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Rationalizing the photochemistry of nucleobases where an oxygen is replaced by a heavier atom is essential for applications that exploit near-unity triplet quantum yields. Herein, we report on the ultrafast excited-state deactivation mechanism of 6-selenoguanine (6SeGua) in water by combining nonadiabatic trajectory surface-hopping dynamics with an electrostatic embedding quantum mechanics/molecular mechanics (QM/MM) scheme. We find that the predominant relaxation mechanism after irradiation starts on the bright singlet S

The excited-state intramolecular proton transfer (ESIPT) phenomenon is nowadays widely acknowledged to play a crucial role in many photobiological and photochemical processes. It is an extremely fast transformation, often taking place at sub-100 fs timescales. While its experimental characterization can be highly challenging, a rich manifold of theoretical approaches at different levels is nowadays available to support and guide experimental investigations. In this perspective, we summarize the state-of-the-art quantum-chemical methods, as well as molecular- and quantum-dynamics tools successfully applied in ESIPT process studies, focusing on a critical comparison of their specific properties.

The investigation of ultrafast molecular dynamics is of great importance towards the understanding of a variety of natural phenomena in physical and chemical sciences. With the rapid development of femtosecond laser systems and precision detection technologies, it is possible now to visualize and steer the motion of molecules in matter as well as the ultrafast dynamics of electrons and nuclei in molecules on a microscopic timescale. When a molecule is exposed to a strong laser field, its electrons can be freed or excited, which often triggers a rapid dissociation of the system, in which the released electrons and nuclei exhibit a strong correlation, while the electronic motion on attosecond timescale is much faster than that of the nuclear ranging from femtosecond to picosecond timescale. One example is the absorbed photon energy shared between electrons and nuclei in a molecular dissociation process. In addition, a lot of physical processes such as electron tunneling ionization show a strong dependence on the spatial direction of the molecular axes with respect to the polarization of the laser field. Confining molecular axes to a specific direction is significant in unambiguously understanding and controlling the ultrafast response of the molecules to the laser fields. Impulsive alignment induced by ultrafast laser pulses with field-free revivals is considered as a powerful method to control the spatiotemporal distribution of the molecules. Field-free alignment is also recognized as an important tool for studying basic physical processes such as rotational dynamics of complex molecules and ultrafast collisional dissipation.By using advanced technologies based on intense femtosecond laser systems, multi-particle coincidence measurements of Cold target recoil ion momentum spectroscopy (COLTRIM), and high-sensitive time-resolved birefringence detections, this thesis reports a variety of studies revealing ultrafast dynamics observed in molecules exposed to strong laser fields, including dissociative Rydberg excitation, tunneling-site-sensitive ionization and dissociation, molecular alignment, rotational echo, and ultrafast collisional dissipation probed by rotational alignment echo., L'étude de dynamiques ultra-rapides dans les molécules revêt une importance particulière pour la compréhension de divers phénomènes naturels en sciences physiques et chimiques. Avec le développement rapide des lasers femtosecondes et de méthodes précises de détection, il est possible de visualiser et de contrôler la dynamique ultra-rapide des électrons et des noyaux de molécules à des échelles microscopiques. Lorsqu'une molécule est exposée à un champ laser intense, ses électrons peuvent être ionisés ou excités, ce qui dans certains cas entraine une dissociation de la molécule, processus dans lequel les mouvements des électrons et des noyaux sont fortement corrélés. Un des exemples est le partage corrélé de l'énergie des photons absorbés entre les électrons et les noyaux dans le processus de dissociation moléculaire. Les processus d’ionisation ou de photodissociation, de même que de nombreux processus d’interaction laser-matière, dépendent de la direction des axes moléculaires par rapport à la polarisation du champ laser excitateur. Les lasers ultra-courts permettent d’aligner efficacement ces axes moléculaires suivant des directions de l’espace, alignement qui perdure un certain temps après extinction du champ laser. Au-delà du fait qu’aligner des molécules permet d’optimiser l’interaction laser matière, ou encore d’analyser le système moléculaire dans son propre référentiel, il permet également d’observer la dynamique rotationnelle des molécules et leur dissipation collisionnelle à des échelles de temps ultra-courtes.En utilisant des techniques avancées basées sur les laser femtosecondes amplifiées, les mesures de coïncidence multi-particules par spectroscopie COLTRIM (Cold target recoil ion momentum) et la détection de biréfringence résolue en temps, cette thèse présente un certain nombre d’études de dynamiques ultra-rapides observées dans des molécules soumises à des champs laser intenses, incluant l’excitation Rydberg dissociative, l’ionisation tunnel, l’alignement moléculaire post-impulsionnel et les échos rotationnels, et la dissipation collisionnelle ultra-rapide sondées par les échos rotationnels.

This study investigates recent advances in photoelectron emission generated by irradiating ultrashort lasers on metallic nanostructures and low-dimensional carbon materials. Recently, primary focus has been on improving the efficiency of emitters, i.e. increasing the number of field-emitted electrons and their respective kinetic energies. An example of this is the modification of the conventional metal nanotip through adiabatic nanofocusing and various plasmonic metal structures, such as nanorods and bowtie antenna. The coherent emission control with two color irradiation enabled modulation in the emission yield. In addition, THz waves near the metallic nanostructure induced a highly accelerated, monochromatic energy. Alternative to metallic nanotips, carbon nanotubes are emerging as efficient photoelectron emitters, due to the large enhancement factor associated with their high aspect ratio and damage threshold. They particularly allowed the use of femtosecond light sources with a relatively short wavelength, resulting in the generation of photoelectrons with a narrow bandwidth. Additionally, electronic control over the single-walled nanotubes band structure added a degree of freedom for controlling the electron emission yield. Finally, we review the strong-field tunneling emission in graphene edge, with the emission yield showing an anomalous increase of nonlinear order, corresponding to the deep strong tunneling regime.

The essential part of electrochemistry is charge transfer. To understand this process in great detail, one needs to probe the relevant kinetics and dynamics on time scales spanning from femtoseconds to seconds or even longer. Although a conventional electrochemical detection scheme is sufficient for nanosecond or slower processes, it does not offer high enough time resolution for probing ultrafast processes, such as solvent reorganization, electron tunneling, and surface isomerization, that occur on faster, for example picosecond or femtosecond, timescales. These are indispensable parameters in the advanced charge transfer theories. In this review, some recent studies using ultrashort lasers to explore the ultrafast dynamics at the metal/solution interface are reviewed. The focus is on optical pump-probe and optical pump-push with electrochemical probe schemes. The connection of these studies with conventional electrochemistry and the limitations of these detection schemes are discussed.

In this thesis different trajectory-based methods for the study of quantum mechanical phenomena are developed. The first approach is based on a global expansion of the hydrodynamic fields in Chebyshev polynomials. The scheme is used for the study of one-dimensional vibrational dynamics of bound wave packets in harmonic and anharmonic potentials. Furthermore, a different methodology is developed, which, starting from a parametrization previously proposed for the density, allows the construction of effective interaction potentials between the pseudo-particles representing the density. Within this approach several model problems are studied and important quantum mechanical effects such as, zero point energy, tunneling, barrier scattering and over barrier reflection are founded to be correctly described by the ensemble of interacting trajectories. The same approximation is used for study the laser-driven atom ionization. A third approach considered in this work consists in the derivation of an approximate many-body quantum potential for cryogenic Ar and Kr matrices with an embedded Na impurity. To this end, a suitable ansatz for the ground state wave function of the solid is proposed. This allows to construct an approximate quantum potential which is employed in molecular dynamics simulations to obtain the absorption spectra of the Na impurity isolated in the rare gas matrix.; Dans cette thèse, différentes méthodes de dynamique quantique basées sur la propagation de trajectoires sont développées. La première approche consiste en une développer global des champs hydrodynamiques sur une base de polynômes de Chebyshev. Ce schéma est utilisé pour étudier la dynamique vibrationnelle unidimensionnelle de paquets d'ondes dans des potentiels harmoniques et anharmoniques. Par la suite, une méthodologie différente est développée, qui, à partir d'un paramétrage précédemment proposé pour la densité quantique, permet de construire des potentiels d'interaction effectifs entre les pseudo-particules représentant la densité. Dans le cadre de cette approche, plusieurs problèmes de modélisation sont étudiés et des effets quantiques importants sont décrits, tels que l'énergie de point zéro, l'effet tunnel, la diffusion et la réflexion sur une barrière. La même approximation est utilisée pour l'étude de l'ionisation des atomes par laser. Dans une troisième approche, un potentiel quantique approximatif à plusieurs corps est dérivé pour décrire des matrices d'argon et de krypton contenant une impureté de sodium. Il est obtenu en proposant un ansatz approprié pour la fonction d'onde de l'état fondamental du solide. Le potentiel est utilisé dans les simulations de dynamique moléculaire pour obtenir les spectres d'absorption de l'atome de Na isolé dans les matrices cryogéniques.

In this thesis different trajectory-based methods for the study of quantum mechanical phenomena are developed. The first approach is based on a global expansion of the hydrodynamic fields in Chebyshev polynomials. The scheme is used for the study of one-dimensional vibrational dynamics of bound wave packets in harmonic and anharmonic potentials. Furthermore, a different methodology is developed, which, starting from a parametrization previously proposed for the density, allows the construction of effective interaction potentials between the pseudo-particles representing the density. Within this approach several model problems are studied and important quantum mechanical effects such as, zero point energy, tunneling, barrier scattering and over barrier reflection are founded to be correctly described by the ensemble of interacting trajectories. The same approximation is used for study the laser-driven atom ionization. A third approach considered in this work consists in the derivation of an approximate many-body quantum potential for cryogenic Ar and Kr matrices with an embedded Na impurity. To this end, a suitable ansatz for the ground state wave function of the solid is proposed. This allows to construct an approximate quantum potential which is employed in molecular dynamics simulations to obtain the absorption spectra of the Na impurity isolated in the rare gas matrix.; Dans cette thèse, différentes méthodes de dynamique quantique basées sur la propagation de trajectoires sont développées. La première approche consiste en une développer global des champs hydrodynamiques sur une base de polynômes de Chebyshev. Ce schéma est utilisé pour étudier la dynamique vibrationnelle unidimensionnelle de paquets d'ondes dans des potentiels harmoniques et anharmoniques. Par la suite, une méthodologie différente est développée, qui, à partir d'un paramétrage précédemment proposé pour la densité quantique, permet de construire des potentiels d'interaction effectifs entre les pseudo-particules représentant la densité. Dans le cadre de cette approche, plusieurs problèmes de modélisation sont étudiés et des effets quantiques importants sont décrits, tels que l'énergie de point zéro, l'effet tunnel, la diffusion et la réflexion sur une barrière. La même approximation est utilisée pour l'étude de l'ionisation des atomes par laser. Dans une troisième approche, un potentiel quantique approximatif à plusieurs corps est dérivé pour décrire des matrices d'argon et de krypton contenant une impureté de sodium. Il est obtenu en proposant un ansatz approprié pour la fonction d'onde de l'état fondamental du solide. Le potentiel est utilisé dans les simulations de dynamique moléculaire pour obtenir les spectres d'absorption de l'atome de Na isolé dans les matrices cryogéniques.

We present efficient high-order harmonic generation (HHG) based on a high-repetition rate, few-cycle, near infrared (NIR), carrier-envelope phase stable, optical parametric chirped pulse amplifier (OPCPA), emitting 6fs pulses with 9{\mu}J pulse energy at 200kHz repetition rate. In krypton, we reach conversion efficiencies from the NIR to the extreme ultraviolet (XUV) radiation pulse energy on the order of ~10^{-6} with less than 3{\mu}J driving pulse energy. This is achieved by optimizing the OPCPA for a spatially and temporally clean pulse and by a specially designed high-pressure gas target. In the future, the high efficiency of the HHG source will be beneficial for high-repetition rate two-colour (NIR-XUV) pumpprobe experiments, where the available pulse energy from the laser has to be distributed economically between pump and probe pulses., Comment: 25 pages, 11 figures

The ultrafast reflectivity of silicon, excited and probed with femtosecond laser pulses, is studied for different wavelengths and energy densities. The confinement of carriers in a thin surface layer delimited by a nanoscale Si-layered system buried in a Si heavily-doped wafer reduces the critical density of carriers necessary to create the electron plasma by a factor of ten. We performed two types of reflectivity measurements, using either a single beam or two beams. The plasma strongly depends on the photon energy density because of the intervalley scattering of the electrons revealed by two different mechanisms assisted by the electron–phonon interaction. One mechanism leads to a negative differential reflectivity that can be attributed to an induced absorption in X valleys. The other mechanism occurs, when the carrier population is thermalizing and gives rise to a positive differential reflectivity corresponding to Pauli-blocked intervalley gamma to X scattering. These results are important for improving the efficiency of Si light-to-electricity converters, in which there is a possibility of multiplying carriers by nanostructurization of Si.

International audience; After UV excitation, gas phase thymine returns to a ground state in 5 to 7 ps, showing multiple time constants. There is no consensus on the assignment of these processes, with a dispute between models claiming that thymine is trapped either in the first (S1) or in the second (S2) excited states. In the present study, a nonadiabatic dynamics simulation of thymine is performed on the basis of ADC(2) surfaces, to understand the role of dynamic electron correlation on the deactivation pathways. The results show that trapping in S2 is strongly reduced in comparison to previous simulations considering only non-dynamic electron correlation on CASSCF surfaces. The reason for the difference is traced back to the energetic cost for formation of a CO π bond in S2.

International audience; The advent of attosecond techniques opens up the possibility to observe experimentally electron dynamics following ionization of molecules. Theoretical studies of pure electron dynamics at single fixed nuclear geometries in molecules have demonstrated oscillatory charge migration at a well-defined frequency but often neglecting the natural width of the nuclear wave packet. The effect on electron dynamics of the spatial delocalization of the nuclei is an outstanding question. Here, we show how the inherent distribution of nuclear geometries leads to dephasing. Using a simple analytical model, we demonstrate that the conditions for a long-lived electronic coherence are a narrow nuclear wave packet and almost parallel potential-energy surfaces of the states involved. We demonstrate with numerical simulations the decoherence of electron dynamics for two real molecular systems (paraxylene and polycyclic norbornadiene), which exhibit different decoherence time scales. To represent the quantum distribution of geometries of the nuclear wave packet, the Wigner distribution function is used. The electron dynamics decoherence result has significant implications for the interpretation of attosecond spectroscopy experiments since one no longer expects long-lived oscillations.

We demonstrate a linear to circular polarization conversion mechanism of a confined polariton condensate created by an optical potential trap. This system features a high degree of circular polarization under linear polarization pumping and is favorable for the development of spin logic operations. The physical mechanism behind this process is explained by the spin precession in the exciton reservoir that results in a condensate of a preferred spin state. Application of a non-resonant below threshold femtosecond pulse on the spinor condensate results in an ultrafast reversal of the spin state. Published version

Für die vorliegende Doktorarbeit wurden enantiosensitive Untersuchungen mit ultrakurzen Laserpulsen im Bereich einiger Femtosekunden (fs) durchgeführt. Dazu wurde die Chiralitätsanalyse mit der Laserionisation-Massenspektrometrie (LIMS) kombiniert. Die unterschiedliche Drehrichtung von links- (LCP) und rechts-zirkular (RCP) polarisierten Laserpulsen erzeugt dabei verschiede Ionenausbeuten für chirale Moleküle. Diese Ionenausbeuten wurden in einem Flugzeitmassenspektrometer detektiert und der CD in Ionenausbeuten berechnet. Ein Vorteil dieser Methode, gegenüber anderen enationsensitiven Chiralitätsanalysen, liegt in der inhärenten Trennung von Stoffen unterschiedlicher Masse zu Ladungsverhältnisse (m/z) im Flugzeit-massenspektrometer, was zeitaufwendige chemische Separationsmethode obsolet macht. Außerdem ermöglicht diese Kombination neben der Chiralitätsanalyse eine chemische Identifikation des Analyten anhand des Masse zu Ladungsverhältnisses des Mutterions oder spezifischer Fragmentationsmuster. Die Verwendung von fs-Laserpulsen für diese Analyse-Methode ist aus wissenschaftlichen und analytischen Fragestellungen interessant, da für solche kurzen Pulse generell höhere Ionenausbeuten für die Mutterionen der Analyten erhalten werden, was die chemische Identifikation erleichtert. Im Rahmen dieser Arbeit wurde der Einfluss von Laserpulsparametern und verschiedener Anregungs- bzw. Ionisationsprozesse auf den CD-Effekt für unterschiedliche chirale Analyten untersucht. Für die Korrektur sowohl von zufälligen wie auch systematischen Abweichungen wurde als interner Standard eine achirale Referenzsubstanz verwendet. Der erste Teil der erhaltenen Ergebnisse betrachtet resonante Ein-Photonenanregungen und die daraus resultierenden CD-Effekte in der Ionisation von 3-Methylcyclopentanon. Der CD in Ionenausbeuten wurde in einer (1+2)-resonanzverstärkten Mehrphotonenionisation (REMPI) im Rahmen meiner Diplomarbeit wellenlängenabhängig charakterisiert. Aus den erhaltenen Unterschieden im Vergleich zu den Experimenten in der ns-LIMS resultierte die Frage inwiefern der CD in Ionenausbeuten in (1+2)-REMPI von der Pulsdauer beeinflusst wird. Zur Untersuchung dieses Sachverhaltes wurde ein Pulsformer für den ultravioletten Spektralbereich aufgebaut und die Pulsdauer zwischen 50 fs und etwa 950 fs anhand des linearen Chirps variiert. Bei einer Zentralwellenlänge von 311 nm zeigte sich ein deutlicher Anstieg des CD-Effektes des Mutterions mit ansteigender Pulsdauer, sowohl für (R)- als auch für (S)‑3‑MCP. Der Verlauf des CD in Ionenausbeuten ist sowohl für negative als auch positive lineare Chirpparameter sehr ähnlich. Im zweiten Teil dieser Dissertation wurden CD-Effekte in nicht-linearen optischen Prozessen untersucht. Anhand des CD in Ionenausbeuten von (R)-Propylenoxid ((R)-PO) bei Zentralwellenlängen von 810 nm und 878 nm wurde in der vorliegenden Arbeit gezeigt, dass CD-Effekte auch in Fünf- bzw. Sieben-Photonenprozessen beobachtet werden können. Weiterhin lässt sich anhand der Photonenenergie bei 878 nm und der Anzahl der im Ionisationsprozess beteiligten Photonen schließen, dass es sich um eine Mehrphotonenionisation ohne resonante Zwischenstufen handelt. Das ist bemerkenswert, da alle bis dahin bekannten Arbeiten auf dem Gebiet der Chiralitätsanalyse mittels LIMS Ein- oder Zwei-Photonenresonanzen nutzen. Weiterhin ist interessant, dass die CD in Ionenausbeuten von (R)-PO bei 810 nm und 878 nm eine Größenordnung größer sind als der CD-Effekt in Ein-Photonenabsorption. Für (1+n)‑REMPI wurden stets CD‑Effekte beobachtet, bei denen der CD in Ein‑Photonenabsorption und der CD in Ionenausbeuten in der Größenordnung. Der nicht-lineare Charakter der beobachteten CD‑Effekte des PO wird dadurch bestätigt. Bei genauerer Untersuchung des CD in Ionenausbeuten der Fragmentionen wurde sowohl bei 810 nm als auch bei 878 nm ein Unterschied in den Vorzeichen der CD-Effekte aller Fragmentionen und des CD-Effekts des Mutterions gefunden. Wiederum die erste Beobachtung eines solchen Effektes und im Gegensatz zu (1+n)‑REMPI, bei denen für das Mutterion und die Fragmentionen die gleichen Vorzeichen im CD in Ionenausbeuten beobachtet wurden.

Received 6 July 2009; published 10 November 2009By using a powerful reductive perturbation technique, or a multiscale analysis, a generic Kadomtsev-Petviashvili evolution equation governing the propagation of femtosecond spatiotemporal optical solitons inquadratic nonlinear media beyond the slowly varying envelope approximation is put forward. Direct numericalsimulations show the formation, from adequately chosen few-cycle input pulses, of both stable line solitons inthe case of a quadratic medium with normal dispersion and of stable lumps for a quadratic medium withanomalous dispersion . Besides, a typical example of the decay of the perturbed unstable line soliton into stablelumps for a quadratic nonlinear medium with anomalous dispersion is also given.DOI: 10.1103/PhysRevA.80.053812 PACS number s : 42.65.Tg, 42.65.Re, 05.45.Yv

Porphyrins have been of interest for a variety of chemical and biological processes. In particular, the heme is the prosthetic group in many proteins such as cytochromes P450, hemoglobin, heme peroxidases. There is a great deal of evidence demonstrating that the biological activity of the hemoproteins is controlled by the conformational and electronic properties of their active site. To acquire a deeper knowledge of the ultrafast processes occurring in biological complex systems, it is helpful to investigate on how porphyrin, both as free base and as metal complex, behaves on its own. We have studied the relaxation dynamics of hemin and protoporphyrin IX in solution by means of UV-vis transient absorption spectroscopy. Upon excitation in the Soret band with pulses of ~100 fs duration, the decay processes at delay times up to 2 ns have been determined and discussed by comparison with the model proposed for the parent porphyrin (1). The results offer the possibility to argue the nature of the short lived states generated after hemoprotein photoexcitation. Finally, the time-resolved behavior of the truncated hemoglobin from Thermobifida fusca (2) are presented.

International audience; By using a reductive perturbation method, we derive from Maxwell-Bloch equations a cubic generalized Kadomtsev-Petviashvili equation for ultrashort spatiotemporal optical pulse propagation in cubic (Kerr-like) media without the use of the slowly varying envelope approximation. We calculate the collapse threshold for the propagation of few-cycle spatiotemporal pulses described by the generic cubic generalized Kadomtsev-Petviashvili equation by a direct numerical method and compare it to analytic results based on a rigorous virial theorem. Besides, typical evolution of the spectrum (integrated over the transverse spatial coordinate) is given and a strongly asymmetric spectral broadening of ultrashort spatiotemporal pulses during collapse is evidenced.

We describe recent improvements in the development of the high power laser system used in the motion induced radiation (MIR) experiment to amplify electromagnetic fields inside a microwave cavity. The improvements made on the oscillator stabilization, the pulse train shaping device, and the spatial beam uniformity are reported.

Knowledge of excited electronic states in semiconductor quantum dots (QDs) is of fundamental scientific interest and is important for application in lasers, optical detectors, LEDs, solar cells, photocatalysis, biomedical imaging, photodynamic therapy etc. In the past few years, carrier multiplication (CM) in QDs has received particular attention, due to promising prospects for exploitation in highly efficient solar cells, photodetectors and possibly photocatalysis. CM can occur when absorption of a high-energy photon leads to production of an excited electron or a hole with an excess energy that exceeds the QD band gap. CM involves transfer of (part of) the excess energy of the excited electron or hole to one or more valence electrons that also become excited across the band gap via a process denoted as impact ionization. In this way absorption of a single photon can lead to excitation of two or more electrons. This thesis describes studies of factors affecting the efficiency of CM in QDs based on PbSe. Ultrafast time-resolved optical pump and probe spectroscopy is used to characterize the nature of photoexcited states, the efficiency of CM, hot exciton cooling and Auger recombination of multiexcitons. The occurrence of CM has been reported by several research groups for QDs consisting of PbX (X = S, Se, Te), CdSe, InAs, and Si. However, in some other studies CM was not observed for CdSe, CdTe, and InAs QDs, thus raising legitimate doubts concerning the occurrence of CM in the other materials. In the work of chapter 2 conclusive evidence is given for the occurrence of CM in PbSe QDs. Possible artifacts due multi-photon absorption and charge trapping are excluded. It is shown that for higher exciton multiplicity a correct determination of the CM efficiency requires spectral integration over the photobleach feature. The CM efficiency of ?CM = 1.7 obtained at a photon energy of 4.8 times the band gap is close to results that have appeared in the literature more recently. Chapter 3 describes studies of the dynamics of hot excitons in PbSe QDs, PbSe/PbS core/shell QDs, and PbSe/PbSexS1-x core/alloyed-shell QDs. The ground state optical absorption exhibits a red-shift on introduction of a shell around a PbSe core, which increases with the thickness of the shell. According to electronic structure calculations, this can be attributed to electron delocalization into the shell. Remarkably, the CM efficiency, the hot exciton cooling rate, and the Auger recombination rate of multiexcitons are similar for PbSe core-only QDs and core/shell QDs with the same core size and varying shell thickness, despite the marked variations in the density of states evidenced by the changes in optical spectra. It is concluded that different effects that may serve to speed up or slow down exciton dynamics, such as variations in density of states, shell-induced asymmetry in the band structure and hot exciton cooling counteract one another. The second transition in the ground state optical absorption spectrum of PbSe QDs is arguably the most discussed optical transition in semiconductor QDs. Ten years of scientific debate have produced many theoretical and experimental claims for the assignment of this feature as the 1Pe1Ph as well as the 1Sh,e1Pe,h transitions. The studies described in chapter 4 show that the strength of the second optical transition in the absorption spectrum of PbSe QDs is not affected by the presence of 1Sh1Se excitons, even if four of those excitons are introduced. Hence, the second optical transition involves neither 1Se nor 1Sh states. This suggests that it is the 1Ph1Pe transition that gives rise to the second peak in the absorption spectrum of PbSe QDs. The transitions causing extinction in the energy region between the 1Sh1Se and 1Ph1Pe transitions was investigated, as described in chapter 5. The ultrafast transient absorption data indicate that the extinction in this region is not due to Rayleigh scattering, nor to local field effects, but to the formally forbidden 1Ph1Se and 1Sh1Pe transitions. These optical transitions can become allowed, due to deviations of the QD shape from ideal spherical symmetry. For applications, it is essential that multiple charges are extracted from multiexcitons generated within a QD, prior to decay by Auger recombination. Therefore the optical properties and decay kinetics of multiexcitons were studied. The results are presented in chapter 6. The first and second optical transitions in the ground state absorption spectrum of PbSe QDs are strongly shifted to the red as the number of 1Sh1Se spectator excitons increases. These red-shifts can be attributed to Coulomb interactions. The lifetimes before Auger decay of 1Sh1Se multiexcitons were determined. The population decay for 6.8 nm PbSe QDs could be described by assuming the Auger recombination rate to increase exponentially with the number of excitons in a QD. For smaller QDs, the exponential and another 3-charge interaction model reproduce the experimental data equally well. Further studies are needed to unravel the effects of QD size on the dependence of Auger recombination on the number of excitons.

Knowledge of excited electronic states in semiconductor quantum dots (QDs) is of fundamental scientific interest and is important for application in lasers, optical detectors, LEDs, solar cells, photocatalysis, biomedical imaging, photodynamic therapy etc. In the past few years, carrier multiplication (CM) in QDs has received particular attention, due to promising prospects for exploitation in highly efficient solar cells, photodetectors and possibly photocatalysis. CM can occur when absorption of a high-energy photon leads to production of an excited electron or a hole with an excess energy that exceeds the QD band gap. CM involves transfer of (part of) the excess energy of the excited electron or hole to one or more valence electrons that also become excited across the band gap via a process denoted as impact ionization. In this way absorption of a single photon can lead to excitation of two or more electrons. This thesis describes studies of factors affecting the efficiency of CM in QDs based on PbSe. Ultrafast time-resolved optical pump and probe spectroscopy is used to characterize the nature of photoexcited states, the efficiency of CM, hot exciton cooling and Auger recombination of multiexcitons. The occurrence of CM has been reported by several research groups for QDs consisting of PbX (X = S, Se, Te), CdSe, InAs, and Si. However, in some other studies CM was not observed for CdSe, CdTe, and InAs QDs, thus raising legitimate doubts concerning the occurrence of CM in the other materials. In the work of chapter 2 conclusive evidence is given for the occurrence of CM in PbSe QDs. Possible artifacts due multi-photon absorption and charge trapping are excluded. It is shown that for higher exciton multiplicity a correct determination of the CM efficiency requires spectral integration over the photobleach feature. The CM efficiency of ?CM = 1.7 obtained at a photon energy of 4.8 times the band gap is close to results that have appeared in the literature more recently. Chapter 3 describes studies of the dynamics of hot excitons in PbSe QDs, PbSe/PbS core/shell QDs, and PbSe/PbSexS1-x core/alloyed-shell QDs. The ground state optical absorption exhibits a red-shift on introduction of a shell around a PbSe core, which increases with the thickness of the shell. According to electronic structure calculations, this can be attributed to electron delocalization into the shell. Remarkably, the CM efficiency, the hot exciton cooling rate, and the Auger recombination rate of multiexcitons are similar for PbSe core-only QDs and core/shell QDs with the same core size and varying shell thickness, despite the marked variations in the density of states evidenced by the changes in optical spectra. It is concluded that different effects that may serve to speed up or slow down exciton dynamics, such as variations in density of states, shell-induced asymmetry in the band structure and hot exciton cooling counteract one another. The second transition in the ground state optical absorption spectrum of PbSe QDs is arguably the most discussed optical transition in semiconductor QDs. Ten years of scientific debate have produced many theoretical and experimental claims for the assignment of this feature as the 1Pe1Ph as well as the 1Sh,e1Pe,h transitions. The studies described in chapter 4 show that the strength of the second optical transition in the absorption spectrum of PbSe QDs is not affected by the presence of 1Sh1Se excitons, even if four of those excitons are introduced. Hence, the second optical transition involves neither 1Se nor 1Sh states. This suggests that it is the 1Ph1Pe transition that gives rise to the second peak in the absorption spectrum of PbSe QDs. The transitions causing extinction in the energy region between the 1Sh1Se and 1Ph1Pe transitions was investigated, as described in chapter 5. The ultrafast transient absorption data indicate that the extinction in this region is not due to Rayleigh scattering, nor to local field effects, but to the formally forbidden 1Ph1Se and 1Sh1Pe transitions. These optical transitions can become allowed, due to deviations of the QD shape from ideal spherical symmetry. For applications, it is essential that multiple charges are extracted from multiexcitons generated within a QD, prior to decay by Auger recombination. Therefore the optical properties and decay kinetics of multiexcitons were studied. The results are presented in chapter 6. The first and second optical transitions in the ground state absorption spectrum of PbSe QDs are strongly shifted to the red as the number of 1Sh1Se spectator excitons increases. These red-shifts can be attributed to Coulomb interactions. The lifetimes before Auger decay of 1Sh1Se multiexcitons were determined. The population decay for 6.8 nm PbSe QDs could be described by assuming the Auger recombination rate to increase exponentially with the number of excitons in a QD. For smaller QDs, the exponential and another 3-charge interaction model reproduce the experimental data equally well. Further studies are needed to unravel the effects of QD size on the dependence of Auger recombination on the number of excitons.

International audience; To shed light on ultra-short laser interactions, we study the laser ionization processes leading to the energy absorption and reflection. In particular, we investigate the ratio of the energy deposited to the material to the total incident energy. The absorbed energy density is studied as a function of pulse width and laser intensity. It is shown that the maximum absorption takes place at a given incident laser intensity that is considered as ablation threshold. For pulses shorter than 100 fs, only a small fraction of laser energy is deposited to the matrix, causing heating and leading to the thermal and/or mechanical modifications of the target material. We connect these results with the electronic excitation and the ionization processes leading to the changes in reflectivity and consuming electron energy. The obtained numerical results explain several recent experiments.

Article de revue (Article scientifique dans une revue à comité de lecture); International audience; By using a reductive perturbation technique applied to a two-level model, this study puts forward a generic two-dimensional sine-Gordon evolution equation governing the propagation of femtosecond spatiotemporal optical solitons in Kerr media beyond the slowly varying envelope approximation. Direct numerical simulations show that, in contrast to the long-wave approximation, no collapse occurs, and that robust (2+1)-dimensional ultrashort light bullets may form from adequately chosen few-cycle input spatiotemporal wave forms. In contrast to the case of quadratic nonlinearity, the light bullets oscillate in both space and time and are therefore not steady-state lumps.

In dieser Arbeit wird ein neuer, umfassender Ansatz für nichtlineare Spektroskopie im Fokus eines Mikroskops vorgestellt. Dabei wird Laserstrahlung mit gezielt geformten Femtosekunden-Impulsen verwendet. Das grundlegende Konzept ist die Funktionalisierung der Femtosekunden-Laseranregung, um eine Reihe von Multiphotonen-Spektroskopie-Methoden in ein und demselben Aufbau umzusetzen, und für die mikroskopische Bildgebung verfügbar zu machen. Die wichtigste Methode dabei ist kohärente anti-Stokes Raman Streuung (CARS), die das Vibrationsspektrum einer Probe nutzt, um sie chemisch spezifisch zu identifizieren. Der hier verfolgte Ansatz stellt eine immense Vereinfachung von CARS-Spektroskopie dar. Dabei wird zum einen ein Schema zur sehr rasche Spektrenaufnahme und quantitativen Analyse komplexer Mehrkomponenten-Spektren vorgestellt, das auf der Anpassung eines physikalischen Modells mittels eines evolutionären Algorithmus basiert. Zudem werden neue Methoden entwickelt, die zum Beispiel CARS-Messungen in die Zeitdomäne verlagern und in der Tat die Molekülschwingungen zeitaufgelöst verfolgen. Das wird nur dadurch möglich, dass durch kohärente Kontrolle mit in Phase, Amplitude und Polarisation geformten Anregungsimpulsen die Licht-Materie-Wechselwirkung auf quantenmechanischem Niveau gezielt beeinflusst wird. Auf diese Weise können die Laserimpulse selbst mit spektroskopischen Funktionen versehen werden. Es wird gezeigt, dass diese funktionale „photonische Integration“ noch weiter getrieben werden kann, indem auch interferometrische Detektion ohne weitere optische Bauelemente durch Formung der Anregungsimpulse verwirklicht wird. Dies vereinfacht Experimente deutlich und erhöht gleichzeitig die Empfindlichkeit von CARS drastisch um mehr als drei Größenordnungen. Zusätzlich zu diesen neu entwickelten Konzepten werden für die Verwirklichung notwendige technische Neuentwicklungen vorgestellt, darunter die Erzeugung von Breitband-Femtosekunden-Impulsen in mikrostrukturierten Fasern und deren genaue Charakterisierung und Kompression durch eine neue, Impulsformer-basierte Variante der SPIDER-Methode. In der Anwendung der neuen Spektroskopietechniken werden Beispiele gezeigt, die von der quantitativen chemischen Bildgebung von Polymer-Blends, über die selektive Detektion von Markermolekülen für biologische Kampfstoffe(Anthrax) bis zu ersten analytischen Anwendungen bei der Messung chemischer Zusammensetzungen in einer mikrofludischen Zelle., This doctoral thesis presents a new, unified approach to nonlinear microspectroscopy employing tailored broadband femtosecond laser radiation. The key concept is to functionalize the femtosecond excitation in order to implement a series of multiphoton spectroscopy techniques, especially for microscopic imaging. The most important application is coherent anti-Stokes Raman scattering (CARS) spectroscopy, which allows chemical identification of untreated samples in microscopy due to their characteristic vibrational spectra. The presented approach allows huge experimental simplifications of CARS, schemes for very rapid spectral acquisition and determination of the chemical composition (based on the quantitative analysis of entangled multiplex spectra by evolutionary algorithm fitting), as well as new methods for microscopic CARS measurements in the time-domain, resolving molecular vibrations temporally. This is possible, because coherent control of the signal generation is applied, manipulating the quantum mechanical processes of the underlying light-matter interaction by shaping the excitation light field in phase, amplitude and polarization. Thus, spectroscopic function and even molecular control is imprinted on the excitation pulses. It is shown that this idea of functional “photonic integration” can be pursued even further by incorporating an interferometric detection scheme in the same pulses without any additional optical elements in the experimental setup, drastically improving the measurement sensitivity by more than three orders of magnitude. In addition to these novel conceptual findings, new technological developments have been invented and pushed forward. These include the generation of ultrabroadband femtosecond radiation in microstructured optical fibres and its precise phase measurement and management, which is a prerequisite for coherent control. In this context, a new pulse-shaper enabled variant of SPIDER was developed, allowing very rapid compression in collinear beam geometry in the microscope. Employing the developed set of tools and concepts, application examples are given ranging from quantitative chemical imaging of polymer blend samples, to the chemical identification of potentially hazardous powdery substances and the microanalytical sensing of the chemical composition in a microfluidic device.

Das Ziel dieser Dissertation ist die Interpretation von Daten, die mit der kürzlich entwickelten optischen zweidimensionalen Fourier-Transform-Spektroskopie (2D-FTS),angewandt auf Halbleiter-Nanostrukturen, gewonnen wurden. Dies erfordert eine Analyse der diagnostischen Möglichkeiten dieser Methode und ihres Anwendungsbereiches. Die Hauptidee der 2D-FTS ist es, Informationen über das angeregte System zu gewinnen und dabei verschiedene Beiträge der Kopplungen innerhalb des angeregten Vielteilchensystems zu entflechten. Es wurde gezeigt, dass 2D-FTS gut geeignet für die Untersuchung von Halbleitermaterialien ist, insbesondere für deren detaillierte Charakterisierung bezüglich der Unordnung und der Coulomb-Korrelationen., The aim of this work is the exploration of the recently developed optical two-dimensional Fourier-transform spectroscopy (2D-FTS) method and its application in the field of semiconductor physics. The main idea of 2D-FTS is to gain information about the excited system and disentangle different contributions to couplings within the excited many-body system. The power of the 2D-FTS method is demonstrated for an example of a semiconductor nanostructure such as quantum wells. It has been shown that by applying 2D-FTS we are able to describe the action of disorder and of the Coulomb correlations in detail.

In einer allgemeinen Sichtweise wird Chemie als die Lehre vom Aufbau, Verhalten und der Umwandlung von Stoffen bezeichnet. Auf der molekularen Skala der Betrachtung bedeutet dies, dass Chemie die Formung, die Dissoziation, im Allgemeinen die Entwicklung von molekularen Bindungen behandelt. Die Beobachtung solcher Vorgänge in Echtzeit und somit ein tiefer gehendes Verständnis von chemischen Reaktionen und ihrer Dynamik ist das Anliegen der Femtochemie. Die Vorsilbe „Femto“ (10-15) ergibt sich aus der natürlichen Zeitskala dieser Phänomene. Die Schwingungsperiode einer C=C-Streckschwingung innerhalb eines Polyengerüsts, die sich in dieser Arbeit als wichtige Größe herausstellen wird, liegt in der Größenordnung von unter 30 Femtosekunden. Die Spektroskopie mit Lichtimpulsen, die eine vergleichbare zeitliche Dauer aufweisen, verspricht Erkenntnisse über die Dynamik auf einer atomaren Skala. Es geht hier um die Untersuchung von molekularen Zuständen fernab vom Gleichgewicht. Phänomene wie die Umverteilung der Schwingungsenergie zwischen den molekularen Normalmoden oder die Kopplung von solchen Freiheitsgraden an die Umgebung wurden durch die Fortschritte in der Ultrakurzimpuls-Lasertechnologie zum Gegenstand physikalisch-chemischer Forschung. Es geht in der Femtochemie demnach um die Beantwortung grundlegender Fragen der Chemie: Wie formen sich chemische Bindungen? Was ist die Natur eines Übergangszustandes? Der Grund warum nach der Interaktion mit ultrakurzen Lichtimpulsen die spektroskopische Information nicht verloren geht oder verwaschen wird, liegt in der Kohärenz zwischen den angeregten Quantenzuständen. Die damit gemeinte Phasenbeziehung zwischen den Energieniveaus bleibt nicht nur erhalten, sie kann auch durch eine Formung des anregenden Lichtimpulses kontrolliert und gesteuert werden. Darin liegt die Grundidee der kohärenten Kontrolle, deren allgemeines Ziel es ist den Verlauf eines photochemischen oder photophysikalischen Prozesses durch die zeitliche Strukturierung der anregenden Lichtwelle zu beeinflussen. Worin liegt nun der Beitrag der vorliegenden Arbeit zu dem oben umrissenen Feld der kohärenten Kontrolle? Es wird ein aus spektroskopischer Sicht relevantes und erkenntnisreiches Teilgebiet weiter vorangetrieben, nämlich das der zustandsselektiven Spektroskopie durch Quantenkontrolle auf elektronisch angeregten Zuständen. Ziel dieses Zweiges ist es nicht, die Ausbeute der einen photogenerierten Spezies zu verstärken, während die andere unterdrückt wird. Es wird vielmehr die Zielsetzung verfolgt, Quantenzustände gezielt zu präparieren, um ihre Entwicklung jener nach unmodulierter Anregung gegenüberzustellen. Dieser Vergleich führte bereits zu neuen Erkenntnissen über das energetische Desaktivierungsnetzwerk in biologischen Systemen. Ziel dieser Arbeit ist es nun, mit den Möglichkeiten der kohärenten Kontrolle die molekulare Dynamik rund um einen entscheidenden Punkt einer molekularen Potentialfläche zu beleuchten, der durch konventionelle spektroskopische Methoden nur schwer zugänglich ist. Es geht um die Steuerung der Dynamik an konischen Durchschneidungen. Dieses wichtige theoretische Konzept der modernen Quantenchemie rüttelt an den konzeptionellen Grundfesten der Beschreibung von Molekulardynamik. Die Vorstellung von adiabatischen elektronischen Potentialflächen fußt auf der Born-Oppenheimer Näherung. Erst durch die Annahme der getrennten Lösbarkeit von elektronischer und nuklearer Wellenfunktion ist die Abbildung von elektronischen Potentialflächen entlang interner Kernkoordinaten möglich. Ultraschnelle Prozesse, die ein Durchdringen zweier solcher Hyperflächen und demnach eine Kopplung zwischen ihnen voraussetzen, lassen keine unabhängige Behandlung von elektronischer und struktureller Dynamik mehr zu. Erst durch das Aufgeben einer so grundlegenden Annahme wie der Born-Oppenheimer Näherung werden eine Vielzahl von quantendynamischen Problemen behandelbar. Wird Benzol als paradigmatisches Beispiel herangezogen, so definieren die geometrischen Eigenschaften einer konischen Durchschneidung die Produktverteilung der Photoreaktion. Die Kontrolle eines solchen nicht-adiabatischen Überganges wird somit zum Schlüsselschritt einer universellen Methode zur Steuerung von Wellenpaketdynamik und Photochemie. Ziel dieser Arbeit ist nun die kohärente Kontrolle von solchen ultraschnellen Vorgängen auf elektronisch angeregten Potentialflächen., In a general perspective, chemistry is concerned with the composition, the behaviour and the conversion of matter. On a molecular scale, this means that chemistry deals with the formation, the dissociation, or more generally the development of molecular bonds. Observing such events in real time, leading towards a deeper understanding of chemical reactions and their dynamics is the aim of Femtochemistry. The prefix “Femto” (10-15) results from the natural time scale of such phenomena. The vibrational period of a C=C stretching vibration in a polyene backbone, which will turn out to be of great importance in this work, is in the order of 30 femtoseconds. Spectroscopy with light pulses of similar duration promises to reveal the inner workings of molecular dynamics far away from thermal equilibrium. Phenomena like internal vibrational redistribution amongst molecular normal modes or the coupling between these degrees of freedom to the surrounding are now part of physical chemistry due to rapid advances in the field of ultrashort laser pulse technology. Femtochemistry deals with questions fundamental to chemistry: How are chemical bonds formed? What is the nature of a transition state? The reason why spectroscopic information is not lost or washed out after interaction with an ultrashort and hence spectrally broad pulse lies within the coherence between the excited quantum states. The phase relation between the energy states associated with this notion is not only preserved, it can be controlled and directed towards a desired target state via modulating the excitation pulse. This is the basic idea behind coherent control, whose general aim it is to influence a photochemical or photophysical process by shaping the temporal structure of the exciting light wave. What is the contribution of this work to the field of coherent control outlined above? From a spectroscopist’s point of view, a relevant and conclusive section is promoted, namely state selective quantum control spectroscopy. It is not the aim of this branch to enhance the yield of a photochemical species while suppressing others. The pursued objective is rather to prepare certain quantum states, in order to contrast their development in time to the one after unmodulated excitation. This comparison already led to the elucidation of the energetic deactivation network in complex biological systems. It is the aim of this work to investigate and control the molecular dynamics around a decisive point of a potential energy surface, which is hard if not impossible to analyze by conventional spectroscopic techniques. This work deals with the coherent control of vibrational dynamics near a conical intersection. This important concept in modern quantum chemistry challenges the conceptual fundamentals of the description of molecular dynamics. The notion of adiabatic electronic potential energy surfaces is based upon the Born-Oppenheimer approximation. It is this assumption of the separate solvability of electronic and nuclear wavefunctions that allows one to describe potential electronic energy surfaces in the space of internal nuclear coordinates. Ultrafast processes presuming an intersection between two such surfaces do not allow for a separate treatment of electronic and structural dynamics. Only after abandoning this fundamental assumption, a variety of quantumdynamical phenomena becomes treatable. Considering benzene as a prototypical system, the geometrical properties of a conical intersection determine the product ratio of a photochemical reaction. Control over such a non-adiabatical passage is therefore the key step in a universal method for influencing wavepacket dynamics and hence photochemistry. The aim of this work is to coherently control such ultrafast phenomena on electronically excited molecular potential energy surfaces.

The equations of motion necessary for the computation of luminescence spectra in realistic semiconductor heterostructures are presented. The combination of these multiband semiconductor luminescence equations on the level of two-particle correlations and scattering terms in 2nd Born-Markov approximation with a k.p-bandstructure calculation allows the quantitative modeling of luminescence spectra. This is demonstrated in theory-to-experiment comparisons. It is also shown why simple rate equations for the luminescence fail at high excitation intensity. Combining bandstructure calculation, semiconductor luminescence equations, and semiconductor Bloch equations it becomes possible to compute a variety of optical properties of semiconductors consistently and quantitatively. This is utilized to compute the appropriate gain spectra from measured luminescence spectra. It is further used to study the expected lasing properties and the carrier loss mechanisms of GaInNAs/GaAs structures in the technologically interesting wavelength range of 1550nm. Finally, the optical properties of a type I to type II transition in the band gap alignment in heterostructures are investigated. In doing so, photomodulated reflectance spectra are computed and analyzed with the help of microscopic theory for the first time., Die Bewegungsgleichungen zur Berechnung der Lumineszenz in realistischen Halbleiterheterostrukturen werden präsentiert. Die Verbindung dieser Multiband-Halbleiterlumineszenzgleichungen auf dem Niveau von Zweiteilchenkorrelationen und Streutermen in zweiter Bornscher und Markovscher Näherung mit einer k.p Bandstrukturrechnung ermöglicht die genaue Berechnung von Lumineszenzspektren. Das wird in Theorie-Experiment Vergleichen gezeigt. Mit dieser mikroskopischen Modellierung wird auch nachgewiesen, warum einfache Ratengleichungen für die Lumineszenz bei starker Anregung versagen. Durch die Kombination: Bandstrukturrechnung - Halbleiterlumineszenzgleichungen - Halbleiter-Bloch-Gleichungen können viele optische Eigenschaften von Halbleitern konsistent und quantitativ berechnet werden. Das wird benutzt, um aus gemessenen Lumineszenzspektren Gewinnspektren zu ermitteln und um die zu erwartenden Lasereigenschaften und die Ladungsträgerverluste von GaInNAs/GaAs Strukturen im technologisch interessanten Wellenlängenbereich von 1550nm zu studieren. Schließlich wird untersucht, wie sich ein Typ I zu Typ II Übergang in der Anordnung der Energielücken in Heterostrukturen auf die optischen Eigenschaften auswirkt. Dafür werden auch zum ersten Mal photomodulierte Reflexionsspektren mit Hilfe mikroskopischer Theorie berechnet und analysiert.

Die laserinduzierte Diffusion von atomarem Sauerstoff auf einer gestuften Pt(111) Oberfläche wurde untersucht, wobei ultrakurze Laserimpulse mit Photonenenergien im nahen Infrarot eingesetzt wurden. Die Arbeit stellt die erste zeitaufgelöste Untersuchung eines Diffusionsprozesses an einer Einkristalloberfläche dar. Sie demonstriert darüberhinaus die Möglichkeit, die Diffusion eines stark chemisorbierten Adsorbats auf einer Metalloberfläche bei tiefen Temperaturen durch elektronische Anregung auszulösen. Für das Experiment wurde die Selektivität der dissoziativen Adsorption von molekularem Sauerstoff für die Stufen der Pt(111) Oberfläche ausgenutzt, um zunächst eine Nichtgleichgewichtsverteilung des Adsorbates zu erzeugen. Die Bedeckung der Stufenkanten mit chemisorbiertem Sauerstoff wurde während der Adsorption und der laserinduzierten Diffusion in-situ mit Hilfe der optischen Frequenzverdopplung (SHG) an Oberflächen verfolgt. Dabei wurde ausgenutzt, dass die Erzeugung der zweiten Harmonischen in Dipolnäherung einen Symmetriebruch erfordert, wie er durch die regelmäßigen Stufen entsteht. Die hohe Empfindlichkeit dieser Detektionsmethode machte es möglich, Hüpfraten für die Bewegung von den Stufenkanten auf die anfangs leeren Terrassenplätze bis herunter zu 10^-7 pro Laserimpuls zu messen. Zur Durchführung dieser optischen Experimente sowie zur Präparation und Charakterisierung der Platinoberfläche wurde eine Ultrahochvakuumapparatur aufgebaut. Um Aufschluss über die Dynamik des Energietransfers von der optischen Anregung in die frustrierte Translationen des Adsorbats zu gewinnen, wurde die Diffusionsrate als Funktion der eingestrahlten Laserleistung und, in einem Korrelationsexperiment, als Funktion der Verzögerung zwischen zwei Pump-Impulsen gemessen. Die Experimente zeigen eine sehr stark nichtlineare Abhängigkeit der Diffusionsrate von der Energiedichte der Laserimpulse (~F^15) sowie eine schnelle elektronische Kopplung zwischen der anfänglichen Anregung und der Adsorbatbewegung mit einer Zeitkonstanten von 1.5 ps. Beide Ergebnisse legen es nahe, dass der prinzipielle Anregungsmechanismus der gleiche ist wie im Fall der bereits intensiv untersuchten, laserinduzierten Desorption: Die kurzen, intensiven Laserimpulse erzeugen eine heiße Elektronenverteilung an der Oberfläche, ohne dass das Substratgitter zunächst nen nenswert aufgeheizt wird. Die heißen Elektronen regen auf der Zeitskala von 1.5 ps mehrfach die Adsorbat-Substratbindung und damit eine Adsorbatbewegung an. Die extrem nichtlineare Abhängigkeit der Hüpfrate von der Energiedichte der Laserimpulse deutet aber darauf hin, dass der Energietransfermechanismus hier komplizierter ist, als im Fall der Desorption. Bei der quantitativen Beschreibung im Rahmen eines elektronischen Reibungsmodells für die nichtadiabatische Kopplung der Adsorbatbewegung an das Elektronensystem des Substrats wird die Einführung eines temperaturabhängigen elektronischen Reibungskoeffizienten notwendig. Als Erklärung wird vorgeschlagen, dass die frustrierten Translationen, die schließlich zur Diffusion führen, hauptsächlich indirekt angeregt werden. Durch die elektronische Reibung werden zunächst primär O-Pt-Streckschwingungen angeregt und diese koppeln dann anharmonisch an die frustrierten O-Pt-Translationen. In diesem Modell entsteht die scheinbare Abhängigkeit des Reibungskoeffizienten von der Elektronentemperatur bzw. der Anregungsdichte durch die Zunahme der anharmonische Kopplung mit der Besetzung höherer Schwingungsniveaus., Electronically induced diffusion of atomic oxygen on a stepped Pt(111) surface has been studied using ultrashort pulses of near-infrared light for the generation of a hot electron distribution at the surface. This study represents the first time-resolved investigation of electronically induced diffusion at surfaces and demonstrates the possibility of initiating diffusion of a strongly chemisorbed adsorbate on a metal surface at low substrate temperatures by an electronic excitation. The dissociative adsorption of molecular oxygen at steps on the Pt(111) surface has been used to generate a non-equilibrium distribution of an adsorbate by decorating the step edges selectively with chemisorbed atomic oxygen. The coverage at the step edges during adsorption and induced diffusion has been monitored by exploiting the sensitivity of optical second-harmonic generation (SHG) on surface symmetry which is macroscopically broken by regular steps. The high sensitivity of this method made it possible to measure rates for hopping from a step site onto an initially empty terrace site down to 10^-7 per laser shot. A UHV chamber has been builded for the realization of the optical experiments as well as for the preparation and the characterization of the platinum surface. The diffusion process has been studied as a function of laser fluence and time delay between two pump-pulses, which made it possible to investigate the energy transfer dynamics from the optical excitation to the frustrated translation. These experiments revealed a strong nonlinear dependence of the diffusion-rate on laser fluence (~F^15) and a fast electronic coupling of the initial excitation and the adsorbate motion with a time-constant of 1.5 ps. Both findings suggests that the excitation mechanism of diffusion on metal surfaces is the same as for the well-studied process of desorption induced by femtosecond laser excitation, which is driven by the hot laser-excited electrons of the metallic substrate. Even for complicated reaction pathways, such experiments can be successfully described by models in which the electronic coupling of the adsorbate to the metal surface is represented by a constant electronic friction coefficient. The extreme nonlinear dependence of the hopping-rate on laser fluence, however, indicates that the energy transfer mechanism is more complicated than for the case of desorption. This is reflected by the fact that a consistent description of the experimental data within a generalized electronic friction model cannot be achieved with a constant electronic friction and a reasonable value for the diffusion barrier, but requires the introduction of a temperature dependent electronic friction coefficient. It is suggested that this temperature dependence appears due to the neglect of the coupling between different vibrational modes. A mechanism of an indirect excitation by anharmonic coupling to the O-Pt stretch is proposed, that introduces a coupling which depends on excitation density and would therefore explain the observed effective dependence of the electronic friction on electron temperature.

International audience; We demonstrate that the capacity of ultrashort high-power laser pulses to trigger and guide high-voltage discharges can be significantly enhanced by a subsequent visible nanosecond laser pulse. The femtosecond pulse induces a bundle of filaments, which creates a conducting channel of low density and cold plasma connecting the electrodes. The subsequent laser pulse photodetaches electrons from O2- ions in the electrode leader. The resulting electrons allow efficient heating by Joule effect in a retroaction loop, resulting in a 5% reduction of the breakdown voltage.

Femtosecond and nanosecond laser flash photolysis was used to determine the photophysical and photochemical processes in aqueous solutions of Fe(III) ion and 5-sulfosalicylic acid (SSA) containing the FeSSA complex and the free ligand. Excitation of the FeSSA complex in the charge transfer band (λmax = 505 nm) is followed by an ultrafast relaxation to the ground electronic state with two characteristic times of 0.26 and 1.8 ps. The shorter time constant is ascribed to internal conversion to the vibrationally hot electronic ground state of FeSSA and the 1.8 ps time constant is assigned to the vibrational cooling of the ground state. The UV irradiation of the solution (308 nm) leads to the excitation of both the free ligand and the FeSSA complex. The latter relaxes rapidly and the free ligand undergoes intersystem crossing to the triplet state. This system undergoes an irreversible photochemical reaction originating from an electron transfer (k = (9 ± 2) × 108 M−1 s−1) from the free ligand in the triplet state to the FeSSA complex. This electron transfer is accompanied by an energy transfer between these species (k = (1.3 ± 0.2) × 109 M−1 s−1).

Die Arbeit stellt die Entwicklung eines Verfahrens zur Erkennung der Niederschlagsfläche in optischen Satellitendaten dar. Die Abgrenzung regnender Wolken beruht dabei auf dem Modellkonzept, dass diese über eine ausreichend große Kombination aus optischer Dicke und effektivem Tropfenradius verfügen müssen., The PhD thesis descirbes the development of a satellite based delineation scheme between raining and non-rainging clouds. This relys on the concept model that precipiating clouds need both a large enough optical depth and a large enough cloud droplet radius.

In French - Acte Ecole thématique "Femto2004", Porquerolles, Sept. 2004; This chapter aims at identifying what is specific to femtosecond laser ablation. It reviews the essential basic processes which contribute to femtosecond laser ablation of various materials : energy absorption by the material's electrons, electronic relaxation processes involving either electron-electron or electron-lattice interactions. A number of widely used models are briefly discussed, as well as some pending questions.

International audience; We demonstrate remote elemental analysis at distances up to 90 m, using a laser-induced breakdown spectroscopy scheme based on filamentation induced by the nonlinear propagation of unfocused ultrashort laser pulses. A detailed signal analysis suggests that this technique, remote filament-induced breakdown spectroscopy, can be extended up to the kilometer range.

International audience; Ultrashort, high-power laser pulses propagating vertically in the atmosphere have been observed over more than 20 km using an imaging 2-m astronomical telescope. This direct observation in several wavelength bands shows indications for filament formation at distances as far as 2 km in the atmosphere. Moreover, the beam divergence at 5 km altitude is smaller than expected, bearing evidence for whole-beam parallelization about the nonlinear focus. We discuss implications for white-light Lidar applications.

Die heutige Halbleiter-Elektronik und Opto-Elektronik basiert fast ausschließlich auf der Ausnutzung der Coulomb-Wechselwirkung zwischen den Ladungen der Elektronen. Elektronen besitzen neben der Elementarladung aber auch noch den Spin als Teilchen-Eigenschaft. Der Elektronenspin als Eigen-Drehimpuls mit einer halbzahligen Quantenzahl hat kein klassisches Analogon. Er kann nur die beiden Zustände ? Spin-up? und ?Spin-down? annehmen. Fast 75 Jahre nach der Einführung des Elektronenspins durch Uhlenbeck und Goudsmith (1925) und seiner korrekten Beschreibung in der relativistischen Quantenmechanik im Jahre 1930 durch Dirac, hält diese Größe Einzug in die moderne Halbleiter- und Informationstechnologie. Auf der Basis von dünnen Metall-Schicht-Systemen sind bereits wichtige Entdeckungen auf dem Gebiet der ,,Magneto-Elektronik?, wie etwa des Riesenmagnetowiderstandes, zur Anwendung gekommen. Ebenso sind der spinabhängige Tunneleffekt, magnetisch steuerbare Widerstände, Magnetfeld-Sensoren und der ,,Magnetic Random Access Memory? Gegenstand der aktuellen Forschung. Alle diese Anwendungen nutzen die quantenmechanischen Eigenschaften des Spins aus, basieren jedoch ausschließlich auf Metallen. Eine Spin-Elektronik auf Halbleiterbasis hat dahingegen den Vorteil, dass sowohl die sehr gut entwickelte Halbleitertechnik der konventionellen Elektronik genutzt werden kann, als auch eine Verbindung mit der Opto-Elektronik möglich ist. Eine der Grundvoraussetzungen für das Betreiben einer Spin-Elektronik (?Spintronik?) in Halbleitern ist die Bereitstellung und Kontrolle von spinpolarisierten Elektronen. Durch die kohärente Kontrolle von zwei Laserstrahlen können spinpolarisierte Ströme in Halbleitern erzeugt werden. Diese Ströme beruhen auf der Interferenz von optischen Übergängen und sind eine makroskopische Manifestation von rein quantenmechanischen Phänomenen. Ein anderer Zugang zur Beeinflussung von Spin-Effekten in Halbleitern ist mit einer steuerbaren Symmetrie-Erniedrigung durch interne und externe elektrische Felder die Spin-Eigenschaften von Ladungsträgern im Leitungsband zu beeinflussen. Die Anisotropie-Effekte werden im Magnetfeld mit der Spin-Quantenschwebungs-Spektroskopie untersucht. Die Spin-Dephasierung von Elektron-Loch-Paaren mit paralleler Spinausrichtung, sogenannten ,,dunklen? Exitonen, in Halbleiter-Quantenpunkten werden untersucht. Halbleiter-Quantenpunkte werden aufgrund ihrer Eigenschaften bezüglich der langen Spin-Kohärenz-Zeiten oft als Kandidaten für eine zukünftige Quanten-Informationsverarbeitung mit Spins herangezogen. Die ,,dunklen'' Exitonen mit einem Gesamtdrehimpuls von J=2 werden durch Zwei-Photonen-Anregung erzeugt und die Umwandlung zu ,,hellen? Exitonen (J=1) mit zeitaufgelöster Ultrakurzzeit-Spektroskopie verfolgt. Im ersten Teil des Anhangs wird der Versuch vorgestellt, aus ferromagnetischen Eisen-Kontakten spinpolarisierte Elektronen über Tunnel-Prozesse in eine Halbleiter-Schichtstruktur zu injizieren. Dazu werden speziell für diesen Zweck prozessierte Dioden-Strukturen verwendet. Der zweite Teil beschäftigt sich mit der Biologie. Blaulicht-Rezeptoren steuern eine Vielzahl von hysiologischen Reaktionen bei Pflanzen, Pilzen und Mikro-Organismen und auch beim Menschen. Die chemischen Substanzen, die zum Beispiel Pflanzen das blaue Licht ,,sehen'' lassen, werden mit Ultrakurzzeit-Spektroskopie untersucht, um die Funktionsweise von lichtabhängigen Prozessen, wie etwa dem Phototropismus, das Wachsen der Pflanzen in Richtung des Lichts, aufzuklären., Today?s semiconductor-electronics and opto-electronics are mainly based on the Coulomb-interaction between the charge of electrons. But electrons own beneath their charge also the spin as an additonal property. The electron-spin is a pure quantum mechanical property which has no classical analogon. It can only be in one of the two states ?spin-up? or ?spin-down?, respectively. About 75 years after the introduction of the spin into physics by Goudsmith and Uhlenbeck in 1925 and its correct description in the relativistical quantum mechanics by Dirac in1930, the property ?spin? gets into the modern semiconductor and information technology. With thin metallic film systems, there have been already succesfull developments in the field of magneto-electronics, e.g. the GMR- and TMR effects. Also the ?Magnetic Random Acces Memory? (MRAM) is subject to intense research. All these application make use of the quantum mechanical property ?spin?, but are base on metallic systems only. A semiconductor based spin-electronic (?Spintronic?) has the advantage of easy integration into the conventional electronics. Also a connection with opto-electronics is possible. One of the main prerequisites for spin-based electronic is the efficient and controlled injection of carrier spin currents. By coherent control of two laser light-fields spin-polarized currents can be generated in semiconductors. These currents are based on the interference of quantum mechanical (opical) transitions and are a makroscopic manifestation of a pure quantum mechanical phenomen, which is shown in this work. A different approach to influence spin-related effects in semiconductors is due a controllable symmetry reduction by internal or external applied electrical fields in semiconductor heterostructures. The resulting anisotropic spin-effects are measured by spin quantum beat spectroscopy in magnetic fields. The spin-depahsing mechanism electron-hole pairs with parallel spin-orientation, so called ?dark excitions? are investigated in semiconductor quantum dots. Those quantum dots are often suggested to play a role in a prosper quantum-computing technology, because of the long decoherece times of their carrier spins. Dark excitons are excited by two-photon absorption by an ultrashort laser pulse and the converion of the dark excitons (J=2) to bright excitons (J=1) is measured time-resolved under different conditions.Today?s semiconductor-electronics and opto-electronics are mainly based on the Coulomb-interaction between the charge of electrons. But electrons own beneath their charge also the spin as an additonal property. The electron-spin is a pure quantum mechanical property which has no classical analogon. It can only be in one of the two states ?spin-up? or ?spin-down?, respectively. About 75 years after the introduction of the spin into physics by Goudsmith and Uhlenbeck in 1925 and its correct description in the relativistical quantum mechanics by Dirac in1930, the property ?spin? gets into the modern semiconductor and information technology. With thin metallic film systems, there have been already succesfull developments in the field of magneto-electronics, e.g. the GMR- and TMR effects. Also the ?Magnetic Random Acces Memory? (MRAM) is subject to intense research. All these application make use of the quantum mechanical property ?spin?, but are base on metallic systems only. A semiconductor based spin-electronic (?Spintronic?) has the advantage of easy integration into the conventional electronics. Also a connection with opto-electronics is possible. One of the main prerequisites for spin-based electronic is the efficient and controlled injection of carrier spin currents. By coherent control of two laser light-fields spin-polarized currents can be generated in semiconductors. These currents are based on the interference of quantum mechanical (opical) transitions and are a makroscopic manifestation of a pure quantum mechanical phenomen, which is shown in this work. A different approach to influence spin-related effects in semiconductors is due a controllable symmetry reduction by internal or external applied electrical fields in semiconductor heterostructures. The resulting anisotropic spin-effects are measured by spin quantum beat spectroscopy in magnetic fields. The spin-depahsing mechanism electron-hole pairs with parallel spin-orientation, so called ?dark excitions? are investigated in semiconductor quantum dots. Those quantum dots are often suggested to play a role in a prosper quantum-computing technology, because of the long decoherece times of their carrier spins. Dark excitons are excited by two-photon absorption by an ultrashort laser pulse and the converion of the dark excitons (J=2) to bright excitons (J=1) is measured time-resolved under different conditions.

Time-resolved x-ray spectroscopy has been used to investigate ionization dynamics of a micrometer-sized nanosecond laser-plasma during the plasma start-up phase. Experimental results are modeled using two-dimensional hydrodynamic simulations and time-dependent collisional-radiative calculations. The study clearly shows that, due to the rapid expansion cooling, x-ray emission originates predominantly from a well-localized plasma region characterized by rapidly evolving hydrodynamic conditions. In this region, ionization dynamics is found to depart substantially from the steady-state regime. The measurements provide clear evidence of this transient ionization regime showing good agreement with the time-dependent calculations.

The energy and electron transfer processes taking place in binuclear polypyridine complexes of ruthenium and osmium based on the tetrapyrido[3,2- a:2¢,3¢- c:3¢¢,2¢¢- h:2¢¢¢-3¢¢¢- j]phenazine bridging ligand (tpphz) have been investigated by ultrafast absorption spectroscopy. In the binuclear complexes, each chromophore is characterized by two spectrally distinguishable metal-to-ligand charge transfer (MLCT) excited states: MLCT1 (with promoted electron mainly localized on the bpy-like portion of tpphz, higher energy) and MLCT0 (with promoted electron mainly localized on the pyrazine-like portion of tpphz, lower energy). In the homodinuclear complexes Ru(II)-Ru(II) and Os(II)-Os(II), MLCT1 f MLCT0 relaxation (intraligand electron transfer) is observed, with strongly solvent-dependent kinetics (ca. 10-10 s in CH2Cl2, ca. 10-12 s in CH3CN). In the heterodinuclear Ru(II)-Os(II) complex, *Ru(II)-Os(II) f Ru(II)-*Os(II) energy transfer takes place by two different sequences of time-resolved processes, depending on the solvent: (a) in CH2Cl2, ruthenium-to-osmium energy transfer at the MLCT1 level followed by MLCT1 f MLCT0 relaxation in the osmium chromophore, (b) in CH3CN, MLCT1 f MLCT0 relaxation in the ruthenium chromophore followed by osmium-to-ruthenium metal-to-metal electron transfer. In the mixed-valence Ru(II)-Os(III) species, the *Ru(II)-Os(III) f Ru(III)-Os(II) electron transfer quenching is found to proceed by two consecutive steps in CH3CN: intraligand electron transfer followed by ligand-to-metal electron transfer. On a longer time scale, charge recombination leads back to the ground state. Altogether, the results show that the tpphz bridge plays an active mechanistic role in these systems, efficiently mediating the transfer processes with its electronic levels.

Several aspects of ultrafast photochemistry in the condensed phase are discussed and illustrated by three examples from our laboratory.

Die vorliegende Arbeit untersucht ultraschnelle Rotations- und Translationsbewegungen in molekularen Flüssigkeiten. Dazu wurde deren Optischer Kerr-Effekt/Raman-Induzierter Kerr-Effekt (OKE/RIKE) zeitaufgelöst mithilfe der Pump/Probetechnik gemessen. Die erzielte Zeitauflösung betrug 30 fs. Langzeitschwankungen des Signals konnten zusätzlich mit einer Echtzeit-Meßtechnik eliminiert werden. In der transienten Doppelbrechung wurde so ein Signal/Rausch Verhältnis von 10^7 erreicht. Das Ziel war eine möglichst genaue Beschreibung der Responsfunktion für den Optischen Kerr-Effekt der Flüssigkeiten Chloroform, Acetonitril, Trichloracetonitril, Tetrachlorkohlenstoff, Methylchloroform und Fluoroform. Die Fourier-Transformation dieser Responsfunktion entspricht dem depolarisierten Raman Spektrum der Flüssigkeit. Die Responsfunktion wurde auf zwei Wegen aus dem gemessenen OKE Signal erhalten: a) in der Zeitdomäne durch Anpassung empirisch gewählter Terme und b) in der Frequenzdomäne durch Anpassung von Brownschen Oszillatormoden. Die Analyse bezieht den gesamten Datenumfang ein, ohne numerisch problematische Entfaltungsverfahren nutzen zu müssen. In den untersuchten Systemen erfolgt die elektronische Antwort instantan auf den anregenden Laserimpuls. Innerhalb der ersten halben Pikosekunde beobachtet man eine intermolekulare Dynamik, die auf Librationen und kollisionsinduzierte Translationsbewegungen zurückgeht. Diese nur schwer unterscheidbare Dynamik verschwindet typischerweise mit Zeitkonstanten bis 250 fs. Die Langzeitrelaxation in den isotropen Zustand wird der diffusiven Reorientierung zugeordnet. Es werden hierfür Zeitkonstanten tau zwischen 1.2 (Methylchloroform) und 3 ps (Chloroform) beobachtet. Durch die hohe Bandbreite des Laserimpulses werden außerdem niederfrequente Raman-Linien (bis ca. 750 cm^-1) angeregt. Diese äußern sich durch untergedämpfte Signaloszillationen (RIKE). Erstmals wird der OKE von flüssigem Fluoroform untersucht. Aufgrund seiner Eigenschaften kommt CHF3 der Modellvorstellung einer polaren harten Kugel sehr nahe. Das beobachtete Signal ist ca. 110 mal schwächer als in Chloroform. Die diffusive Reorientierung verläuft auß erdem deutlich schneller (tau = 0.8 ps). Der (intermolekulare) OKE wächst nun mit steigender Polarisierbarkeitsanisotropie, während der (intramolekulare) RIKE von der änderung der Polarisierbarkeitsanisotropie mit der Schwingungskoordinate des Moleküls abhängt. Durch vergleichende Messungen mit isotropen Tetrachlorkohlenstoff kann auch prinzipiell der Einfluß der kollisionsinduzierten Effekte (CI) in den untersuchten Flüssigkeiten abgeschätzt werden. Rotations- und Translationsbewegungen korrelieren allerdings miteinander. Die daraus resultierenden Kreuzterme sind dann aus den Responsfunktionen allein nicht mehr zu ermitteln. Sie sind nur aus weiterführenden moleküldynamischen Simulationen erhältlich. Für Acetonitril wird eine übereinstimmung zwischen experimenteller und theoretisch vorhergesagter OKE Responsfunktion gefunden. Die OKE Responsfunktionen sind mit den Relaxationsfunktionen der dielektrischen Relaxation und der Solvatation von polaren Sondenmolekülen verknüpft. Die Messungen des zeitaufgelösten Optischen Kerr-Effektes sollen hier künftig dazu dienen, die reinen Flüssigkeitsbeiträge zur polaren und nichtpolaren Solvatation zu erkennen., Within this work the ultrafast rotational and translational motions of molecular liquids are investigated. Therefore their Optical Kerr effect/Raman induced Kerr effect (OKE/RIKE) was measured time-resolved with the pump/probe technique. Achieved time resolution was 30 fs. Long-time fluctuations of the signal might be eliminated by an additional realtime measurement technique. Hence a signal/noise ratio of ca. 10^7 in the transient birefringence was obtained. A detailed description of the response function derived from Optical Kerr effect was targeted for the liquids chloroform, acetonitrile, trichloroacetonitrile, carbon tetrachloride, 1,1,1-trichloroethane and fluoroform. The Fourier-Transformation of the response functions of the liquids is eqivalent to their depolarized Raman spectra. The response function is obtained from measured signals in two ways: a) by fitting empirical terms in time domain and b) fitting Brownian oscillator modes in frequency domain. Analysis includes complete data range without using numerical deconvolution techniques. In all investigated systems an electronical response follows to the stimulating laser pulse instantaneously. Within the first half pico second one observes intermolecular dynamics due to librational and collision-induced translational motions. These (hard to distinguish) dynamics disappear with typical time-constants of up to 250 fs. The long-time relaxation into an isotropic distribution of molecules is termed diffusive reorientation. Here time-constants tau between 1.2 (CCl3CH3) and 3 ps (CHCl3) are observed. Additionally low-frequent Raman lines (up to 750 cm^-1) may be stimulated due to the high pulse bandwidth, resulting in underdamped signal oscillations (RIKE). The OKE of liquid fluoroform is investigated for the first time. It comes closest to the Mean Spherical Approximation model for a fluid composed of polar, nonpolarizable hard spheres. The observed signal is ca. 110 times weaker than in chloroform. The diffusional reorientation occurs also faster (tau = 0.8 ps). Generally (intermolecular) OKE rises with growing polarizability anisotropy, while (intramolecular) RIKE depends from the change of polarizability anisotropy with the vibrational coordinate. Influence of collision-induced effects (CI) is derived in principle from compared measurements in isotropic carbon tetrachloride. However a correlation exists between rotational and translational motion. Resulting cross-terms cannot be obtained from response functions alone. These are only available in molecular-dynamic simulations in literature. In acetonitril one finds similar response functions derived from OKE measurements and predicted theoretical simulation. The OKE respons functions are related to relaxation functions of the dielectrical relaxation and the solvation of polar molecules. Time-resolved measurements of the OKE reveal here contributions of the pure liquid in polar and nonpolar solvation.

Harmonic radiation generated in a neon gas jet by sub-10-fs laser pulses was investigated both experimentally and theoretically. The spectral profile of the harmonics with respect to the order, their intensity and relative spectral shifts were measured as a function of the position of the gas jet. The results point out spectral features typical of the quasi-single-cycle excitation regime. A nonadiabatic three-dimensional numerical model was developed, which provides harmonic spectra in remarkable agreement with the experiments.

We report the realization of a vacuum-ultraviolet radiation source based on high-order harmonic generation in noble-gas samples, operating at high repetition rate. In particular, we observed up to the 13th harmonic (lambda = 61 nm) of the fundamental frequency of a short pulse, high repetition rate titanium-sapphire laser after its interaction with a Xe gas jet. The effects of the propagation of the fundamental and harmonic beams through an ionized medium are studied by analysing the spectral profile of the 9th and 7th harmonics. Finally, we report a study of the dependence of the harmonic conversion efficiency on relative position of the focus and the gas target.

The basic considerations underlying the optical transmission-correlation technique based upon the high-repetition-rate 40 fs laser system are reviewed. Recent results on the relaxation dynamics of nonequilibrium carriers in GaAs and related compounds and structures and photoexcited dye molecules are summarized.

The aim of this thesis is to investigate the ultrafast inter¿ and intra¿molecular processes, occurring in both metal¿containing molecular complexes and in proteins, by means of ultrafast Transient Absorption (TA) spectroscopy. The first part of the thesis focuses on understanding the origin of the solvent dependent Intersystem Crossing (ISC), from the lowest excited singlet state (1A2u) to the lowest excited triplet state (3A2u), in [Pt2 (P2O5H2)4]4¿ (Pt(pop)) and its perfluoroborated derivative [Pt2 (P2O5(BF2)2)4]4¿ (Pt(pop)¿BF2). Our UV pump¿Visible probe TA experiments (exciting both in the 1A2u state and in higher¿lying states) highlight the presence of an intermediate state, in agreement with the previous hypothesis of Milder and Brunschwig. The energy of the latter state is strongly modulated by both the solvent and the ligands present in the pop cage, causing the large difference in ISC timescales (upon excitation in the 1A2u state) between Pt(pop) in acetonitrile (MeCN) (< 1 ps), Pt(pop) in water (~13 ps) and Pt(pop)¿BF2 in MeCN (1.6 ns). Excitation at higher¿lying excited states populates both the 1A2u and the 3A2u state in different ratios, depending on the investigated system. Finally, we report on the presence of wave¿packets in the ground and excited states, as well as the coherence transfer from the initially excited state to the 3A2u state. The second part focuses on the photo¿excited tryptophan (*Trp) quenching by metal complexes in protein systems, such as ferrous Myoglobins (Mbs) and Prion Proteins (PrPs). Mb is a small globular protein composed by 153 amino acids and containing two Trp residues (positions 7 and 14). Trp7 is ~20 Å far away from the heme and its fluorescence is only quenched via Förster energy transfer (FRET), while Trp14 is closer (~15 Å) and it is quenched via both FRET and electron transfer to the heme. Our results demonstrate that *Trp14¿to¿heme electron transfer process occurs, surprisingly, also in ferrous Mbs (e.g. deoxy¿Mb and MbCO) and highlight the generation of a long¿lived product, namely a FeII¿porphyrin¿¿. We also report the interesting case of MbNO, in which the *Trp14 transfers the electron on the NO instead of the heme. Prion proteins are also globular proteins composed of 209 amino acids (in humans) and involved in metal binding. Even though a large amount of experiments and theoretical work have been performed on these proteins their main physiological role is not completely determined, but it is clear that the formation of a scrapie isoform (PrPSC) of the cellular PrPs (PrPC) is the infectious agent and functions as a template for the PrPC ¿ PrPSC conversion. Our UV pump¿Visible probe TA experiments aim to unravel the ultrafast processes occurring in the octarepeat region of PrPs and fill the gap between ultrashort and biological timescales. We find that both the minimum Cu binding sequence HGGGW (OPS) and the total octarepeat region (PHGGGWGQ)4 (OP4) display a non¿exponential quenching of the Trp residues, which was assigned to *Trp¿to¿nearby amino acids (H, G or Q) in different peptide¿s conformations. Upon Cu2+ complexation by OPS, a feature related to the formation of a Cu+ complex was detected. The same was not detected upon Cu2+ complexation by OP4, showing longer quenching timescales than the free peptide. Our results highlight the relationship between structural conformation of the peptide and quenching timescales of the *Trp.

We present efficient high-order harmonic generation (HHG) based on a high-repetition rate, few-cycle, near infrared (NIR), carrier-envelope phase stable, optical parametric chirped pulse amplifier (OPCPA), emitting 6 fs pulses with 9 μJ pulse energy. In krypton, we reach conversion efficiencies from the NIR to the extreme ultraviolet (XUV) radiation pulse energy on the order of ∼10-6 with less than 3 μJ driving pulse energy. This is achieved by optimizing the OPCPA for a spatially and temporally clean pulse and by a specially designed high-pressure gas target. In the future, the high efficiency of the HHG source will be beneficial for high-repetition rate two-colour (NIR-XUV) pump-probe experiments, where the available pulse energy from the laser has to be distributed economically between pump and probe pulses. © 2017 IOP Publishing Ltd.