Dissertations / Theses on the topic 'Static and time resolved spectroscopies'

2011/2012
Nel vasto scenario dei materiali fortemente correlati gli ossidi dei metalli di transizione hanno attratto enorme interesse a causa delle loro interessanti proprietà fisiche, come ad esempio, la superconduttività nei cuprati e la magnetoresistenza gigante nelle manganiti. In particolare, il mio interesse è stato rivolto ad una specifica classe di materiali, per i quali la dimensionalità è il parametro più importante. Le attività sperimentali sono state focalizzate verso due sistemi: la manganite Pr0.5Ca1.5MnO4 dopata a metà e a strato singolo (hd-PCMO) e la famiglia dei rutenati Srn+1RunO3n+1 (n=1,2,3). Entrambi questi sistemi esibiscono fenomeni affascinanti strettamente legati ad una complicata interazione tra i gradi di libertà del reticolo cristallino, di spin, di carica, ed orbitale, dove la dimensionalità cristallina gioca un ruolo cruciale. Con il mio progetto di dottorato ho studiato alcune proprietà dei materiali sopracitati per mezzo di spettroscopie con raggi X, come l’emissione risonante di raggi X (RXES) e l’assorbimento di raggi X (XAS) statico e risolto in tempo. Tutte le misure sono state condotte utilizzando la linea di luce BACH (linea di luce per dicroismo avanzato) dell’anello di accumulazione Elettra della Elettra-Sincrotrone Trieste. Il sistema hd-PCMO presenta una transizione di ordinamento di carica ed orbitale (CO-O) ad una temperatura TCO relativamente elevate, i.e. 340 K, accompagnata da una distorsione strutturale ortorombica, dove i portatori di carica fortemente correlati eg del Mn si ordinano in sotto-reticoli cristallografici separati (stato di carica ordinato) con un carattere orbitale specifico (stato di ordinamento orbitale). Inoltre, hd-PCMO presenta anche una risposta reticolare anomala ad una temperatura 20 K sopra la temperatura di Neél TN, che è associata ad un inatteso accoppiamento spin-reticolo. Poiché mancava uno studio degli stati elettronici non occupati del PCMO, misure dipendenti dalla temperatura per mezzo del dicroismo lineare (XLD) sono state realizzate alle soglie K dell’ossigeno e L3 del Mn al fine di spiegare il ruolo della topologia orbitale dei Mn 3d – O 2p. I dati sperimentali, supportati da calcoli ab-initio LDA+U, ci danno informazioni sulla ridistribuzione di carica e sui cambiamenti delle p-DOS alla transizione CO-O e a quella antiferromagnetica (AFM). I risultati ottenuti mostrano che l’interazione competitiva tra la distorsione locale atomica, necessaria per permettere l’ordinamento CO, e le dinamiche di carica del meccanismo di hopping regolano lo stato orbitale dei portatori di carica. Inoltre, sulla base di studi teorici che predicono la formazione di fasi orbitali e strutturali transienti “nascoste” per mezzo della stimolazione ottica, abbiamo studiato le DOS non occupate dello stato metastabile indotto otticamente nel PCMO per mezzo della XAS risolta in tempo, che offre uno strumento unico per misurare le DOS proiettate in sito ed in simmetria degli stati metastabili della materia. Le misure XAS risolte in tempo alla soglia K dell’ossigeno sono state realizzate per mezzo di un nuovo apparato sperimentale disponibile a BACH, che si basa su un laser Ti:zaffiro (impulsi di pompa) con tasso di ripetizione variabile sincronizzato con gli impulsi a 500 MHz dei raggi X (impulsi di sonda). L’evoluzione temporale degli spettri XAS attraverso la transizione CO-O fotoindotta otticamente risulta differente rispetto alle misure XAS adiabatiche, dimostrando l’esistenza di una “fase nascosta” fotoindotta nel PCMO, la cui natura è ancora sconosciuta. I rutenati Srn+1RunO3n+1 (n=1,2,3) sono emersi come una famiglia importante di peroschiti a causa dell’evoluzione inattesa e senza precedenti dal comportamento anisotropico ferro- o metamagnetico del Sr4Ru3O10 (n=3) dipendente dalla direzione del campo magnetico, all’ aumentato paramagnetismo di Pauli vicino all’ordinamento magnetico del Sr3Ru2O7 (n=2) e, infine, alla superconduttività a bassa temperature in Sr2RuO4 (n=1). Nonostante vengano riportati numerosi studi sulle proprietà strutturali e magnetiche di questi composti, l’evoluzione delle strutture elettroniche occupate e non occupate non è stata investigata in dettaglio. Quindi, la dipendenza delle strutture elettroniche e l’ibridizzazione degli stati 2p dell’ossigeno sono state investigate combinando la spettroscopia XAS alla soglia K dell’ossigeno (transizione 2p-1s) dipendente dalla polarizzazione e la spettroscopia RXES. Una sezione del capitolo 3 è dedicata ad illustrare un setup sperimentale sviluppato recentemente per esperimenti XAS risolti in tempo sfruttando la struttura temporale “multibunch” dell’anello di accumulazione del sincrotrone. Sfruttando le potenzialità di questo setup, la transizione di superficie semiconduttore-metallo nel germanio cristallino è stata fotoindotta ed il set completo di dati viene discusso. Lo schema della mia tesi di dottorato è il seguente. Il primo capitolo presenta una panoramica dell’intero lavoro. Il secondo capitolo è diviso in due sezioni. La prima sezione introduce il lettore alla fisica orbitale ed alle transizioni di fase elettroniche nei metalli di transizione a ridotta dimensionalità, con un excursus sullo stato dell’arte dei composti 3d del manganese e la famiglia 4d dei rutenati. L’intento della seconda sezione è quello di spiegare l’importanza delle tecniche spettroscopiche nei raggi X molli come strumenti per investigare le proprietà elettroniche dei solidi. La descrizione delle spettroscopie XAS e RXES vengono riviste più in dettaglio nel capitolo 3, che include anche la descrizione dell’apparato sperimentale della beamline BACH e del laboratorio T-ReX al Sincrotrone Elettra. Il capitolo 4 è dedicato alla teoria funzionale di densità (DFT) ed alla approssimazione locale di densità più U (LDA+U) ed ai dettagli del modello del sistema hd-PCMO. Il capitolo 5, che presenta i casi studiati, è diviso in due sezioni: il caso del PCMO, che include le misure XAS statiche e risolte in tempo, ed il caso della serie Ruddlesden-Popper dei rutenati di Sr investigate per mezzo della RXES. Nel capitolo finale vengono presentati i commenti finali su questo lavoro.
In the vast scenario of strongly correlated-electron materials transition-metal oxides have attracted enormous interest because of their interesting physical properties, including for example, superconductivity in cuprates and colossal magnetoresistance in manganites. In particular, my interest was directed to a particular class of materials, whose dimensionality is the most defining material parameter. With my Ph.D. project I deepened into some physical properties of these materials by means of core-levels spectroscopies such as resonant x ray emission (RXES) and static and time-resolved x ray absorption (XAS). All the measurements have been carried out at the beamline BACH (Beamline for Advanced diCHroism) at the Elettra light source facility in Trieste. The experimental activities focused on two case-study systems: the single layered half-doped Pr0.5Ca1.5MnO4 (hd-PCMO) and the layered Srn+1RunO3n+1 (n=1,2,3) family. Both these systems exhibit fascinating phenomena intimately related to a complicated interplay between the crystal lattice, spin, charge, and orbital degrees of freedom, where crystal dimensionality plays a crucial role. hd-PCMO exhibits a charge-orbital ordering (CO-O) transition at a remarkably high TCO, slightly above room temperature, accompanied by an orthorhombic structural distortion, where the strongly correlated Mn eg charge carriers order onto separate crystallographic sub-lattices (charge-ordered state) with a specific orbital character (orbital ordered state). Furthermore, hd-PCMO also displays an anomalous lattice response at temperatures 20K above the Neél temperature TN, which is associated to an unexpected spin-lattice coupling. Since a study of the PCMO unoccupied electronic states was lacking, temperature dependence measurements by XAS linear dichroism (XLD) have been performed at the O-K and Mn-L3 thresholds in order to elucidate the role of Mn 3d - O 2p orbital topology. The experimental data, supported by ab-initio LDA+U, shed light on the charge redistribution and p-DOS changes at the CO-O and antiferromagnetic (AFM) transitions. The results obtained show that the competitive interplay between the local atomic distortion, necessary for accomodating the CO-ordering, and the charge dynamics of the hopping mechanism regulates the orbital state of the charge carriers. Furthermore, on the basis of theoretical studies that predict the formation of transient “hidden” orbital and structural phases by optical stimulation, we have studied the unoccupied DOS of the optically induced metastable state in PCMO by means of time resolved XAS, which offers a unique tool to measure site and symmetry projected DOS of metastable states in matter. Tr-XAS measurements at the O-K edge have been carried out by means of a novel experimental apparatus available at BACH, which is based on a variable repetition rate Ti:sapphire laser (pump pulse) synchronized with the ∼ 500 MHz X-ray photon pulses (probe pulses). The time evolution of the XAS lineshapes across the optically photoinduced CO-O transition results different respect to the adiabatic XAS measurements, demonstrating the existence of a photoinduced “hidden phase” in PCMO, whose nature is still unknown. The layered Srn+1RunO3n+1 (n=1,2,3) have emerged as an important family of perovskites because of the unexpected and unprecedented evolution from anisotropic ferro- or metamagnetic behavior of Sr4Ru3O10 (n=3) dependent on the direction of the magnetic field, enhanced Pauli paramagnetism close to magnetic order of Sr3Ru2O7 (n=2) and, finally, to low-temperature superconductivity in Sr2RuO4 (n=1). Although numerous studies have been reported on the structural and magnetic properties of these compounds, the evolution of the occupied and unoccupied electronic structures were not investigated in detail. Thus, the dependence of electronic structures and the hybridization of O 2p states have been investigated by combining polarization dependent O K (2p-1s transition) XAS and RXES spectroscopies. A section of the chapter 3 is dedicated to illustrate a newly developed experimental setup for time-resolved XAS experiments by exploiting the multibunch time structure of a synchrotron storage ring. By exploiting the capabilities of this setup, the surface semiconductor-metal transition in crystalline germanium has been photoinduced and the complete set of data discussed. The outline of my Ph.D. thesis is the following. The first chapter presents an overview of the entire work. The second chapter is divided into two sections. The first section introduces the reader into the orbital physics and the electronic phase transitions in low dimensional transition metal oxides, with an excursus on the state of the art of 3d manganese compounds and the family of 4d Ruthenates. The second section is aimed to explain the importance of soft x-ray spectroscopic techniques as tools to investigate the electronic properties of solids. The description of XAS and RXES are reviewed in more details in chapter 3, which includes also the description of the experimental apparatus of BACH beamline and T-ReX lab at the Elettra synchrotron light source. Chapter 4 is dedicated to the Density Functional Theory (DFT) and Local Density Approximation plus U (LDA+U) theories and to the details of the modelling of the hd-PCMO system. Chapter 5, which presents the cases studied, is divided into two sections: the case of PCMO, including static and time resolved XAS measurements, and the case of Ruddlesden-Popper series of Sr Ruthenates investigated by means of RXES. In the final chapter the concluding remarks on this work are presented.
XXV Ciclo
1983

Les matériaux semiconducteurs de pérovskite halogénée se sont révélés très prometteurs en raison de leurs propriétés exceptionnelles et de leur capacité d’élaboration à faible coût. Malgré les progrès réalisés, une meilleure compréhension des dynamiques de relaxation et de recombinaison dans ces matériaux photo-actifs est nécessaire afin d’améliorer les performances des dispositifs opto-électroniques. Cette thèse porte sur la relaxation des porteurs de charge/excitons « chauds » après excitation au-dessus de la bande interdite, et la recombinaison Auger advenant après une excitation de forte puissance ou à haute énergie. La spectroscopie de fluorescence résolue en temps et d'absorption transitoire (AT) femtoseconde sont utilisées afin d’étudier les effets du confinement et de la composition sur ces processus, en particulier dans les nanoplaquettes (NPLs) colloïdales de pérovskite bidimensionnelles (2D), dont la synthèse a été développée et optimisée au préalable. Les dynamiques de relaxation ont été étudiées par AT en utilisant une méthode d'analyse globale où l'évolution temporelle des différentes compositions spectrales est modélisée suivant une cinétique de réactions séquentielles. Cette méthode a pu être appliquée afin de décrire efficacement la relaxation progressive dans les nanostructures de FAPbI₃ (FA= formamidinium) faiblement confinées, et a permis de dissocier les processus de retardement de relaxation à haute puissance d’excitation dus aux effets de « hot phonon bottleneck » et de ré-excitation par recombinaison Auger (RA). Par ailleurs, l'analyse globale de l’évolution spectrale a été essentielle afin d’examiner les dynamiques de relaxation dans les NPLs 2D à fort confinement quantique et diélectrique, présentant des effets Stark importants et une transition de bord de bande excitonique discrète. Comme dans les systèmes faiblement confinés, le taux de relaxation dans les NPLs diminue avec la puissance d'excitation. Cependant, il est plus rapide dans les systèmes présentant un confinement plus important, mettant alors en évidence l'absence de ”phonon bottleneck” intrinsèque. La nature des cations internes (FA, MA=méthylammonium ou Cs) n’influe pas ce taux, ni son évolution en puissance. Toutefois, les mesures dans les films de pérovskite 2D présentant une épaisseur de puits quantique équivalente, suggèrent un rôle des ligands de surface dans la capacité à libérer l'excès d'énergie dans l'environnement. Ensuite, les recombinaisons multi-excitoniques dominées par la RA non radiative ont été étudiées dans les NPLs de pérovskite 2D fortement confinées. Dû à la géométrie asymétrique de ces nanostructures et de la délocalisation limitée de l'exciton, le temps de RA dépend fortement de la densité d'excitons via la distance inter-exciton moyenne : A faible puissance d’excitation, la RA est limitée par la diffusion de l'exciton dans le plan 2D et se produit sur plusieurs centaines de picosecondes (dépendant ainsi de la géométrie de l'échantillon). En revanche, une puissance d'excitation élevée produit des excitons dont les fonctions d’onde se recouvrent spatialement, entraînant des temps de RA inférieurs à 10 ps. Finalement, les dynamiques excitoniques dans les NPLs 2D ont été mesurées par AT après excitation dans l’ultraviolet afin d'observer le processus de multiplication d’excitons. Ce dernier implique la génération de plusieurs excitons “géminés”, et donc proches spatialement, suite à l'absorption d'un unique photon de haute énergie
Halide perovskites have emerged as very promising photoactive materials due to their outstanding optoelectronic properties combined with low-cost processability. In spite of their successful implementation in photovoltaic or light-emitting devices, a deep understanding of the dynamics of relaxation and recombination is still missing in order to enhance the device performances. This thesis focuses on the study of two major fundamental processes occurring in colloidal halide perovskite nanostructures: the hot charge carrier/exciton relaxation (“cooling”), after excitation above the optical bandgap, and the non-radiative Auger recombination, taking place after high-fluence or high-photon energy excitation. In particular, time-resolved photoluminescence and femtosecond transient absorption spectroscopy were used to investigate the confinement and composition effects in strongly confined two-dimensional (2D) lead iodide perovskite nanoplatelets (NPLs), that were synthesized following the development and optimizations of colloidal methods. For the investigation of the cooling dynamics, a global analysis method based on single value decomposition was used, where the temporal evolution of the spectral lineshapes was modeled with a sequential kinetic scheme. This method was succesfully applied to effectively describe the continuous energy relaxation in weakly-confined thick FAPbI₃ nanoplates (FA=formamidinium) and allowed disantangleting the hot phonon bottleneck from the Auger reheating effects at high excitation fluence. Furthermore, the global analysis was essential to investigate the cooling dynamics in strongly confined 2D NPLs presenting large Stark effects and discrete excitonic band-edge transition far away from the continuum of states (exciton binding energy in several hundreds of meV). As in the weakly confined samples, the cooling rate of the NPLs decreases with the excitation fluence. However, it is faster in more-strongly confined samples, evidencing the absence of an intrinsic phonon bottleneck. Furthermore, the cooling rate and its evolution with the exciton density were found independent of the nature of the internal cations (FA, MA=methylammonium and Cs=cesium). However, when comparing with the rate measured in 2D layered perovskite thin film with equivalent quantum well thickness, the results strongly suggest a role of the surface ligands in the possibility to release the excess energy to the surrounding environment. This ligand-mediated relaxation mechanism becomes dominent in the thinner NPL samples with enhanced exciton/ligand vibrational mode coupling. Then, the multiple exciton recombination dominated by non-radiative Auger recombination (AR) was studied in the strongly-confined 2D perovskite NPLs. Due to the large asymmetric geometry and the limited exciton wavefunction delocalization, the AR rate strongly depends on the exciton density via the initial average inter-exciton distance. At low fluence, this distance is in several tens of nanometers such as the AR is limited by the exciton diffusion in the 2D plane. It thus occurs on a timescale of several hundreds of picoseconds and depends on the sample dimensionality (thickness and lateral sizes). In contrast, high excitation fluences produce “overlapping” excitons with inter-exciton distances of only a few times the exciton Bohr radius, resulting in AR times of less than 10 ps and independent of the NPL composition nor geometry. Finally, the exciton population dynamics of 2D NPLs after excitation in the ultraviolet was measured. The strong dependence of the AR with the inter-exciton distance allows the identification of multiple exciton generation (MEG), which involves the reaction of “geminate biexcitons” produced by the absorption of a single high-energy photon

Les brins d'ADN riches en guanine, comme ceux présents à l'extrémité des chromosomes humains, sont capables de s'associer entre eux pour former des structures G-quadruplexes, résultant de l'association de quatre guanines (G-tétrade). Ces structures sont actuellement l'objet d'un intérêt particulier pour le développement de nouvelles thérapies anti-cancéreuses et des applications potentielles pour l'électronique moléculaire. Il n'existe cependant que très peu d'études des propriétés photophysiques des G-quadruplexes. L'objectif de ce travail de thèse est d'étudier l'influence de la structrure des G-quadruplexes sur leurs propriétés photophysiques au moyen de la spectroscopie de fluorescence résolue en temps sur une gamme temporelle allant de la centaine de femtosecondes à la centaine de nanosecondes. Nous avons examiné l'effet de la taille de structures G-quadruplexes tétramoléculaires sur leurs propriétés photophysiques. Nous avons pu montrer que le caractère collectif des états ππ* des guanines est renforcé lorsque le nombre de tétrades augmente et qu'un transfert d'énergie ultra-rapide, en moins de 100 fs a lieu entre ces états. Nous avons ensuite mis en évidence le rôle des cations métalliques situés dans la cavité centrale des quadruplexes dans le processus de désactivation des états excités. En présence de K+, l'émission provient principalement des états délocalisée ππ* des guanines, alors qu'en présence de Na+, l'émission dominée par la contribution d'états excités à caractère de transfert de charge. Enfin, nous avons abordé l'effet de la topologie, en comparant les propriétés photophysiques des G-quadruplexes tétramoléculaires avec celles de structures formées par le repliement d'un simple brin d'ADN. Les différences observées peuvent s'expliquer par la rigidité accrue des structures simple-brins et l'orientation relative différente des tétrades qui détermine la force du couplage électronique entre les bases.

Ce travail de thèse est centré sur l'étude expérimentale de deux types de transformations photo-induites observées dans différents complexes métalliques (Fe, Ru, ...) à ligand nitrosyle: (i) la photo-isomérisation au cours de laquelle le ligand NO passe d'un état lié par l'azote (M-NO, état stable GS) à un état lié par l'oxygène (M-ON, état métastable MSI). C'est un processus réversible et un second état métastable, 'side-on' MSII est observable au cours du processus inverse, MSI --> 'side-on' MSII --> GS; (ii) la libération photo-induite du radical NO°. La photo-isomérisation a été étudiée dans le système [Ru(py)_4Cl(NO)](PF_6)_2.1/2H_2O (sur monocristal) par la diffraction des RX, l'absorption visible statique et l'absorption UV-Vis résolue en temps. Nous avons pu mettre en évidence un processus d'absorption de photons (473 nm) en deux étapes, suivre la dynamique des transformations directes et inverses par l'évolution des paramètres de maille et des l'absorption optique visible, caractériser structuralement l'état 'side-on' MSII. La nature de l'état intermédiaire, MSII, lors du processus direct semble être différente de l'état 'side-on' mais la très faible population de cet intermédiaire n'a pas permis de trancher définitivement. Le rôle de la molécule d'eau dans la maille du complexe [Ru(py)_4Cl(NO)](PF_6)_2.1/2H_2O a été testé par diffraction des RX sur les échantillons hydratées, déshydratées et réhydratées. L'effet de déshydratation sur l'évolution des paramètres de maille et sur la génération d'espèces MSII au cours de la photo-commutation inverse, MSI --> MSII --> GS, est présenté. La libération photo-induite du radical NO° a été étudiée dans une solution d'acétonitrile du complexe trans-(Cl,Cl)[Ru^{II}(FT)Cl_2(NO)]PF_6 par spectroscopies d'absorption UV-Vis et Mi-IR, statiques et résolues en temps. En observant la dynamique ultra-rapide de la photo-transformation, nous avons mis en évidence un processus d'absorption de photons (406 nm) en deux étapes. La nature de l'état intermédiaire est discuté par rapport à de récents calculs théoriques et au processus de photo-isomérisation
This Ph. D. thesis is focused on the experimental study of two photo-induced processes observed in various [ML_5NO] complexes, where M=Fe, Ru, ..., L=CN, Cl, ... : (i) photo-isomerization, where the system goes from N-bound state (M-NO, stable GS state) to O-bound state (M-ON, metastable MSI state) and a second metastable state, 'side-on' MSII is observed during the inverse photo-transformation, MSI --> 'side-on' MSII --> GS; (ii) photo-induced release of NO° radical. The photo-isomerization is investigated in single crystals of [Ru(py)_4Cl(NO)](PF_6)_2.1/2H_2O compound by means of X-Ray diffraction, steady state visible and time-resolved UV-visible absorption spectroscopies. During these experiments we observed the evidence of a two-step photon absorption process (473 nm), we followed the dynamics of direct and inverse photo-switching by the evolution of lattice parameters and absorption in the visible range and we caracterised the structure of 'side-on' MSII state. The nature of MSII in the direct photo-switching, GS --> MSI, seems to be different from the 'side-on' configuration, however, we do not have a direct access to this state due to its low population. The role of water molecule in the unit cell of [Ru(py)_4Cl(NO)](PF_6)_2.1/2H_2O complex was tested by X-Ray diffraction studies on hydrated, dehydrated and rehydrated samples. The effect of sample dehydration on the unit cell parameters evolution and MSII species generation during the MSI --> MSII --> GS inverse photo-switching is presented. The photo-induced NO° release in trans-(Cl,Cl)[Ru^{II}(FT)Cl_2(NO)]PF_6 system in acetonitrile solution is studied by the UV-visible and mid-IR absorption spectroscopies, both steady state and time-resolved. By observing the ultra-fast dynamics of photo-transformation we evidenced a two-step photon absorption process (406 nm). The nature of intermediate state is discussed within the recent theoretical calculations and the photo-isomerization process

La focalisation de lasers à impulsions ultra-brèves dans les verres a montré des potentialités importantes pour la structuration des matériaux transparents, permettant d'envisager la réalisation de dispositifs tout-optique en une seule étape. Le développement de nouveaux matériaux vitreux de composition 40P2O5-55ZnO-xAg2O-(5-x)Ga2O3 permet de tirer avantage de la photosensibilité de l'ion Ag+ pour la structuration de propriétés optiques au cœur du matériau. L'augmentation du taux d'oxyde de gallium provoque la dépolymérisation des chaînes phosphates et s'accompagne de la réticulation du réseau vitreux par des polyèdres GaOx (4 ≤ x ≤ 6). La formation de des derniers n'affecte pas les propriétés d'émission lumineuses, qui proviennent des ions Ag+ isolés dans différents sites cristallographiques. L'exposition des verres sous faisceau laser intense provoque la formation de centres-trous Ag2+, responsables d'une luminescence intense, et de pièges d'électrons Ag0. Un traitement thermique consécutif ou simultané à l'irradiation provoque la dissociation de ces espèces, et s'accompagne de la formation d'agrégats moléculaires de la forme Agmx+. L'utilisation d'un laser femtoseconde à haute cadence permet, en combinant création de paires électron-trou et élévation locale de la température, de réaliser des architectures complexes conduisant à la structuration de propriétés optiques.

Les espèces réactives de l'oxygène sont générées dans l'environnement biologique dans le cadre du métabolisme, mais elles peuvent aussi être produites en excès dans le cas de stress oxydatif provoqué par exemple par une exposition aux rayons UV. Dans le travail présenté ici, nous sommes intéressés par l'oxydation des protéines par deux de ces espèces réactives de l'oxygène : le peroxyde d'hydrogène, oxydant plutôt faible avec un temps de vie long, et l'oxygène singulet, oxydant fort avec un temps de vie court. L'action de ce dernier sur les protéines est étudiée en utilisant la spectroscopie de phosphorescence résolue en temps et l'oxydation des protéines par le peroxyde d'hydrogène est suivie par spectroscopie Raman. Dans ce cas, un travail préliminaire a été nécessaire afin d'attribuer de manière précise les bandes Raman des chaînes latérales des résidus d'acides aminés. Pour les deux types d'oxydations, les constantes de vitesse des réactions ont été déterminées pour trois protéines modèles. La stratégie suivie est d'utiliser de petits fragments de protéines tels que des acides aminés libres et des tripeptides pour comprendre ce qui se passe à l'échelle de la protéine. Cela nous aide à souligner l'importance de l'environnement protéique. Dans le cas de l'étude par spectroscopie Raman, l'influence du nombre de liaisons peptidiques sur les spectres obtenus depuis l'acide aminé libre, au tripeptide, jusqu'à la protéine est aussi mis en évidence.

In this diploma thesis was studied time-resolved fluorescence in polymer-surfactant system. At first aggregation numbers of cationic (cetyltrimethylammonium bromide), anionic (sodium dodecylsulfate) and nonionic (Triton X-100) surfactants were studied by steady-state and time-resolved fluorescence spectroscopy. These two methods were compared. Aggregation numbers by steady-state method were always lower than aggregation numbers measured by time-resolved method. Steady-state method of determination aggregation numbers is useless for surfactants with high aggregation number and for aerated samples. Addition of hyaluronan to surfactant system was studied. There was observed change in aggregation number after addition of hyaluronan and change in percentage of dynamic quenching after addition of hyaluronan. Hyaluronan affected aggregation number of cetyltrimethylammonium bromide and Triton X-100. Hyaluronan increased percentage of dynamic quenching in cetyltrimethylammonium bromide and in Triton X-100. Pyren in sodium dodecylsulfate was quenched by sphere of action with negligible percentage of dynamic quenching and addition of hyaluronan had no effect on quenching. As next goal of this thesis, the determination of the position of fluorescence probe pyrene in cetyltrimethylammonium bromide, sodium dodecylsulfate and Triton X-100 micelles was chosen. Position of pyrene changed with charge and structure of micelles. Next was studied how percentage of dynamic quenching by iodide compounds changes with different charge of micelle. In all cases majority of dynamic quenching was calculated.

This thesis collects the results of a series of experiments aimed at the characterization of energy and charge transfer processes in organic systems, carried out during my PhD course. The complexity of the subject is connected to the identification of the micro-scale phenomena that occur after visible excitation of the systems. The goal is to control these processes in order to maximize the energy migration throughout the systems, aiming to their potential application in OPV devices. Stationary and time-resolved spectroscopic techniques are excellent experimental methods to investigate ET in organic molecular complexes, because the electronic properties are revealed in the absorption and emission spectra. The characterization of the spectral features and their time-evolution, on femtosecond timescale, gives important pieces of information about the nature and dynamics of the excited state, and its interaction with the surrounding. The first experimental work I carried out in the Ultrafast group of Prof. Paolo Foggi at LENS was focused on the spectroscopic investigation of a substituted Zn-Phtalocyanine (ZnPc) dye, that has a potential application as sensitizer in Dye Sensitized Solar Cells (DSSCs). Transient Absorption Spectroscopy (TAS), an ultrafast pump-probe technique with sub-picosecond time resolution, was employed to investigate the excited state dynamics of the dye in ethanol (EtOH) solution and adsorbed on nanocrystalline films of Titania (TiO2) and Zirconia (ZrO2). TAS results allowed us to identify the timescales of the different relaxation processes occurring after visible excitation, and to propose a scheme of the energetic levels involved in the excited state dynamics. Furthermore, we had evidence electron injection in the Titania conduction band, which makes the molecule suitable for DSSCs applications. Many research group are focused toward the realization of artificial light-harvesting antennae for energy capture, based on organic molecules. The optimal electronic properties of such systems are still object of scientific debates, because different ET mechanisms take part in the overall exciton migration, which interplay is not trivial. During a one-year period spent in the M. G. Bawendi group at MIT, I studied the exciton transport properties in self-assembled light-harvesting nanotubes (LHNs), quasi one-dimensional aggregates consisting of ordered amphiphillic cyanine dyes. These systems present interesting excitonic properties, which derive from the unusual electronic coupling between the monomers. Because of the low static disorder, large exciton delocalization and the negligible reorganization energy (low coupling with the environment), LHNs represent an ideal model system to explore the quantum contribution and the influence of energetic disorder on the ET. The experimental investigation of LHNs was carried out by stationary and time resolved spectroscopy, at room temperature and as a function of temperature, from room temperature to 5K: the change in the spectral lineshape was interpreted in terms of homogeneous broadening. Intensity dependent fluorescence measurements gave evidence of exciton-exciton nnihilation (EEA). In order to get insight the effect of energetic disorder, a ballistic model for exciton diffusion was developed: two regimes of exciton migration were identified, depending on the relative value of the 2homogeneous and inhomogeneous timescales. Stationary fluorescence was measured as a function of the visible light irradiation, over several minutes, showing Photobrightening (PB) phenomena followed by Photodarkening (PD) on longer timescales. This results, confirmed also by 2D electronic spectroscopy (2DES), have been interpreted in terms of exciton migration mediated by super-radiance, a collective emission process arising from the coherent exciton delocalization. In order to deeply investigate the view on the molecular aggregate and their influence on exciton dynamics, I performed 2DES measurements on mixed monomer / H-aggregate systems made of ZnPc molecules. When dissolved in particular solvents, such as Cloroform (CHCl 3 ), the ZnPc molecules forms aggregates. Aggregation is one of the main issues that prevents a high efficiency of electron injection in the semiconductor conduction band. In the aggregation process, radiative channels of deactivation from the dye excited state are introduced. 2DES is a powerful multidimensional coherent spectroscopic technique that allows to: investigate the exciton dynamics and the lineshape time-evolution on femtosecond timescale, identifying the homogeneous and inhomogeneous timescales; highlight the coupling between electronic states and the possible coherent contribution. Broadband excitation of the sample allowed to discriminate between the monomer and aggregate kinetic traces. We had evidence of electronic coupling between the aggregate and the isolated molecules in close proximity. Furthermore, electronic coupling mediated by an internal vibrational mode was observed between two non-degenerate Q-states of the monomer.

This Ph.D. thesis is an exploration of carotenoids by ultrafast, time-resolved absorption spectroscopy to investigate their complicated relaxation processes, means of energy transfer, and dependence on structure. The introduction begins with an overview of carotenoids, intended for the reader to appreciate their importance and their complexity as revealed by decades of research in carotenoid photophysics. To understand the primary concerns of this research field, the reader is guided through basic theory of energetic processes, the experimental method, and methods of analysis. The main body of the text is the Research Chapter, containing four sections, each describing research using varied ultrafast transient absorption spectroscopies on carotenoids in solution and when bound to a host protein. Section 2.1 concerns an equilibration phenomenon in the lowest excited state of the carotenoid fucoxanthin in various solutions and temperatures by a multi-pulse transient absorption method. The same method is applied to fucoxanthin in a host antennae protein of the pennate diatom Phaeodactylum tricornutum to investigate the function of the equilibration in energy transfer to Chlorophyll a in Section 2.2. The next two sections regard the effect of carotenoid structure on its relaxation dynamics. Section 2.3 investigates the effect of the non-conjugated acyloxy group of two fucoxanthin derivatives in various solvents. Here, one of the energetic states involved in the equilibrium mentioned above is seen drastically affected. Lastly, Section 2.4 investigates alloxanthin, a carotenoid with an unusual pair of carbon-carbon triple bonds. Their effect on the conjugation is evaluated based upon the molecules' decay dynamics. A general summary and conclusion is provided at the end.

In this work the transient non-instantaneous polarization, i.e., laser-pulse injected small polarons and self-trapped excitons, is studied in the perovskite-like ferroelectric lithium niobate. The investigations span a time scale from femtoseconds to several hours. It is shown that the established small polaron picture is not able to describe transient absorption and photoluminescence of lithium niobate consistently. Several strong indications are presented demonstrating that the photoluminescence cannot be caused by geminate small polaron annihilation. Instead, the idea of radiatively decaying self-trapped excitons at the origin of the blue-green photoluminescence is revived. Excitons pinned on defect sites are proposed to lead to the already observed long-lived transient absorption in the blue spectral range in Mg- and Fe-doped crystals. Excitons pinned on iron-defects are studied in more detail. Their spectral fingerprint and absorption cross section is determined. Furthermore, it is shown that the occurrence of these pinned STEs can be tailored by chemical treatment of the samples and the experimental parameters such as the pump pulse intensity and photon energy. Based on the new experimental results and reviewing data published in literature, an atomistic picture of hopping and pinning of self-trapped excitons in lithium niobate is proposed. The question is addressed whether small polarons and self-trapped excitons in lithium niobate are coupled species in the sense that oppositely-charged polarons may merge into self-trapped excitons or STEs break into small polaron pairs. Decay kinetics of transient absorption and luminescence assigned to free small polarons and STEs indicate that this is not the case. For a more complete picture the ultrafast time scale is investigated as well. The formation times of small polarons and STEs are determined, which both lie in the range of 200 fs. No indications are found on the (sub)picosecond time scale indicating a coupling of both quasi-particle species either. In order to gain access to the formation of self-trapped excitons a custom-built femtosecond broadband fluorescence upconversion spectrometer is installed. Based on an already existing scheme, it is adapted to the inspection of weakly luminescent solid samples by changing to an all reflective geometry for luminescence collection. To avoid the necessity for an experimentally determined photometric correction of the used setup, an already established calculation method is extended considering the finite spectral bandwidth of the gate pulses. The findings presented here are important not only as fundamental research, but also regarding the technical application of lithium niobate and other similar nonlinear optical crystals. The simultaneous occurrence of both small polarons and self-trapped excitons is a rather rarely described phenomenon. Usually, the optical response of wide band gap oxide dielectrics is associated with only one of these quasi-particle species. This work may therefore be a stimulus to review the existing microscopic models for transient phenomena in other oxide dielectrics, which may help to improve their application in nonlinear optical and electro-optical devices. In this context the ultrafast transient photoluminescence spectroscopy established here for weakly luminescing solid samples may again provide valuable insight. With respect to lithium niobate, the results do not only resolve inconsistencies between the microscopic pictures described in literature, but also provide information regarding the extends to which the propagation of ultrashort laser pulses may be affected by (pinned-)STE absorption. It is shown that tailoring of the long-lived absorption center in the blue spectral range is possible, which may be used to avoid optical damage when high repetition rates are applied. It is important to emphasize that the microscopic model proposed in this work is mainly based on experimental indications. It is the task of further detailed theoretical investigations, e.g., via time-dependent density functional theory, to test whether the proposed model is justified. From an experimental perspective the important question remains whether (pinned-)STEs contribute to a photorefractive effect. In the experimentally easily accessible spectral range no absorption feature of mobile STEs is observed. As a complementary experimental technique, ultrafast holographic spectroscopy may reveal an excitonic contribution to photorefraction and provide further insight to STE transport and pinning phenomena.