Doctor of Philosophy / Department of Physics / Carlos A. Trallero / Ultra-fast spectroscopy has become a common tool for understanding the structure and dynamics of atoms and molecules, as evidenced by the award of the 1999 Nobel Prize in Chemistry to Ahmed H. Zewail for his pioneering work in femtochemistry. The use of shorter and more energetic laser pulses have given rise to high intensity table-top light sources in the visible and infrared which have pushed spectroscopic measurements of atomic and molecular systems into the strong-field limit. Within this limit, there are unique phenomena that are still not well understood. Many of such phenomena involve a photoionization step. For three decades, there has been a steady investigation of the single ionization of atomic systems in the strong-field regime both experimentally and theoretically. The investigation of the ionization of more complex molecular systems is of great interest presently and will help with the understanding of ultra-fast spectroscopy as a whole. In this thesis, we explore the single ionization of molecules in the presence of a strong electric field. In particular, we study molecular isomer pairs, molecules that are the same elementally, but different structurally. The main goal of this work is to compare the ionization yields of these similar molecular pairs as a function of intensity and gain some insight into what differences caused by their structure contribute to how they ionize in the strong-field limit. Through our studies we explore a wavelength dependence of the photoionization yield in order to move from the multi-photon regime of ionization to the tunneling regime with increasing wavelength. Also, in contrast to our strong-field studies, we investigate isomeric molecules in the weak-field limit through single photon absorption by measuring the total ionization yield as a function of photon energy. Our findings shed light on the complexities of photoionization in both the strong- and weak-field limits and will serve as examples for the continued understanding of single ionization both experimentally and theoretically.

Orientador: René Alfonso Nome Silva / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Química / Made available in DSpace on 2018-08-27T06:54:35Z (GMT). No. of bitstreams: 1 Dantas_WillianFranciscoCordeiro_M.pdf: 2446830 bytes, checksum: a6ef77a86d65956736e20e7c5e22ff59 (MD5) Previous issue date: 2015 / Resumo: A espectroscopia vibracional de femtossegundos da vizinhança é ideal para caracterizar os movimentos moleculares da vizinhança acoplados com o sistema eletrônico captador de luz. No caso dos movimentos nucleares intramoleculares, isto pode ser realizado tanto por infravermelho quanto por Raman, ambos de femtossegundos. No caso de movimentos intermoleculares, a dinâmica de femtossegundos somente pode ser caracterizada com experimento Raman coerente, e, por essa razão, é importante sabermos qual é o comportamento do clorofórmio em um experimento de femtossegundo. Dessa forma, pode-se realizar a comparação entre os dados experimentais e teóricos e concluir se o comportamento observado experimentalmente é o mesmo que o esperado. Este trabalho explora a análise de Franck-Condon para os espectros de emissão do pireno com dependência da temperatura. Assume-se que uma progressão vibrônica de bandas no formato de Lorenzianas pode representar o formato das bandas de emissão do fluoróforo. Consequentemente, é possível obter alguns parâmetros, como a largura de linha das bandas obtidas, as intensidades relativas dos picos observados (valores que são utilizados para encontrar os fatores de Huang-Rhys), a variação do comprimento de onda de emissão com o aumento da temperatura e a área integrada dos espectros / Abstract: The femtosecond vibrational spectroscopy of the neighborhood is ideal to characterize the molecular movements of the neighborhood coupled with the electronics pickup light. In the case of intra-molecular nuclear movements, this can be accomplished either by infrared and Raman both femtosecond. In the case of intermolecular movements, the dynamics of femtosecond can only be characterized with coherent Raman experiment, and so it is important to know the behavior of chloroform in a femtosecond experiment. Thus, it is possible to make a comparison between experimental and theoretical data and conclude that the observed experimentally is the same behavior expected. This work explores the Franck-Condon analysis for the emission spectra of pyrene in dependence on temperature. It is assumed that a vibronic bands in the progression Lorenzianas shape may represent the format of fluorophore emission bands. Consequently, it is possible to obtain some parameters such as the line width of the bands obtained, the relative intensities of the observed peaks (values that are used to find the Huang-Rhys factors), the variation of emission wavelength with increasing temperature and the integrated area of the spectra / Mestrado / Físico-Química / Mestre em Química

III-nitride semiconductors suffer significant efficiency limitations; ‘efficiency’ being an umbrella term that covers an extensive list of challenges that must be overcome if they are to fulfil their vast potential. To this end, it is imperative to understand the underlying phenomena behind such limitations. In this dissertation, I combine powerful optical and structural characterization techniques to investigate the effect of different defects on the carrier dynamics in III-nitride materials for light emitting devices. The results presented herein will enhance the current understanding of the carrier mechanisms in such devices, which will lead to device efficiency improvements. In the first part of this dissertation, the effects of some important types of crystal defects present in III-nitride structures are investigated. Here, two types of defects are studied in two different III-nitride-based light emitting structures. The first defects of interest are V-pit defects in InGaN/GaN multiple quantum well (MQW) blue LEDs, where their contribution to the high-efficiency of such LEDs is discussed. In addition, the effect of these defects on the efficiency droop phenomenon in these LEDs is elucidated. Secondly, the optical effects of grain boundary defects in AlN-rich AlGaN/AlGaN MQWs is studied. In this study, it is shown that grain boundary defects may result in abnormal carrier localization behavior in these deep ultraviolet (UV) structures. While both defects are treated individually, it is evident from these studies that threading dislocation (TD) defects are an underlying contributor to the more undesirable outcomes of the said defects. In the second part, the dissertation reports on the carrier dynamics of III-nitride LED structures grown on emerging substrates—as possible efficiency enhancing techniques—aimed at mitigating the effects of TD defects. Thus, the carrier dynamics of GaN/AlGaN UV MQWs grown, for the first time, on (2̅01) – oriented β-Ga2O3 is studied. It is shown to be a candidate substrate for highly efficient vertical UV devices. Finally, results from the carrier dynamics investigation of an AlGaN/AlGaN MQW LED structure homoepitaxially grown on AlN substrate are discussed, where it is shown that its high-efficiency is sustained at high temperatures through the thermal redistribution of carriers to highly efficient recombination sites.

In this work, optical pump – near-infrared probe and near-infrared pump – mid-infrared probe spectroscopy are used for the investigation of pressure-induced insulator-to-metal transitions in transition metal oxide compounds. The materials under study are α-Fe₂O₃, also known as hematite, and VO₂. Both materials undergo pressure-induced metallization. However, the physical mechanisms of this phase transition are very different for these systems and have not been fully understood up to now. Using ultrafast pump-probe spectroscopy we obtain an insight into the evolution of the band structure and electron dynamics across the insulator-to-metal transition. In the case of VO₂, our near-infrared pump – mid-infrared probe experiments reveal a non-vanishing pumping threshold for photo-induced metallization even at our highest pressures around 20 GPa. This demonstrates the existence of localized charge carriers and the corresponding persistence of a band gap. Besides the threshold behaviour for photo-induced metallization, the carrier relaxation time scale, and the linear reflectivity and transmissivity have been studied under pressure increase. An anomaly in the threshold behaviour as well as the linear reflectivity and transmissivity at a critical pressure around 7 GPa indicates band gap filling under pressure. This is further supported by results obtained under decompression, where the changes of the linear reflectivity turned out to be almost fully reversible. The observations on VO₂ are highly reproducible and can be explained in terms of a pressure-induced bandwidth-driven insulator-to-metal transition. Fe₂O₃ has been studied via optical pump – near-infrared probe spectroscopy up to pressures of 60 GPa. In the pressure range up to 40 GPa, the changes of the response can be explained by photo-induced absorption and bleaching. The pressure dependent study of the relaxation dynamics allows to identify cooling of the electron system as origin of the picosecond relaxation process. A sharp anomaly found in the response of Fe₂O₃ at 40 GPa indicates a strong rearrangement of the electronic band structure which could be explained by an insulator-to-metal phase transition induced by pumping. The successful demonstration of pump-probe experiments in diamond anvil cells using pulses from optical to mid-infrared wavelengths and reaching pressures of several tens of GPa is a good basis for further experimental high-pressure studies. Our results obtained on VO₂ and Fe₂O₃ can serve as a benchmark for the development of advanced material models. / In der vorliegenden Arbeit wird der druckinduzierte Isolator–Metall-Phasenübergang in den Übergangsmetalloxiden α-Fe₂O₃ (Hämatit) und VO₂ mittels ultraschneller Anrege-Abfrage-Spektroskopie (engl. pump-probe spectroscopy) untersucht. Hämatit wird dazu im sichtbaren Spektralbereich angeregt und im nahen Infrarot (NIR) abgefragt, bei VO₂ wurde zur Anregung NIR und zur Abfrage mittleres Infrarot (MIR) verwendet. Beide Materialien werden bei hinreichend hohem Druck metallisch, wobei die jeweils dem Isolator–Metall-Phasenübergang zugrundeliegenden Mechanismen verschieden und noch nicht vollständig verstanden sind. Dies motiviert den Einsatz von ultraschneller Anrege-Abfrage-Spektroskopie, die einen Einblick in die Änderung der Bandstruktur und der Ladungsträgerdynamik während des Isolator–Metall-Übergangs gewährt. Beim Überschreiten eines Schwellenwertes der Anregung wird VO₂ photoinduziert metallisch. In unseren NIR-MIR Anrege-Abfrage-Experimenten zeigt sich, dass der Schwellenwert auch bei den höchsten Drücken dieser Messreihe (ca. 20 GPa) nicht verschwindet. Dies weist auf die Existenz lokalisierter Ladungsträger hin und damit verbunden auf das Fortbestehen der Bandlücke. Neben dem Schwellenwert für photoinduzierte Metallisierung wurden auch die Druckabhängigkeiten der Relaxationsdynamik der Ladungsträger sowie des linearen Reflexions- und Transmissionsvermögens untersucht. Eine Anomalie im druckabhängigen Verlauf des Anrege Schwellenwertes sowie des linearen Reflexions- und Transmissionsvermögens bei einem kritischen Druck von ca. 7 GPa deutet darauf hin, dass durch das Anlegen von Druck Zustände innerhalb der Bandlücke induziert werden. Diese Interpretation wird auch durch während der Dekompression gewonnene Messdaten unterstützt. Die druckinduzierte Änderung des linearen Reflexionsvermögens erwies sich als nahezu vollständig reversibel. Unsere Beobachtungen an VO₂ sind reproduzierbar und lassen sich als druckinduzierter, Bandbreiten-getriebener Isolator–Metall-Übergang nachvollziehen. Fe₂O₃ wurde mittels Anrege-Abfrage-Spektroskopie bei Drücken bis zu 60 GPa untersucht. Änderungen im Druckbereich bis 40 GPa können als Wechselspiel eines photo-induzierten Absorptionsbandes und der photoinduzierten Unterdrückung eines anderen Absorptionskanals erklärt werden. Die druckabhängige Untersuchung der Relaxationsdynamik ermöglicht es, der Relaxation auf der Zeitskala weniger Pikosekunden Kühlungsdynamik als Ursache zuzuordnen. Eine scharfe Anomalie im qualitativen Verlauf des Anrege-Abfrage-Signals von Fe₂O₃ bei einem Druck von 40 GPa weist auf deutliche Änderungen in der elektronischen Bandstruktur hin, welche als Signatur eines photoinduzierten Isolator–Metall Phasenübergangs interpretiert werden können. Die erfolgreiche Demonstration von Anrege-Abfrage-Experimenten in Diamantstempeldruckzellen mit Laserimpulsen vom sichtbaren Spektralbereich bis hin zum mittleren Infrarot und bei Drücken von 20 GPa bis zu 60 GPa liefert die solide Basis für weitergehende Hochdruck-Experimente. Die an VO₂ und Fe₂O₃ erzielten Ergebnisse sind eine gute Grundlage für die Weiterentwicklung der theoretischen Beschreibung solcher Materialsysteme.

This thesis deals with infrared studies of impurity states, ultrafast carrier dynamics as well as coherent intersubband polarizations in semiconductor quantum structures such as quantum wells and superlattices, based on the GaAs/AlGaAs material system. In the first part it is shown that the 2pz confined impurity state of a semiconductor quantum well develops into an excited impurity band in the case of a superlattice. This is studied by following theoretically the transition from a single to a multiple quantum well or superlattice by exactly diagonalizing the three-dimensional Hamiltonian for a quantum well system with random impurities. Intersubband absorption experiments, which can be nearly perfectly reproduced by the theory, corroborate this interpretation, showing that at low temperatures in the low doping density regime all optical transitions originate from impurity transitions. These results also require reinterpretation of previous experimental data. The relaxation dynamics of interminiband transitions in doped GaAs/AlGaAs superlattices in the mid-IR are studied. This involves single-color pump-probe measurements to explore the dynamics at different wavelengths, which is performed with the Rossendorf freeelectron laser (FEL), providing picosecond pulses in a range from 3-200 µm and are used for the first time within this thesis. In these experiments, a fast bleaching of the interminiband transition is observed followed by thermalization and subsequent relaxation, whose time constants are determined to be 1-2 picoseconds. This is followed by an additional component due to carrier cooling in the lower miniband. In the second part, two-color pump-probe measurements are performed, involving the FEL as the pump source and a table-top broad-band tunable THz source for probing the transmission changes. These measurements allow a separate specification of the cooling times after a strong excitation, exhibiting time constants from 230 ps to 3 ps for different excitation densities and miniband widths. In addition, the dynamics of excited electrons within the minibands is explored and their contribution quantitatively extracted from the measurements. Intersubband absorption experiments of photoexcited carriers in single quantum well structures, measured directly in the time-domain, i.e. probing coherently the polarization between the first and the second subband, are presented. From the data we can directly extract the density and temperature dependence of the intersubband dephasing time between the two lowest subbands, ranging from 50 up to 400 fs. This all optical approach gives us the ability to tune the carrier concentration over an extremely wide range which is not accessible in a doped quantum well sample. By varying the carrier density, many-body effects such as the depolarization and their influence on the spectral position as well as on the lineshape on the intersubband dephasing are studied. Also the difference of excitonic and free-carrier type excitation is discussed, and indication of an excitonic intersubband transition is found.

Ultrafast spectroscopy allows us to probe and understand material properties. With it, we can measure phonon-polaritons (optical phonons coupled with light) and the resulting dispersion curve in lithium niobate. Customizing the excitation source in ultrafast measurements can excite phonon modes to large amplitudes, allowing the experimental exploration of the Potential Energy Surface in solids. However, stronger pump fluences and bigger signal isn't always the answer in ultrafast spectroscopy. When sample signals and their nonlinear and mechanisms cannot be distinguished with 1D measurements, simple 2D THz measurements are a great place to start searching for distinct factors as was the case in cadmium tungstate. 2D measurements when paired with modeling and first principles calculations can reveal cutting edge information about exciting materials.

Chromophore is a molecule or a part of a molecule which is responsible for its appearance color. This definition has been evolving over time with the progress of science. Contemporary scientific advances have expanded its meaning: to an inclusive level, chromophore is an irreducible collective of fundamental particles, which can represent the photophysical (optical physical) properties of the macroscopic matter. Previous studies have already found that the same molecule can have different photophysical properties under different condensed states. Therefore, it is straight forward to conclude that the definition of chromophore should take such extrinsic influencing interactions of this given molecule into consideration, thus simply taking the smallest unit such as a molecule is not accurate. A good example is quantum dots. Same species of quantum dots possess the identical smallest chemical unit but can emit very differently due to quantum confinement effect, thus defining the smallest unit as the chromophore is apparently fallacious. In solid polymeric compositions, the chemical unit or building blocks may differ from the spectroscopic unit depending on how these chemical units interacts within their ensemble to evolve new properties such as a new transition dipole. As thus, understanding the evolution of photophysical behaviors between the targeted unit and neighbors is of much importance to determine whether they should be considered as one chromophore or many. This requires a thorough understanding towards the evolution of photophysical properties of a collective, and the construction of such collective will need to pay extra attention to, as any structural factor could have changed some photophysical interactions of the collective. The introductory chapter discusses the material platform and fundamental photophysics investigated in this dissertation. Chromophore assembly (CA) as a sylloge of several classes of self-assembled materials, including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), porous organic polymers (POPs). Among them, MOF-based CAs (MOF-CAs) featuring with the ease of synthesis, demonstrate incomparable promises to construct such collective with several appealing characteristics, including component diversity, chemical stability, structural porosity, and post-synthetic versatility (Chapter 1.1). As for here, the main target to achieve using these assemblies is to understand the interaction between adjacent chemical monomeric units, therefore their spatial arrangements are of the paramount importance. As modern theory discovered, both ordered and random systems can be very important for novel quantum material developments. Both crystalline and amorphous arrangements of monomeric units can be achieved by adopting different classes of materials. MOF-CAs could achieve the precise control of spatial arrangement including distance, direction, and dihedral angle by its crystalline structures, whereas porous organic polymer-based CAs (POP-CAs) could feature a total randomness. Photophysics, as the research topic targeting the firsthand knowledge gained by interrogating the information provided by the propagating light after its interaction with matters, could provide crucial knowledge of the targeted matter. Hence, photophysical properties could provide fundamental understanding of the targeted matter (Chapter 1.2). State-of-the-art spectroscopic methods and instrumentation have made it possible to critically examine new structures to correlate photophysics with the chemical structure of their assemblies. By combining multiple spectroscopic techniques along with theoretical study, several correlations between the electronic properties of the matter, such as structural features, have been investigated. To illustrate, some unique topology-dependent photophysical behaviors found in chromophore assemblies are introduced (Chapter 1.3). In this dissertation, the feasibility of using specific types of MOF-CAs to conduct unique photophysical studies has been carefully chosen and verified (Chapter 2). Next, with the help of first principles computations, the nature of several electronic excited states as a function of different extent of Van der Waals or electronic interaction in MOF-CAs is unveiled, and experimentally studied with several environmental variates (Chapter 3). The knowledge was then articulated to devise a strategy to improve resonance energy transfer process in MOF-CAs. Here, low electronic symmetry of linker and directionally aligned transition dipoles of their collective ensembled are found beneficial to improve such photophysical process in a bottom-up manner (Chapter 4). Then, a series of MOFs were rationally designed to examine the feasibility and extent of a nonlinear excitonic process, singlet fission, to promote the generation of carriers usable for many applications including light-harvesting applications. The outcome demonstrated MOF-CA is a powerful tool to design such materials and is more capable in terms of its tunability (Chapter 5). At last, a set of randomly oriented CAs in POP were examined for underlying excited state dynamic process that highlights a thermal activated delayed fluorescence (TADF) involving S1 and low-lying T2 excited states (Chapter 6). This dissertation has highlighted unique yet tunable excited-state features and photophysical processes within the well-defined molecular ensemble realized via porous frameworks. These photophysical properties differ from those of their respective molecular system in their solubilized forms. Studies in this dissertation demonstrates a reliable platform to investigate multibody chromophore systems and suggested several valuable discoveries and lights the way for the study of novel chromophore assembly systems.

Successful enzyme catalysis depends on many noncovalent interactions between the enzyme, cofactors, and substrate that poise the system to access a productive transition state. Motions on a variety of timescales contribute to this, but some controversy exists surrounding the role of ultrafast dynamics on catalysis. Site-specific 2D IR spectroscopy using probes of vibrational dynamics provides the opportunity to explore ultrafast motions in an enzyme active site owing to the technique’s spatial and temporal resolution. In this work, a series of aryl-azide vibrational labels were assessed using a variety of 2D IR techniques for their sensitivity to solvent and energy transfer processes, and their ability to be adapted to experiments in biomacromolecules. One of these labels, 4-azido-N-phenylmaleimide, is a substrate analog for the promiscuous ene-reductase from Pyrococcus horikoshii (PhENR). The label was covalently attached in two orientations in the enzyme active site, occupying the same position as native substrates based on X-ray crystallography and molecular dynamics simulations. FTIR and 2D IR spectroscopy were used to identify close-lying conformational states based on the strong anharmonic coupling of the label, revealing that the active site itself modulates the probe’s internal vibrational coupling. More commonly used analogous aryl-nitrile labels, however, were not sensitive to such small structural and lineshape changes. This demonstrates the importance of thoughtful label design to maximize the amount of information that can be gleaned from 2D IR studies. Using the methods herein—both spectroscopic and biochemical—provides a strategy for probing ultrafast motions that could possibly be catalytically relevant.

No description available.

No description available.