Methoden der transienten Vierwellenmischung mit Femtosekunden-Zeitauflösung werden angewendet, um die Phasen- und Energierelaxation optisch selektiv erzeugter Anregungszustände im Lichtsammelkomplex II höherer Pflanzen (LHC II) sowie im Halbleiter Cadmiumsulfid (CdS) bei verschiedenen Temperaturen zu untersuchen. Für den LHC II werden die Ergebnisse der Messungen des zeitaufgelösten und integrierten Zweipuls-Photonenechos mit Resultaten aus Pump-Test-Experimenten verglichen, um unter Einbeziehung von Literaturdaten Rückschlüsse über den Charakter der phasenzerstörenden Prozesse zu ziehen und Zusammenhänge zu Strukturdaten des Komplexes aufzudecken. Die vorliegende Arbeit liefert erstmals einen systematischen Überblick über die totalen Phasenrelaxationszeiten T2 im Bereich der Qy-Bande des LHC II von 640 bis 685 nm bei 5 K. Das bei 5 K beobachtete Photonenechosignal am LHC II zeigt in Abhängigkeit von der Verzögerung der beiden Anregungsimpulse ein multiexponentielles Abklingen, das auf die Überlagerung der Einflüsse mehrerer Relaxationsprozesse zurückgeführt wird. Dabei lassen sich drei charakteristische Bereiche der Phasenrelaxationszeit unterscheiden, die verschiedenen phasenzerstörenden Prozessen zugeordnet werden. Ein Vergleich mit Resultaten aus Pump-Test-Experimenten führt zu der Schlußfolgerung, daß die Phasenrelaxation im LHC II bei 5 K für Wellenlängen £ 675 nm im wesentlichen durch den Energietransfer auf einer sub-ps Zeitskala bestimmt wird. Für Wellenlängen > 675 nm steigt die Phasenrelaxationszeit stark an und wird insbesondere im Bereich der tiefsten Anregungszustände um 680 nm durch reine Phasenzerstörung dominiert. Ab 20 K setzt bei dieser Wellenlänge ein zusätzlicher phasenzerstörender Prozeß ein, der mit steigender Temperatur zu einem mäßigen linearen Anstieg der Phasenrelaxationsrate (T2)-1 führt. Die Ursache ist vermutlich ein Aufwärts-Energietransfer. Im Bereich der Chlorophyll a-Absorption vernichten außerdem (physiologisch irrelevante) Multiexzitoneneffekte die Kohärenz der angeregten Zustände, verursacht durch die hohe Anregungsintensität. Zusammenfassend kann festgestellt werden, daß die Erhaltung der Kohärenz für die Funktionalität des LHC II eine untergeordnete Rolle spielt. Die wesentlichen Prozesse sind der schnelle räumliche Energietransfer und die Energierelaxation auf das Niveau des primären Elektrondonators P680 im Reaktionszentrum. Am Halbleiter CdS wird erstmals ein mittels Zwei-Photonen-Absorption angeregtes Photonenecho beschrieben, das in Abhängigkeit von der Wellenlänge charakteristische Quantenbeats mit einer Periode von 700 bis 800 fs zeigt. Das stark gedämpfte periodische Echosignal tritt sowohl für positive als auch für negative Verzögerungszeiten t der Anregungsimpulse auf, wobei die Abklingzeit für t>0 mit 170±10 fs doppelt so groß ist wie für t / Transient four-wave-mixing experiments with femtosecond resolution are performed in order to investigate phase and energy relaxation processes of optically excited states in the light harvesting complex II of higher plants (LHC II) and in the semiconductor cadmium sulfide (CdS) at different wavelengths and temperatures. Extensive studies of the time resolved and integrated two-pulse photon echo on LHC II are combined with pump-probe experiments. Results of both methods together with literature data are used to characterize the nature of dephasing processes and to reveal connections with structural data of the complex. This study gives the first systematic survey of total dephasing times T2 in the spectral region of the Qy-absorption band of LHC II from 640 to 685 nm at 5 K. In the case of LHC II, the photon echo signal at 5 K monitored as a function of delay between both excitation pulses shows a multi-exponential decay which is attributed to the superposition of several relaxation processes. Three characteristic dephasing time domains can be distinguished, ascribed to different dephasing processes. Comparing photon echo and pump-probe results it can be concluded that dephasing in LHC II at 5 K and for wavelengths £ 675 nm is dominated by the fast excitation energy transfer on a sub-ps time scale. At wavelengths > 675 nm the total dephasing time increases drastically. The loss of coherence of the lowest excited states around 680 nm at 5 K is mainly determined by pure dephasing. An additional dephasing process, probably uphill energy transfer, occurs at temperatures higher than 20 K leading to a moderate linear rise of the dephasing rate (T2)-1 with increasing temperature. Furthermore, the dephasing in the spectral region of chlorophyll a absorption is affected by (physiologically irrelevant) multi-excitonic effects caused by the high excitation energy. In summary, it can be concluded that the preservation of coherence plays a minor role in the functionality of LHC II. The main processes are the fast spatial excitation energy transfer and the energy relaxation down to the energetic level of the primary electron donor P680 of the reaction center. Investigations of four-wave mixing signals of the semiconductor CdS resulted in the first description of a two-photon excited photon echo in CdS showing characteristic quantum beats with a period of 700 to 800 fs in dependence on wavelength. The strongly damped periodical echo signal is found for both positive and negative delay times t between the excitation pulses. The decay time for t>0 amounts to 170±10 fs and is twice as large as for t

Ce mémoire présente une étude expérimentale de la fragmentation en phase gazeuse des biomolécules, adénine (H5C5N5) et porphyrine FeTPPCl (C44H28N4FeCl), induite par collision avec des ions à basse énergie. La distribution de population pour chaque voie de dissociation a été mesurée en fonction de l'énergie d'excitation des ions moléculaires parents avec la méthode CIDEC (Collision Induced Dissociation under Energy Control). Dans les collisions entre Cl+ à 3keV et adénine (Ade), le schéma de fragmentation de Ade2+ est dominée par la perte de H2CN+ et les émissions successives de HCN. La distribution de l'énergie des Ade2+ parents confirme la dynamique des émissions successives. Une voie de dissociation spécifique, à savoir l'émission successive de H2CN+ et HC2N2 est observée. Les schémas de fragmentation des ions moléculaires FeTPPCl1+, 2+, 3+ sont étudiés dans des collisions avec Kr8+ à 80 keV. Il est constaté qu’indépendante de l'état de charge initiale de FeTPPClr+ (r=1, 2, 3), la perte de Cl0 constitue la première étape de la chaîne de dissociation, tandis que l’état de charge initiale des molécules joue un rôle important dans les étapes suivantes de la dissociation. Dans les collisions avec H+ et F+ à 3keV, dû à un effet de fenêtre de réaction dans les processus de production d’ions négatifs, des schémas de fragmentation très différents sont observés pour FeTPPCl2+. Grâce à la mesure de l’énergie interne des molécules parents, la perte de nH2 est observée et analysée. De plus, le rendement de production d'ions négatifs, mesuré à environ 1% dans des collisions F2+-Ade à 30 keV, est étudié dans ce travail en utilisant une nouvelle approche expérimentale. / In this work, the Collision Induced Dissociation under Energy Control method was extended to study the fragmentation of gas-phase biomolecules adenine (H5C5N5) and porphyrin FeTPPCl (C44H28N4FeCl). The population distribution for each dissociation channel has been experimentally determined as a function of the excitation energy of the parent molecular ions at a well-determined initial charge state. In collisions between Cl+ and adenine (Ade) at 3keV, the fragmentation pattern of Ade2+ is dominated by the loss of H2CN+ and the successive emission of HCN. The energy distribution of the parent dications confirms the successive emission dynamics. A specific decay channel is observed, i.e., the emission of a charged H2CN+ followed by the emission of HC2N2. In Kr8+-FeTPPCl collisions at 80keV, parent ions FeTPPCl1+,2+,3+ are observed, along with the corresponding decay patterns. It is found that in the first step the dominant low-energy-cost decay channel is the emission of Cl0 independent of the initial charge state of FeTPPClr+ (r=1-3). For the resulted dication FeTPP2+, the dominant fragmentation channel is the neutral evaporation; for the trication however, the dominant fragmentation channel is the asymmetrical fission. In the case of H+ and F+ impact at 3keV, due to the different reaction windows opened in the two collision systems, different fragmentation patterns are observed. Furthermore, nH2 loss processes are observed. Additionally, the production yield of the negative ion emerged in F2+-Ade collision at 30keV is measured to be about 1% using a new experimental approach.

Title: Artificial light-harvesting antenna based on an aggregation of bacteriochlorophyll c with selected pigments Author: Tomáš Malina Department: Department of Chemical Physics and Optics Supervisor of the master thesis: doc. RNDr. Jakub Pšenčík, Ph.D., KCHFO MFF UK Abstract: Solar energy is one of the most important energy sources for all living organisms. The light harvesting takes place in specialised photosynthetic complexes called antennas; they typically contain pigments held by a protein scaffold. Antennas of green bacteria, chlorosomes, are unique in this respect, for they do not need proteins to organise the pigments. The pigments contained in chlorosomes, bacteriochlorophyll (BChl) c, d or e, aggregate spontaneously. This self-aggregation can be used to form an artificial light-harvesting antenna the absorption spectrum of which can be extended by addition of other pigments. Antennas based on aggregation of BChl c with β-carotene and BChl a were prepared by a fast and slow method. The excitation energy transfer efficiency between these pigments was studied. The efficiency of energy transfer from BChl c to BChl a reached up to 95 %, the efficiency of energy transfer from β-carotene to BChl c was lower. An important role of β- carotene in artificial aggregates as well as in chlorosomes is its...

Les nanoparticules polymériques fluorescentes apparaissent comme des outils importants pour l'imagerie en temps réel des processus biologiques au niveau moléculaire et cellulaire. L’objectif de mon projet de doctorat a été d’optimiser les nanoparticules polymériques fluorescentes pour l’imagerie biologique. Premièrement, nous avons pu, en faisant varier la chimie des polymères, obtenir un très bon contrôle de leur taille. Ceci a permis de mettre en évidence l’importance de la taille des NPs pour des applications intracellulaires avec une taille maximale de 23 nm pour une distribution dans tout le cytosol. Deuxièmement, nous avons pu montrer que la simple adsorption d’un amphiphile PEGylé de type Pluronic permet la stabilisation des nanoparticules dans des milieux biologiques. Le nombre de molécules incorporées et leur stabilité ont été étudiés en combinant des techniques de FRET et de FCS. Les meilleures formulations résultent en une stabilité des nanoparticules in vivo, ce qui a permis leur imagerie en tant que particules individuelles dans les vaisseaux sanguins du cerveau de souris. Troisièmement, le transfert d’énergie entre différents fluorophores encapsulés dans les NPs a été étudié et optimisé. / Fluorescent polymeric nanoparticles appear as important tools for real-time imaging of biological processes at the molecular and cellular level. The objective of my PhD project was to optimize fluorescent polymeric nanoparticles for biological imaging. First, by varying the chemistry of the polymers, we have been able to obtain a very good control of their size. This made it possible to highlight the importance of NPs size for intracellular applications with a maximum size of 23 nm for optimal distribution throughout the cytosol. Secondly, we have shown that simple adsorption of a PEGylated amphiphiles pluronic family allows the stabilization of nanoparticles in biological media. The number of incorporated molecules and their stability has been studied by combining FRET and FCS techniques. The best formulations result in nanoparticle stability in vivo, which allowed their imaging as individual particles in the blood vessels of the mouse brain. Third, energy transfer among different fluorophores encapsulated in NPs has been studied and optimized.

Die vorliegende Arbeit untersucht den Anregungsenergie-Transfer in Nano-hybrid-Systemen, welche zweidimensionale Übergangsmetall-Dichalkonid-Schichten (TMDCs) enthalten. Heterostrukturen, welche TMDC-Schichten mit sogenannten nulldimensionalen Systemen kombinieren, werden als wesentlich für die nächste Generation von elektronischen und photonischen Bauelementen angesehen. Trotz dieser großen Bedeutung existieren wenige theoretische Untersuchungen. Insbesondere ist der Anregungsenergie-Transfer in diesen Hybridsystemen nicht umfassend erklärt, und die Behandlung von TMDC-Schichten bezieht sich auf sehr kleine oder periodische Systeme. Daher wird in der Arbeit der Versuch unternommen, existierende Theorien zu verbessern, und es werden Transferprozesse in zwei Typen von Heterostrukturen simuliert. Die berechneten Systeme enthalten tausende von Atomen und kommen damit in den Bereich experimentell untersuchter Strukturen. In dem einen Nanohybrid-System ist eine MoS2-Monoschicht mit einem einzelnen Para-Sexiphenyl-Molekül kombiniert, wogegen im zweiten System ein CdSe-Nanokristall an der MoS2-Mono-schicht plaziert ist. Dabei ermöglicht die Coulomb-Wechselwirkung zwischen Monoschicht und Molekül bzw. Nanokristall den Anregungsenergie-Transfer. In allen untersuchten Heterostrukturen ist die Stärke der Anregungsenergie-Transfer-Kopplung auf den sub-meV-Bereich beschränkt. In diesem Bereich ist der Anregungsenergie-Transfer inkohärent und bestimmt durch Raten, die aus Fermi's Goldener Regel folgen. Auch wird eine Abhängigkeit der Transferrate von der relativen Position des para-Sexiphenyl-Moleküls gefunden. Durch die Analyse der Übergangsladungsdichte des CdSe-Nanokristalls kann aufgezeigt werden, dass die energetisch tiefliegenden Exziton-Niveaus mit ausgeprägtem Dipolcharakter zu einer stärkeren Transferkopplung führen. Die resultierenden Transferzeiten erstrecken sich vom Piko- zum Nanosekunden-Bereich und decken sich mit entsprechend gemessenen Werten. / This thesis explores the excitation energy transfer in two-dimensional transition metal dichalcogenides (TMDCs) based nanohybrid systems. Such heterostructures combining TMDC layers with zero-dimensional materials are considered in next-generation electronics and photonics. However, there exists a shortage of current theoretical work, because the general process of excitation energy transfer in these hybrid systems has rarely been explored and the treatment of TMDCs is limited to a small size. We therefore improve the existing theories and investigate the transfer phenomena in two types of heterostructures. The considered systems contain thousands of atoms close to the experimental system size. In the first nanohybrid system, a MoS2 monolayer is combined with a single para-sexiphenyl molecule. In the second hybrid, a CdSe semiconductor spherical nanocrystal is placed close to the MoS2 monolayer. The MoS2 monolayer is coupled to the para-sexiphenyl molecule or the CdSe spherical nanocrystal via Coulomb interaction, which makes the excitation energy transfer mechanism possible. In our heterostructures, all excitation energy transfer coupling strengths lie in the meV-range or below. Within this limitation, the non-coherent excitation transfer is determined by rate expressions derived from Fermi’s Golden Rule. An effective transfer rate dependency on the relative positions of the para-sexiphenyl molecule is found. For the case of the CdSe spherical nanocrystal , by visualizing the shape of transition charge densities of CdSe excitons, we find that the low-lying exciton levels with more obvious dipole character lead to a stronger transfer coupling. The resultant transfer times range from picoseconds to nanoseconds and coincide with experimental data.

No description available.

Ultraschnelle Energietransferprozesse zwischen den Anregungszuständen organischer Pigmentmoleküle in photosynthetischen Lichtsammelkomplexen gehören zu den schnellsten bisher untersuchten biologischen Ereignissen. Diese Vorgänge wurden insbesondere auch für den Haupt-Antennenkomplex der höheren Pflanzen (LHCII) beobachtet, der mehr als die Hälfte des pflanzlichen Chlorophylls (Chl) bindet (5 Chl b und 7 Chl a pro Monomer). Offenbar ist dieser Pigment-Protein-Komplex entscheidend für Regulationsmechanismen verantwortlich, die eine schnelle Adaptation des Photosyntheseapparats an wechselnde Licht- bedingungen ermöglichen. Die Struktur von LHCII ist mit einer Auflösung von 3.4 Å bekannt und erlaubt (im Prinzip) die Berechnung des Anregungsenergietransfers auf Basis eines Förster-Mechanismus. In diesem Zusammenhang gibt es jedoch noch zahlreiche ungeklärte Fragen, die vor allem die Orientierung der Pigmente zueinander sowie deren mögliche starke (exzitonische) Wechselwirkung betreffen. Allerdings sind konventionelle spektroskopische Methoden nicht geeignet, diese Merkmale ausreichend aufzuklären. Aus diesem Grund wird in dieser Arbeit untersucht, inwieweit neuere laserspektroskopische Methoden wie die nichtlineare Polarisationsspektroskopie in der Frequenzdomäne (NLPF) zur Ermittlung unbekannter Parameter beitragen können. Anfänglich ergaben sich besonders Fragen der Anwendbarkeit der NLPF auf solche hoch- komplexen Untersuchungsobjekte sowie der Signifikanz eventuell erzielbarer Ergebnisse. Aufbauend auf einer parallel verfaßten Dissertation zu theoretischen Aspekten der NLPF- Methode [1] wurde daher ein vereinfachtes System modelliert, das die Heterogenität der individuellen Chl(e) im LHCII widerspiegelt. Die gewonnenen Resultate ließen vermuten, daß die reine Simulation von NLPF-Spektren nicht ausreicht, um eindeutige Aussagen über die Molekülparameter zu gewinnen. Um den benötigten zusätzlichen Erkenntnisgewinn zu erreichen, wurden daher Paralleluntersuchungen mit anderen laserspektroskopischen Methoden (nichtlineare Absorption mit fs-Pulsen, intensitätsabhängige NLPF, Einzelmolekülspektroskopie, Tieftemperatur-NLPF) sowie mit in vitro rekonstituierten Protein-Mutanten durchgeführt. Als Ergebnis konnte die Subbstruktur der Qy- Absorptionsbande der ersten angeregten Zustände der Chl(e) für LHCII ausreichend beschrieben werden. Darüber hinaus ergaben sich Aussagen zu exzitonischen Wechselwirkungen zwischen bestimmten Chl(en), die unter anderem Einfluß auf das Energie- transferverhalten haben. Diese zusätzlichen Untersuchungen erlaubten letztendlich eine Modellierung der bei Raum- temperatur an LHCII gemessenen NLPF-Spektren. Neben dem dabei implizit gewonnenen Verständnis der nichtlinearen optischen Eigenschaften im Bereich der Qy-Absorption ließen sich so Aussagen über bestimmte Modellparameter, besonders über die Orientierung von Übergangsdipolmomenten, ableiten. Abschließend wurde die Auswirkung der Erkenntnisse auf das Verständnis der Struktur-Funktionsbeziehungen für intra- und inter-komplexen Energietransfer erläutert. / Ultra-fast excitation energy transfer (EET) between excited states of organic pigment molecules in photosynthetic antenna complexes belongs to the fastest observed biological processes. Such EET phenomena has been studied to a large extent for the main light- harvesting complex of the higher plants (LHCII), which appears to play an exceptional role for the regulatory function (i.e. light adaptation) of the plant photosynthetic apparatus. The structure of this pigment-protein complex harboring more than 50 % of the total chlorophyll (Chl) content is known with 3.4 Å resolution and reveals the binding sites of 5 Chl b and 7 Chl a per monomeric unit. Based on this structure analysis, EET calculations are (in principle) available on the molecular level under the assumption of Förster-type transfer. However, several molecular features like mutual pigment orientations and electronic interactions between their transition dipoles are still rather uncertain. Since conventional spectroscopic techniques can hardly reveal the corresponding parameters, this work was aimed at the evaluation of newly introduced laser spectroscopic techniques with respect to these questions. In the beginning, suitability and significance of the method when applied to highly complicated structures like pigment-protein complexes were studied by modeling heterogeneous, LHCII-like absorption systems in NLPF experiments. Based on recent improvements in the NLPF theory by a parallel theoretical investigation [1], these simulations clarified the sensitivity of the NLPF method on numerous physical parameters. As a major consequence, unambiguous evaluations of NLPF measurements appear to require substantial additional information about the investigated system. Accordingly, several supplementary methods like nonlinear absorption (using fs-pulses), intensity-dependent NLPF, single- molecule spectroscopy, and NLPF at low temperatures were employed. These investigations revealed unique information about excitonic interaction between certain Chl(s), including implications for the overall EET scheme. The sub-structure model for the Qy-absorption region of LHCII was further essentially improved by the analysis of reconstituted proteins with selectively modified Chl binding residues in the amino-acid sequence. The sum of all complementary investigations allowed finally the evaluation of room temperature NLPF measurements of trimeric LHCII. Due to the unique selectivity of the spectra to individual transition-dipole directions, several orientation parameters have been obtained. Under this point of view, the NLPF method has indeed revealed a high potential as compared to conventional techniques like circular dichroism spectroscopy. Moreover, the understanding of nonlinear phenomena in the Qy-absorption region of LHCII as a consequence of molecular interaction provides further knowledge for the application of other nonlinear optical experiments. Concluding, implications of the obtained results for the structure-function relationship of intra- and inter-complex EET were elucidated.

Die Arbeit untersucht den Anregungsenergietransfer in supramolekularen Phäophorbid-a-Komplexen. Das P4- und das P16-Molekül bestehen aus vier bzw. sechzehn Phäophorbid-a-Molekülen. Die Komplexe werden in explizitem Lösungsmittel im Rahmen einer gemischt quanten-klassischen Methode untersucht. Klassische Molekulardynamik-Simulationen werden durchgeführt. Die zeitabhängige Schrödingergleichung wird gelöst, der entsprechende Hamiltonoperator hängt parametrisch von den Kernkoordinaten ab. Es wird eine Methode vorgestellt, die die Berechnung des Schwingungsbeitrags der Koordinatenabhängigkeit in harmonischer Näherung ermöglicht. Die Qualität der Methode wird bewiesen. Es werden drei verschiedene Ansätze benutzt, um das Zeitverhalten des Anregungsenergietransfers innerhalb der Chromophorkomplexe zu charakterisieren. Es werden zunächst Transferraten berechnet und entsprechende Ratengleichungen gelöst. Desweiteren werden gemittelte zeitabhängige Populationen aus der Lösung der Schrödingergleichung bestimmt. Zudem wird die Zeitskala des Anregungsenergietransfers aus der Anisotropie erhalten. Die Berechnung der Anisotropie beruht auf der Lösung einer Schrödingergleichung, welche das elektromagnetischen Feldes explizit enthält. Für alle drei Ansätze ergibt sich die gleiche Dynamik des Anregungsenergietransfers. Es werden zudem lineare und transiente Spektren der Qy-Banden der Chromophorkomplexe berechnet. Für ein einzelnes Phäophorbid-a-Molekül in Ethanol werden zusätzlich die Qx-Bande und die Schwingungsprogression bestimmt. Außerdem wird die lineare Absorption von Phäophorbid a und P16 neben einem Gold-Nanopartikel untersucht, die erwartete Verstärkung des Absorptionssignals durch die Präsenz des Nanoteilchens wird gezeigt. Abschließend wird eine neue Methode vorgestellt, die es erlaubt, die abstands- und orientierungsabhängige Abschirmung der exzitonischen Kopplung parametrisch in die gemischt quanten-klassische Methode zu integrieren. / This thesis investigates the excitation energy transfer in pheophorbide a complexes. The P4 and the P16 molecule consist of four and sixteen pheophorbide a molecules, respectively. The complexes in explicit ethanol solution are investigated utilizing a mixed quantum-classical methodology. Classical molecular dynamics simulations are carried out. The time-dependent Schrödinger equation is solved for a Hamiltonian that depends parametrically on the classical nuclear coordinates. In this thesis a method is introduced which allows the computation of the vibrational contribution in harmonic approximation. The high quality of the method is proven. Three different ansatzes were utilized to compute the time development of the excitation energy transfer within the chromophore complexes. The expansion coefficients that result from the solution of the time-dependent Schrödinger equation are utilized to compute averaged time-dependent populations. Also, the expansion coefficients are used to compute excitation energy transfer rates in second order of the excitonic coupling. Thirdly, the time scale of the excitation energy transfer is derived from the delay-time dependent transient anisotropy. In order to compute the anisotropy, the electromagnetic field is included directly in the Hamiltonian of the system. The excitation energy transfer dynamics is exactly the same for the three approaches. In addition, linear and transient spectra of the chromophor complexes Qy band are computed. For a single pheophorbide a in ethanol, the Qx band and the vibrational progression are calculated. Furthermore, the linear absorption of pheophorbide a and P16 next to a gold nanoparticle is studied. The amplification of the molecular absorption signal due to the presence of the nanoparticle is shown. Finally, a new method is introduced to treat distance and conformation dependent screening of the excitonic coupling parametrically within a mixed quantum-classical description.

Photosynthesis involves the absorption of photons by light-harvesting pigments and the subsequent transfer of excitation from the absorption centre to the reaction centre. This highly efficient phenomenon of excitation transfer has traditionally been explained by the Forster mechanism of incoherent hopping of excitation from one chromophore to another. Recently 2D electronic spectroscopic evidences were gathered by Fleming and coworkers on the photosynthetic Fenna-Matthews-Olson (FMO) complex in green sulfur bacteria [1]. Subsequent simulation studies by the same group [2] led to the proposition of a quantum-mechanical, coherent, wave-like transfer of excitation among the chromophores. However, Fleming's conclusions regarding retention of coherence appeared surprising because, the complex would interact with the numerous degrees of freedom of the protein scaffold surrounding it, leading to decoherence, which is expected to be rapid. Thus, we were interested in proposing an analytical treatment to rationalize the excitation transfer. Traditional approaches employed for studying excitation energy transfer involve the master equation techniques where the system-bath coupling is perturbative and is truncated after a few orders. It is important to note that the system-bath coupling causes both decoherence and population relaxation. Such a perturbative approximation is difficult to justify for the photosystem, as the system-bath coupling and the interchromophoric electronic coupling have comparable values. Also, these treatments are largely numerical studies and demand involved calculations. Thus, exact calculations for such a system (7-level) are very difficult. Consequently, we were interested in developing an analytical approach where the coupling is treated as non-perturbative. We devised a novel analytical treatment which employs a unitary transformation analogous to the one used for the theory of nonadiabatic effects in chemical reactions [3]. Our treatment rests on an adiabatic basis which are eigenstates calculated at each nuclear position (i.e. at each configuration of the bath) bearing a parametric dependence in Qi, where Qi denotes the shift of the exciton at site `i' due to the environment. The treatment is justified because in the case of coherent transfer, the excitation would travel mostly amongst the adiabatic states and the effects of non-adiabaticity are small. We observed that the system-bath coupling, after the unitary transformation, could be decoupled at the lowest order into two parts: a) an adiabatic contribution, which accounts solely for decoherence (this is evaluated almost exactly in our approach) and b) a non-adiabatic contribution which accounts for population relaxation from one adiabatic state to another (treated by a Markovian master equation). When we applied our technique to the FMO complex, our prediction for population evolution at the chromophores showed excellent correspondence with those obtained by Nalbach and coworkers using path-integral calculations [4], which are exact. These were calculations where the environment was modelled using a Drude spectral density. Our method allowed the calculations to be readily performed for different temperatures as well. It should be specifically emphasized that, unlike the involved and cumbersome path-integral calculations by Nalbach and coworkers [4] or the hierarchical equation calculations by Ishizaki et al. [2], our method is simple, easy to apply and computationally expedient. Further it became evident that the ultra-efficiency of energy transfer in photosynthetic complexes is not completely captured by coherence alone but is the result of an interplay of coherence and the dissipative influence of the environment (also known as ENAQT or Environment Assisted Quantum Transport [5]). An added advantage of our analytical treatment was the flexibility it offered. Thus, we could use our formalism to perform expedient analyses on the behavior of the system under various conditions. For example, we may wish to evaluate the consequences of introducing correlations among the bath degrees of freedom on the efficiency of transfer to the reaction centre. To this end, we applied our formalism by introducing correlations among the bath degrees of freedom and then by introducing anticorrelations among the bath degrees of freedom. The conclusions were interesting, for they suggested that the efficiency of transfer to the reaction centre was enhanced by the presence of anti-correlations, when compared with an uncorrelated bath. Uncorrelated baths, in turn, had a higher efficiency of energy transfer than correlated baths [6]. Thus, the population evolution is fastest for the anti-correlated bath, followed by the uncorrelated bath and is slowest for the correlated bath. Similar conclusions have been reached at by Tiwari et al. [7]. We could also extend the formalism for studying the system under different spectral densities for the environment, apart from just the Drude spectral density which is popularly used in literature associated with FMO calculations. For instance, the FMO system could be analyzed for the Adolphs-Renger spectral density [3, 8]. Once again our results showed excellent agreement with those reported by Nalbach. We also analyzed the FMO system under the spectral density proposed by Kleinekathofer and coworkers [9]. It was found that these latter spectral densities had more profound participation from the environment, therefore coherences were destroyed more effectively and population relaxation was faster. The excitation transfer to the final site (site closest to the reaction centre in the FMO complex) was found to be faster for the Adolphs and Renger spectral density and the spectral density proposed by Kleinekathofer and coworkers, when compared to the Drude spectral density. Also, the excitation transfer was fastest when we modelled the environment using the Kleinekathofer spectral density. This reinforced the previous conclusions that the dissipative effects of the environment promote a faster energy transport. Being an almost analytical approach, our technique could be applied to systems with larger number of levels as well. A good example of such a case is the MEH-PPV polymer. 2D electronic-spectroscopic experiments performed on this polymer in solution speculate that the excitation energy transfer might be coherent even at physiological temperatures [10]. A prototype for studying this system might be a conjugated polymer with around 80-100 chromophores. Linewidths and Lineshapes in the vicinity of Graphene It has been reported that a vibrating dipole may de-excite by transferring energy non-radiatively to a neighboring metal surface [11]. It is also understood that due to its delocalized pi-cloud, graphene has a continuum of energy states and can behave like a metal sheet and accept energies. Thus, we proposed that if a vibrationally excited dipole de-excites in the vicinity of a graphene sheet, graphene may get electronically excited and thus serve as an effective quencher for such vibrational excitations. Depending on the distance of the dipole from the graphene sheet, the transfer might be intense enough to be spectroscopically probed. We have investigated the rate of such an energy transfer. We use the Dirac cone approximation for graphene, as this enables us to obtain analyt-ical results. The Fermi Golden rule was used to evaluate the rate of energy transfer from the excited dipole to the graphene sheet [12]. The calculations were performed for both the instances: a) energy transfer from a dipole to undoped graphene and, b) energy trans-fer from a dipole to doped graphene. For undoped graphene, the carrier (electron) charge density in the conduction band is zero and we would only have transitions from the valence band to the conduction band. As a consequence of absence of carrier charge density in CB (conduction band), the screening of Coulombic interactions in the graphene plane is ineffective. Thus, one could use the non-interacting polarizability for undoped graphene in the rate expression [13]. However, when we consider the case of doped graphene where EF is shifted upwards into CB, the conduction band electrons will contribute to screening. In this case, we have two sets of transitions: a) from ki in VB (valence band) to kf in CB and b) ki in CB to kf in CB, where ki and kf are the wavevectors which correspond to the initial and final electronic states in graphene. So we have used the polarizability propagator in the random phase approximation [14] to calculate the rate following the approach of [13]. It is also known that the imaginary part of the frequency domain dipole-dipole corre-lation function is a measure of the lineshape [15]. We were, thus, interested in evaluating the lineshape for these transitions. For evaluating the correlation function, we used the partitioning technique developed by L•owdin [16] and subsequently extracted the lineshape from its imaginary part. Using this method, we calculated lineshape for the vibrational excitation of CO molecule in the vicinity of an undoped graphene lattice. The linewidth for this system also was obtained. It could be seen that the vibrational linewidth for 1 CO in the vicinity (5 A) of undoped graphene (EF = 0:00eV ) is small (0:012 cm ) but could be observed experimentally. The lineshape calculations were also extended to cases where it is possible to have atomic transitions by placing an electronically excited atom in the vicinity of the graphene sheet. We considered the following two cases: a) 3p ! 2s transition in hydrogen atom, at a distance of 12 A from the graphene sheet and, b) 4p ! 3s transition in hydrogen atom, at a distance of 20 A from the graphene sheet. The linewidths for atomic transitions could be easily probed in these cases ( 55 cm 1 for 3p ! 2s and 56 cm 1 for 4p ! 3s). In the preceding calculations, the transi-tion dipoles were considered perpendicular to the graphene surface. It is worthwhile to note that if the transition dipoles are considered parallel to the graphene surface, the respective linewidths would be half of those obtained for the case where the transition dipoles are perpendicular. Another interesting possibility would be to consider a lanthanide metal complex placed within a few nanometers from graphene. Lanthanides are known to have sharp f-f transitions [17] and consequently, one could easily observe the effects of broadening due to energy transfer to the electronic system of graphene. Energy Eigenmodes for arrays of Metal Nanoparticles In the final part of the thesis we consider organized assemblies of metal nanoparti-cles, specifically helical and cylindrical assemblies and investigate the plasmonic excitation transfer across these assemblies. These were motivated by recent studies which reported growth of chiral asymmetric assemblies of nanoparticles on D and L- isomers of dipheny-lalanine peptide nanotubes [18]. The plasmons in the helical/cylindrical assemblies are expected to couple with each other via electromagnetic interactions. We construct the Hamiltonian for such systems and evaluate the eigenmodes and energies pertaining to these modes in the wave vector space. We also perform calculations for the group velocity for each eigenmode as this gives us an idea of which eigenmode transports excitation the fastest.

Le noyau est un objet quantique complexe formé de protons et neutrons. Dans l'approche champ moyen, les nucléons sont considérés comme des particules indépendantes évoluant dans un potentiel moyen. Cependant dans certaines conditions, des nucléons peuvent se regrouper pour former des amas ou « clusters ».Pour comprendre ce phénomène de « clusters » dans les noyaux, nous avons étudié la structure dans l'état fondamental du 40Ca et de l'40Ar. En effet, des calculs théoriques tendent à montrer que les noyaux N = Z pourraient plus facilement adopter une structure en « clusters » que les noyaux N ≠ Z en raison d'un plus grand recouvrement des fonctions d'onde des neutrons et protons. Dans ce cas, l'émission de particules α par les noyaux N = Z sous l'effet du potentiel nucléaire attractif d'un noyau projectile, appelée « Towing-Mode » sera plus important que pour un noyau N ≠ Z. Dans ce but, nous avons réalisé une expérience au GANIL utilisant un faisceau d'40Ar à 35 MeV/A et une cible de 40Ca. Le spectromètre SPEG a permis d'identifier avec une très bonne résolution les ions lourds produits durant la réaction. Les détecteurs silicium MUST2 furent placés tout autour de la cible pour mesurer les particules α émises par la cible et le projectile et le prototype de calorimètre EXL fut utilisé pour la détection des photons de décroissance des noyaux résiduels d'36Ar et de 36S.Un modèle théorique basé sur la résolution de l'équation de Schrödinger dépendante du temps (TDSE) a été utilisé pour reproduire certains résultats expérimentaux comme les distributions angulaires. L'analyse des données a permis de reconstruire des spectres d'énergies d'excitation et des sections efficaces différentielles. De la comparaison entre ces distributions expérimentales et celles calculées par le modèle théorique, nous avons pu extraire des facteurs spectroscopiques Sα pour les deux noyaux d'intérêt. Les taux de « clusterisation » observés pour ces deux noyaux semblent indiquer que la structure en « clusters » n'est pas plus favorisée dans le 40Ca que dans l'40Ar. / Nuclei are complex self-bound systems formed by nucleons. Conjointly to a mean-field picture in which nucleons can be regarded as independent particles, few nucleons might self-organize into compact objects, called clusters, inside the nucleus. It is theoretically predicted that it should manifest itself most strikingly for N = Z nuclei close to the emission thresholds and has been studied extensively in this region. We propose to study α-clusterization in the ground state of the N = Z 40Ca nucleus and the N ≠ Z 40Ar nucleus. We have studied the nuclear break-up of 40Ca when the 40Ar projectile passes by. If α clusters are preformed in 40Ca, the probability of α-emission through nuclear break-up will be enhanced as compared to 40Ar N ≠ Z nuclei. The nuclear break-up of 40Ca was studied with an 40Ar beam produced at GANIL at 35 MeV/A. The SPEG spectrometer was used to detect the heavy projectile with accurate resolution. The MUST2 Silicon detectors were placed around the target to measure the emitted α and the EXL calorimeter prototype was used to identify the γ rays from the decay of the residual 36Ar and 36S.A theoretical approach based on Time-Dependent Schrödinger Equation (TDSE) theory has been used to reproduce some experimental results like angular distributions.From the data analysis, we reconstructed excitation energy spectra and angular distributions which are compared to TDSE theory to extract some spectroscopic factors Sα. These factors show that there is no more clusterization state in the ground state of the 40Ca than in the ground state of 40Ar.