redistribution processes after photoexcitation. The LDM beamline is ideal for performing experiments over a broad range of topics (e.g. photofragmentation or even ultrafast demagnetization) because of its ability to take advantage of the full control of the FEL pulse polarization. The design and characterization of the LDM photon transport system is described, detailing the optical components of the beamline.

Photoionization experiments in the study of energy transfer in nanostructured materials and their precursors

GRAZIOLI, CESARE
2017-04-05

Abstract

This thesis deals with the study of the electronic structure of substances relevant to molecular materials technology. Detailed investigations were carried out by means of core and valence photoionization spectroscopy supported by Density Functional Theory (DFT) calculations. An approach similar to the “bottom-up” strategy was adopted, starting from the study of the simpler molecular building blocks as model systems and increasing complexity to larger molecular systems and deposited films. In the first part, three simple organic system where characterized in detail:oligothiohenes, biphenylene and pyridines. A systematic study of oligothiophenes was performed as a function of the chain length. The evolution of the spectral features is analyzed as a function of the number of thiophene rings; a tendency to a stabilization for increasing chain length is found. Theoretical computation assisted in the assignment the spectral features to the different carbon sites. For biphenylene, the characters of the non-equivalent carbons were separated in gas phase core hole spectra. In a film, the adsorbed biphenylene is characterized by important intermolecular interactions but maintains, substantially, a molecular character. The molecules choose an ordered arrangement and switch their orientation from lying to standing on the surface for increasing coverage. In the study of pyridines and fluorinated derivatives the contributions of molecular vibrations was been added in the analysis and simulation of the experiments. This study is particularly appealing in view of the sufficient reliability and low computational cost of our computational protocol. The second part is dedicated to Transition Metal Phthalocyanines (Pc), which are systems of higher complexity than the previous ones. They are widely studied for the possibility to deposit them in molecular films suitable for a variety of technological applications. Fe and Mn Pc’s were chosen since the different metal in the molecular center gives rise to Highest Occupied Molecular orbitals (HOMO) with different atomic characters. Our analysis reveals that the electronic structure of the Pc molecules rises from the combination of orbitals of all the atoms in the molecule - carbon, nitrogen and the metal in the molecular center - and that the HOMO and HOMO-1 features depend on the hybridization between the metal atom and mostly C 2p and N 2p orbitals. In the third part, a parallel research activity towards the development of novel light sources in view of their application for time resolved photoionization studies of energy transfer processes in novel materials. CITIUS, a new light source for ultrafast science is presented. CITIUS provides tunable, intense, femtosecond pulses in the spectral range from infrared to extreme ultraviolet (XUV). The generated pulses are produced by laser induced high order harmonic generation (HHG) in gas and then monochromatized by a time preserving monochromator. We present the results of two pump-probe experiments: one to characterize the temporal duration of harmonic pulses; the second one to demonstrate the capability for selective investigation of the ultrafast dynamics in a magnetic compound. Secondarily, the Low Density Matter (LDM) beamline of the FERMI free electron laser (FEL) is described. LDM was designed for experiments with supersonic beams of atoms, molecules or clusters to explore nonlinear multiple ionization processes and energy redistribution processes after photoexcitation. The LDM beamline is ideal for performing experiments over a broad range of topics (e.g. photofragmentation or even ultrafast demagnetization) because of its ability to take advantage of the full control of the FEL pulse polarization. The design and characterization of the LDM photon transport system is described, detailing the optical components of the beamline.
FRONZONI, GIOVANNA
29
2015/2016
Settore FIS/03 - Fisica della Materia
Università degli Studi di Trieste
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11368/2908130
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