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We theoretically demonstrate the control of the polarization direction of isolated attosecond pulses (IAPs) with inhomogeneous two-color fields synthesized by an 800-nm fundamental pulse and a 2000-nm control pulse having crossed linear polarizations. The results show that by using the temporally and spatially shaped field, the high-order harmonic generation (HHG) process can be efficiently controlled. An ultra-broad supercontinuum ranging from 150th to 400th harmonics which covers the water window region is generated. Such a supercontinuum supports the generation of a 64-as linearly polarized IAP, whose polarization direction is at about 45° with respect to the x axis. Moreover, we analyze the influence of the inhomogeneity parameters and the relative angle of the fundamental and control pulses on the IAP generation. It is shown that the polarization direction of the IAP can rotate in a wide range approximately from 8° to 90° relative to the x axis when the inhomogeneity parameters and the relative angle vary.

Quantum manipulation of populations and pathways in matter by light pulses, so-called coherent control, is currently one of the hottest research areas in optical physics and photochemistry. The forefront of coherent control research is moving rapidly into the regime of extreme ultraviolet wavelength and attosecond temporal resolution. This advance has been enabled by the development of high harmonic generation light sources driven by intense femtosecond laser pulses and by the advent of seeded free electron laser sources. Synchrotron radiation, which is usually illustrated as being of poor temporal coherence, hitherto has not been considered as a tool for coherent control. Here we show an approach based on synchrotron radiation to study coherent control in the extreme ultraviolet and attosecond regime. We demonstrate this capability by achieving wave-packet interferometry on Rydberg wave packets generated in helium atoms., Synchrotron light sources have wide range of tunable parameters like frequency, intensity. Here the authors demonstrate quantum control of Rydberg states of helium using delay controlled XUV wavepackets generated from synchrotron radiation.

Subjecting atoms or molecules to intense laser pulses gives rise to a variety of highly nonlinear phenomena, such as for instance the ionization of electrons and the radiation of high-frequency photons. The distributions of the velocity of the ionized electrons or the frequency of the radiated photons measured at the detector encode relevant informations on the target atoms and molecules at the natural time scale of the electrons, the attosecond–that is, million, million, millionths of a second. Understanding the dynamics of the ionized electrons and identifying the mechanisms of high-frequency radiation are essential steps toward interpreting and decoding the informations encrypted in the experimental measurements.In this thesis, atoms subjected to intense and elliptically polarized laser fields in the infrared regime are theoretically studied. Despite their fundamental quantal nature in atoms, electrons display some classical behaviors when subjected to intense laser pulses. We exploit these classical features to understand and picture, with the help of trajectories, the physical mechanisms at play in order to interpret experimental measurements. We show the interdependent role of the quantum tunnel ionization of the electron and its subsequent classical motion for interpreting measurements in attosecond science.After tunnel ionization of the electrons, the interplay between their interactions with the laser and their parent ion, by yielding their dynamics highly nonlinear, gives rise to rich and diverse ionization channels. Changing the ellipticity of the driving laser, which acts as a simple control knob in experiments, changes the prioritized ionization channel taken by the electrons. In this way, for instance, the electrons can probe different characteristics of the target atoms. The motion of the ionized electrons is analyzed using pertur- bative and nonperturbative techniques from nonlinear dynamics and Hamiltonian systems. This thesis work demonstrates the complementarity of quantum mechanics and nonlinear dynamics for understanding and illustrating the mechanisms involved when atoms are subjected to intense and elliptically polarized laser pulses.; Assujettir des atomes ou des molécules à des impulsions laser de fortes intensités donne lieu à une variété de phénomènes hautement non-linéaires, tels que l’ionisation d’électrons et la radiation de photons de fréquences élevées. Les distributions des vitesses des électrons ionisés ou les distributions des fréquences des photons radiés encodent des informations pertinentes sur les atomes ou les molécules ciblés à l’échelle temporelle na- turelle des électrons, l’attoseconde–qui est un millionième, d’un millionième, d’un millionième d’une seconde. Comprendre la dynamique des électrons ionisés ainsi qu’identifier les mécanismes de radiation de fréquences élevées sont des étapes essentielles afin d’interpréter et décoder les informations cryptées dans les mesures expérimentales.Dans cette thèse, des atomes sujets à des impulsions laser de fortes intensités polarisées elliptiquement dans le régime infra-rouge sont étudiés théoriquement. Malgré leur nature fondamentalement quantique dans les atomes, les électrons manifestent certains comportements classiques lorsqu’ils sont sujets à des impulsions laser de fortes intensités. Nous exploitons ces traits classiques pour comprendre et illustrer, à l’aide des trajectoires, les mécanismes physiques en jeu afin d’interpréter les résultats expérimentaux. Nous montrons le rôle interdépendant de l’ionisation quantique par effet tunnel de l’électron et ultérieurement de son mouvement classique pour interpréter les mesures en science attoseconde.Après l’ionisation par effet tunnel des électrons, la conjugaison entre leurs interactions avec le laser et leurs interactions avec leur ion parent, en rendant leurs dynamique hautement non-linéaire, donne lieu à de riches et variés canaux d’ionisation. Changer l’ellipticité du laser, qui agit comme un simple bouton de contrôle en expérience, change les canaux d’ionisation prioritairement empruntés par les électrons. De cette façon, par exemple, les électrons peuvent sonder différentes caractéristiques des atomes ciblés. Le mouvement des électrons ionisés est analysé en utilisant des techniques perturbatives et non-perturbatives issues de la dynamique non-linéaire et des systèmes hamiltoniens. Ce travail de thèse démontre la complémentarité de la mécanique quantique et de la dynamique non-linéaire pour comprendre et illustrer des mécanismes impliqués lorsque des atomes sont sujets à des impulsions laser de fortes intensités polarisées elliptiquement.

Since the advent of femtochemistry, at the end of 1980's, we are able to probe the nuclear dynamics underlying chemical reactions down to the scale of a femtosecond. More recently, the production of isolated attosecond pulses allows to reach an even bigger temporal resolution. It is now possible to probe the ultrafast electronic dynamics following a photo-excitation. In this context, with the developpement of time-resolved spectroscopy techniques, it is important to have reliable theorectical approaches in order to apprehend the increasing number of results in this field. This task is made difficult by the intrinsic multi-electronic nature processes at play. The precise treatment of such effects requires a considerable computing power, and have thus limited the availables studies to relatively small systems. In this thesis, I tried to explain the outcome of a "pump-probe" (XUV-IR) experiment on Caffeine molecule realized by our collaborators at the Insitut Lumière Matière. To do so, I used a method based on density functional theory, the TD-DFTB, which lower numerical cost with respect to TD-DFT allows calculation on bigger compounds. I present in the document a study of the energetical landscape of Caffeine, and 2 approaches to simulate ionization. The first one, the so called sudden-ionization approximation consist to retrieve "by hand" an electron from the occupied Kohn-Sham orbitals of the neutral system without taking the laser field into account. The other one is based on the introduction of a complex absorbing potential (CAP) to account for electron loss and take explicitely the laser field into account.; Depuis la fin des années 80 et l'avènement de la femto-chimie nous pouvons sonder la dynamique nucléaire à l’œuvre au cours de réactions chimiques à l'échelle de la femtoseconde. Plus récemment, la production d'impulsions lasers attosecondes isolées permet d'atteindre une résolution temporelle plus grande encore. Par elle, il devient possible de sonder la dynamique d'origine purement électronique induite par photo-excitation, et notamment photo-ionisation. Dans ce contexte, avec le développement des techniques de spectroscopie résolue en temps, il est important de disposer d'approches théoriques fiables aidant à l'appréhension de résultats toujours plus nombreux dans ce domaine. La tâche et néanmoins rendue difficile par le caractère profondément multi-électronique des processus en jeu. Traiter de tels effets précisément requiert une grande puissance de calcul, ce qui a limité les études disponibles aujourd'hui à de petits systèmes. Au cours de cette thèse, j'ai tenté d'expliquer les résultats d'une expérience de type "pompe-sonde" (UVX-IR) sur molécule de Caféine menée par une équipe de collaborateurs à l'Institut lumière matière. J'ai utilisé pour cela une méthode basée sur la théorie de la fonctionnelle de la densité dépendante du temps, la TD-DFTB dont le coût numérique réduit par rapport à cette dernière permet des calculs sur de gros systèmes en temps raisonnable. J'y présente une étude du paysage énergétique de la Caféine ainsi que le résultat de 2 approches distinctes pour simuler l'ionisation de ce composé. La première, l'approximation de l'ionisation soudaine cosiste à retirer "à la main" un électron à l'une des orbitales Kohn-Sham occupées du système neutre et ne tient pas compte du champ laser. La seconde à recours à un potentiel imaginaire (ou CAP - Complex Absorbing Potential) pour simuler la perte d'electrons, et tiens explicitement compte du champ laser

Attosecond probing of core-level electronic transitions provides a sensitive tool for studying valence molecular dynamics with atomic, state, and charge specificity. In this report, we employ attosecond transient absorption spectroscopy to follow the valence dynamics of strong-field initiated processes in methyl bromide. By probing the 3d core-to-valence transition, we resolve the strong field excitation and ensuing fragmentation of the neutral σ* excited states of methyl bromide. The results provide a clear signature of the non-adiabatic passage of the excited state wavepacket through a conical intersection. We additionally observe competing, strong field initiated processes arising in both the ground state and ionized molecule corresponding to vibrational and spin-orbit motion, respectively. The demonstrated ability to resolve simultaneous dynamics with few-femtosecond resolution presents a clear path forward in the implementation of attosecond XUV spectroscopy as a general tool for probing competing and complex molecular phenomena with unmatched temporal resolution., Excited-state molecular dynamics may be too complex to be resolved by femtosecond spectroscopic studies. Here the authors resolve the competing pathways in the excited state dynamics of methyl bromide by attosecond transient absorption spectroscopy, from excitation to fragmentation.

The strong coupling between intense laser fields and valence electrons in molecules causes distortions of the potential energy hypersurfaces which determine the motion of the nuclei and influence possible reaction pathways. The coupling strength varies with the angle between the light electric field and valence orbital, and thereby adds another dimension to the effective molecular potential energy surface, leading to the emergence of light-induced conical intersections. Here, we demonstrate that multiphoton couplings can give rise to complex light-induced potential energy surfaces that govern molecular behavior. In the laser-induced dissociation of H2+, the simplest of molecules, we measure a strongly modulated angular distribution of protons which has escaped prior observation. Using two-color Floquet theory, we show that the modulations result from ultrafast dynamics on light-induced molecular potentials. These potentials are shaped by the amplitude, duration and phase of the dressing fields, allowing for manipulating the dissociation dynamics of small molecules., Conical intersections, a hallmark of polyatomic molecules, can be induced with light, leading to new reaction pathways. Here, the authors show that intense fields can create complex, beyond-conical intersections even in diatomics, resulting in an unexpected angular distribution of fragment ions.

We study double ionization of He driven by a near-single-cycle laser pulse at low intensities at 400 nm. Using a three-dimensional semiclassical model, we identify the pathways that prevail non-sequential double ionization (NSDI). We focus mostly on the delayed pathway, where one electron ionizes with a time-delay after recollision. We have recently shown that the mechanism that prevails the delayed pathway depends on intensity. For low intensities slingshot-NSDI is the mechanism that prevails. Here, we identify the differences in two-electron probability distributions of the prevailing NSDI pathways. This allows us to identify properties of the two-electron escape and thus gain significant insight into slingshot-NSDI. Interestingly, we find that an observable fingerprint of slingshot-NSDI is the two electrons escaping with large and roughly equal energies., 9 pages, 5 figures

Subjecting atoms or molecules to intense laser pulses gives rise to a variety of highly nonlinear phenomena, such as for instance the ionization of electrons and the radiation of high-frequency photons. The distributions of the velocity of the ionized electrons or the frequency of the radiated photons measured at the detector encode relevant informations on the target atoms and molecules at the natural time scale of the electrons, the attosecond–that is, million, million, millionths of a second. Understanding the dynamics of the ionized electrons and identifying the mechanisms of high-frequency radiation are essential steps toward interpreting and decoding the informations encrypted in the experimental measurements.In this thesis, atoms subjected to intense and elliptically polarized laser fields in the infrared regime are theoretically studied. Despite their fundamental quantal nature in atoms, electrons display some classical behaviors when subjected to intense laser pulses. We exploit these classical features to understand and picture, with the help of trajectories, the physical mechanisms at play in order to interpret experimental measurements. We show the interdependent role of the quantum tunnel ionization of the electron and its subsequent classical motion for interpreting measurements in attosecond science.After tunnel ionization of the electrons, the interplay between their interactions with the laser and their parent ion, by yielding their dynamics highly nonlinear, gives rise to rich and diverse ionization channels. Changing the ellipticity of the driving laser, which acts as a simple control knob in experiments, changes the prioritized ionization channel taken by the electrons. In this way, for instance, the electrons can probe different characteristics of the target atoms. The motion of the ionized electrons is analyzed using pertur- bative and nonperturbative techniques from nonlinear dynamics and Hamiltonian systems. This thesis work demonstrates the complementarity of quantum mechanics and nonlinear dynamics for understanding and illustrating the mechanisms involved when atoms are subjected to intense and elliptically polarized laser pulses., Assujettir des atomes ou des molécules à des impulsions laser de fortes intensités donne lieu à une variété de phénomènes hautement non-linéaires, tels que l’ionisation d’électrons et la radiation de photons de fréquences élevées. Les distributions des vitesses des électrons ionisés ou les distributions des fréquences des photons radiés encodent des informations pertinentes sur les atomes ou les molécules ciblés à l’échelle temporelle na- turelle des électrons, l’attoseconde–qui est un millionième, d’un millionième, d’un millionième d’une seconde. Comprendre la dynamique des électrons ionisés ainsi qu’identifier les mécanismes de radiation de fréquences élevées sont des étapes essentielles afin d’interpréter et décoder les informations cryptées dans les mesures expérimentales.Dans cette thèse, des atomes sujets à des impulsions laser de fortes intensités polarisées elliptiquement dans le régime infra-rouge sont étudiés théoriquement. Malgré leur nature fondamentalement quantique dans les atomes, les électrons manifestent certains comportements classiques lorsqu’ils sont sujets à des impulsions laser de fortes intensités. Nous exploitons ces traits classiques pour comprendre et illustrer, à l’aide des trajectoires, les mécanismes physiques en jeu afin d’interpréter les résultats expérimentaux. Nous montrons le rôle interdépendant de l’ionisation quantique par effet tunnel de l’électron et ultérieurement de son mouvement classique pour interpréter les mesures en science attoseconde.Après l’ionisation par effet tunnel des électrons, la conjugaison entre leurs interactions avec le laser et leurs interactions avec leur ion parent, en rendant leurs dynamique hautement non-linéaire, donne lieu à de riches et variés canaux d’ionisation. Changer l’ellipticité du laser, qui agit comme un simple bouton de contrôle en expérience, change les canaux d’ionisation prioritairement empruntés par les électrons. De cette façon, par exemple, les électrons peuvent sonder différentes caractéristiques des atomes ciblés. Le mouvement des électrons ionisés est analysé en utilisant des techniques perturbatives et non-perturbatives issues de la dynamique non-linéaire et des systèmes hamiltoniens. Ce travail de thèse démontre la complémentarité de la mécanique quantique et de la dynamique non-linéaire pour comprendre et illustrer des mécanismes impliqués lorsque des atomes sont sujets à des impulsions laser de fortes intensités polarisées elliptiquement.

Plasma-based accelerators (PBAs), having demonstrated the production of GeV electron beams in only centimetre scales, offer a path towards a new generation of highly compact and cost-effective particle accelerators. However, achieving the required beam quality, particularly on the energy spread for applications such as free-electron lasers, remains a challenge. Here we investigate fundamental sources of energy spread and bunch length in PBAs which arise from the betatron motion of beam electrons. We present an analytical theory, validated against particle-in-cell simulations, which accurately describes these phenomena. Significant impact on the beam quality is predicted for certain configurations, explaining previously observed limitations on the achievable bunch length and energy spread. Guidelines for mitigating these contributions towards high-quality beams are deduced.

The many-body quantum nature of molecules determines their static and dynamic properties, but remains the main obstacle in their accurate description. Ultrashort extreme ultraviolet pulses offer a means to reveal molecular dynamics at ultrashort timescales. Here, we report the use of time-resolved electron-momentum imaging combined with extreme ultraviolet attosecond pulses to study highly excited organic molecules. We measure relaxation timescales that increase with the state energy. High-level quantum calculations show these dynamics are intrinsic to the time-dependent many-body molecular wavefunction, in which multi-electronic and non-Born−Oppenheimer effects are fully entangled. Hints of coherent vibronic dynamics, which persist despite the molecular complexity and high-energy excitation, are also observed. These results offer opportunities to understand the molecular dynamics of highly excited species involved in radiation damage and astrochemistry, and the role of quantum mechanical effects in these contexts., The many-body quantum nature of molecules determines their static and dynamic properties, but remains the main obstacle in their accurate description. Here, the authors employ ultrafast spectroscopic methods to explore the dynamics of highly excited organic molecules, revealing many-body effects and hints of coherent vibronic dynamics which persist despite their molecular complexity.

Since the advent of femtochemistry, at the end of 1980's, we are able to probe the nuclear dynamics underlying chemical reactions down to the scale of a femtosecond. More recently, the production of isolated attosecond pulses allows to reach an even bigger temporal resolution. It is now possible to probe the ultrafast electronic dynamics following a photo-excitation. In this context, with the developpement of time-resolved spectroscopy techniques, it is important to have reliable theorectical approaches in order to apprehend the increasing number of results in this field. This task is made difficult by the intrinsic multi-electronic nature processes at play. The precise treatment of such effects requires a considerable computing power, and have thus limited the availables studies to relatively small systems. In this thesis, I tried to explain the outcome of a "pump-probe" (XUV-IR) experiment on Caffeine molecule realized by our collaborators at the Insitut Lumière Matière. To do so, I used a method based on density functional theory, the TD-DFTB, which lower numerical cost with respect to TD-DFT allows calculation on bigger compounds. I present in the document a study of the energetical landscape of Caffeine, and 2 approaches to simulate ionization. The first one, the so called sudden-ionization approximation consist to retrieve "by hand" an electron from the occupied Kohn-Sham orbitals of the neutral system without taking the laser field into account. The other one is based on the introduction of a complex absorbing potential (CAP) to account for electron loss and take explicitely the laser field into account., Depuis la fin des années 80 et l'avènement de la femto-chimie nous pouvons sonder la dynamique nucléaire à l’œuvre au cours de réactions chimiques à l'échelle de la femtoseconde. Plus récemment, la production d'impulsions lasers attosecondes isolées permet d'atteindre une résolution temporelle plus grande encore. Par elle, il devient possible de sonder la dynamique d'origine purement électronique induite par photo-excitation, et notamment photo-ionisation. Dans ce contexte, avec le développement des techniques de spectroscopie résolue en temps, il est important de disposer d'approches théoriques fiables aidant à l'appréhension de résultats toujours plus nombreux dans ce domaine. La tâche et néanmoins rendue difficile par le caractère profondément multi-électronique des processus en jeu. Traiter de tels effets précisément requiert une grande puissance de calcul, ce qui a limité les études disponibles aujourd'hui à de petits systèmes. Au cours de cette thèse, j'ai tenté d'expliquer les résultats d'une expérience de type "pompe-sonde" (UVX-IR) sur molécule de Caféine menée par une équipe de collaborateurs à l'Institut lumière matière. J'ai utilisé pour cela une méthode basée sur la théorie de la fonctionnelle de la densité dépendante du temps, la TD-DFTB dont le coût numérique réduit par rapport à cette dernière permet des calculs sur de gros systèmes en temps raisonnable. J'y présente une étude du paysage énergétique de la Caféine ainsi que le résultat de 2 approches distinctes pour simuler l'ionisation de ce composé. La première, l'approximation de l'ionisation soudaine cosiste à retirer "à la main" un électron à l'une des orbitales Kohn-Sham occupées du système neutre et ne tient pas compte du champ laser. La seconde à recours à un potentiel imaginaire (ou CAP - Complex Absorbing Potential) pour simuler la perte d'electrons, et tiens explicitement compte du champ laser

Elektronien liikkeen havainnointi ja ohjaaminen on attosekuntitieteen keskiössä. Attosekuntiluokan elektroniprosessit ovat esimerkiksi kemiallisten reaktioiden takana, selittävät aineen optiset ominaisuudet sekä ovat pohjana useille ultranopeille nanomittaluokan kuvantamismenetelmille. Useat mielenkiintoiset attosekuntiluokan ilmiöt aiheutuvat vahvasta ulkoisesta sähkömagneettisesta kentästä. Tällaisia kenttiä saadaan femtosekuntilasersykäyksillä, joiden kenttien vahvuus on samaa suuruusluokkaa kuin atomin elektroniinsa kohdistama sähkökenttä. Voimakas sähkömagneettinen vuorovaikutus aiheuttaa atomien, molekyylien ja kiinteän aineen epälineaarisia ilmiöitä kuten korkeaenergisten fotonien tuottoa (HHG), nopeiden elektronien emissiota sekä esimerkiksi atomielektronin virittymistä korkeille sidotuille tiloille. Edellämainitut ilmiöt ovat myös pohjana useille teknisille edistyksille: HHG:lla tuotetaan koherentteja röntgensykäyksiä, joiden kesto on vain muutamien attosekuntien suuruusluokkaa; nopeita fotoemittoituneita elektroniaaltopaketteja käytetään aineen kuvantamiseen; ja Rydberg-tiloille viritettyjä atomeita käytetään kvanttilaskennassa kubitteina. Attosekuntiluokan ilmiöitä voidaan ohjata haluttuun suuntaan käyttämällä femtosekuntilasersykäyksiä, joiden sähkökentän aikariippuvuutta voidaan säätää. Tämä väitöskirja on laskennallinen tutkimusretki, jolla pyritään löytämään menetelmiä ennustamaan sellaisten femtosekuntilasersykäysten aikaprofiileja, joilla aiemmin mainittuja ilmiöitä – HHG:ta, elektroniemissiota sekä Rydbergtilojen virityksiä – voidaan tehostaa, optimoida. Väitöskirjan alussa esitellään työn kannalta oleelliset vahvojen kenttien attosekuntiluokan ilmiöt keskittyen etenkin niiden teoreettisiin ja laskennallisiin malleihin. Tutkielmassa annetaan myös yleiskatsaus femtosekuntisykäysten käytöstä atomifysiikan ilmiöiden ohjaamisessa ja optimoinnissa avaten sekä alan kokeellista että laskennallista puolta. Tutkimuksessamme käytetyt laskennalliset mallit käydään yksityiskohtaisesti läpi, ja väitöskirjan oheismateriaali (saatavilla internetistä) sisältää oleellisimmat työssä käytetyt ohjelmistot ja analyysityökalut. Tutkimusten tuloksina on löydetty menetelmiä femtosekuntilasersykäysten suunnittelua varten. Näillä menetelmillä saadaan kasvatettua sekä HHG:n että korkeaenergisen elektroniemission hyötysuhdetta ja maksimienergiaa. Työssä tutkittiin myös femtosekuntilasersykäysten käyttöä alkalimetalliatomien virittämiseksi kvanttilaskentaa varten. Optimointimenetelmämme ja femtosekuntilasersykäysten käyttö vähentää viritykseen käytettävää aikaa huomattavasti perinteisiin tekniikoihin verrattuina, mutta nykyisessä muodossaan menetelmä ei ole tarpeeksi tarkka, jotta sillä voitaisiin miehittää vain yksi tietty tila. Väitöskirjassa kehitetään myös uusi elementtimenetelmään pohjautuva laskentaohjelmisto, joka on suunniteltu nanorakenteiden attosekunti-ilmiöiden mallinnukseen. Nanorakenteet muuttavat niihin kohdistetun femtosekuntilasersykäyksen paikkariippuvuutta, mitä useimmat aiemmat mallinnusohjelmistot eivät kykene huomioimaan. Kehittämämme ohjelmisto mallintaa näitä tilanteita tehokkaasti ja ottaa huomioon femtosekuntilasersykäysten epähomogeenisen paikkariippuvuuden. Väitöskirjan lopussa on yhteenveto löydöksistämme, joita käsitellään suhteessa muihin alan tuoreisiin tutkimuksiin. Pohdimme myös mahdollisia kehityskohteita sekä suuntaa tuleville tutkimuksille. Attosecond science deals with monitoring and control of electron dynamics in their native, attosecond time scale. Ultrafast electron dynamics is the driving force behind chemical reactions, it determines the optical response of matter, and it is the cornerstone of multiple ultrafast nanoscale imaging techniques. Attosecond phenomena are often driven by strong-field light-matter interaction. Femtosecond laser pulses with electric fields rivaling those of atomic binding forces drive complex nonlinear phenomena in atoms, molecules, and solid state. They include electron excitations, nonlinear frequency up-conversion known as high-order harmonic generation (HHG), and emission of ultra-energetic electrons via above-threshold ionization (ATI). These processes have important roles in ultrafast technologies. For example, HHG is used as a source for coherent X-ray pulses with durations down to attoseconds, ATI is used for building electron wave packets for self-interrogation spectroscopy of matter, and excited Rydberg-states of atoms are prime candidates for multi-qubit quantum computing. Control of strong-field attosecond phenomena can be achieved by shaping the temporal profile of the driving femtosecond pulse in modern light-field synthesizers. This dissertation is a computational expedition to shaping the driving laser pulses for optimizing strong-field light-matter interaction in HHG, ATI, and Rydberg-state preparation in atoms. We begin this dissertation with a brief reviewof relevant strong-field attosecond phenomena with an emphasis on their theoretical modeling. We continue with an overview of control and optimization of these phenomena both from an experimental and a computational point of view. Later, we describe in detail the computational models we have used. The corresponding software is provided in the online supplementary material. Our optimization studies deliver experimentally feasible optimization/control schemes for shaping the driving femtosecond laser pulses to increase the maximum energy and signal strength of HHG and ATI in atomic gases. We also demonstrate how the optimized processes behind the optimized HHG and ATI can be understood with a semiclassical three-step model. The excitation of alkali metals to their Rydberg states is shown to be feasible with multicolor femtosecond fields, decreasing the excitation time by several orders of magnitude compared to traditional methods. On the downside, in its current form the proposed scheme lacks the finesse to populate only a single final state. We also develop a new finite element simulation suite for studying attosecond phenomena in nanostructures. Nanostructures shape the spatial profile of the driving laser field, something existing simulation software cannot easily model. Our software suite is designed for simulating these systems efficiently, and it can incorporate the spatial inhomogeneity of the driving field with ease. We close this dissertation with a summary of our optimization studies and obtained results. They are discussed in the context of other recent work in the field, and we also reflect on possible improvements and directions for future work.

Die Entdeckung und Erkl��rung des photoelektrischen Effekts war einer der ersten Beweise der Lichtquantenhypothese. Dar��ber hinaus stellt die Absorption eines hochenergetischen Photons, und die darauf folgende Emission eines Elektrons, bis heute einen fundamentalen Baustein der Wechselwirkung von Licht mit Materie dar. Obwohl Photoionisation schon sehr genau untersucht worden ist, konnte seit der Jahrtausendwende mit Hilfe der Attosekundenphysik ein noch tiefergehendes Verst��ndnis des photoelektrischen Effekts erzielt werden. So gelang es, unter anderem, die Zeit zu messen, die ein Elektron ben��tigt, um das Atom zu verlassen, nachdem es ein Photon absorbiert hat. Diese charakteristische Zeit ist in der komplexen Phase der quantenmechanische Wellenfunktion des Elektrons eingepr��gt. Eine pr��zise Messung der Elektronenwellenfunktion nach der Ionisation, wie durch die Attosekundenphysik erm��glicht, erlaubt es direkte R��ckschl��sse auf die zugrundeliegenden physikalischen Prozesse zu ziehen. Die quantenmechanische Phase der Wellenfunktion ist allerdings nicht direkt experimentell bestimmbar, sondern kann nur mit Hilfe von aufwendigen Messprotokollen gemessen werden. Zusammen mit theoretischen Rechnungen und numerischen Simulationen, ist es jedoch oft m��glich die korrelierte Elektronendynamik zu rekonstruieren. Die Untersuchung von einfachen atomaren Systemen, f��r die numerisch pr��zise Simulationen m��glich sind, bilden die Grundlage unseres Verst��ndnisses der Wechselwirkung von ultrakurzen und starken Laserpulsen mit Materie. Wir untersuchen, teilweise in Kooperation mit experimentellen Arbeitsgruppen, unterschiedliche Photoionisationsmessprotokolle zur Charakterisierung von Photoionisation mit Hilfe von hochpr��zisen ab initio Simulationen. Der Gro��teil der untersuchten Protokolle verwendet die Winkelverteilung der emittierten Elektronen als Observable zur Charakterisierung prototypischer Ionisierungsprozesse. Im einfachsten Fall f��hrt die Absorption eines hochenergetischen Photons zur Emission eines einzelnen Elektrons. Um die quantenmechanische Phase der ionsierten Elektronen zu bestimmen, wurden Messprotokolle entwickelt, bei denen das bereits ionisierte Elektron ein weiteres Photon absorbiert. Eben dieser Kontinuum-Kontinuum ��bergang bewirkt eine zus��tzliche Phasenverschiebung der Wellenfunktion, welche mit Hilfe des winkelaufgel��sten Elektronenspektrums bestimmt werden kann. Neben der Anregung eines Einteilchenkontinuum-Zustandes kann die Absorption eines Photons auch die Anregung einer sogenannten Fano-Resonanz bewirken. Wir zeigen im Detail wie ultrakurze Laserpulse verwendet werden k��nnen, um die Streuphase in der N��he einer solchen Resonanz zu bestimmen, und wie der zeitliche Aufbau der charakteristischen Absorptionslinie unter Einsatz von kurzen, aber starken, Laserpulsen verfolgt werden kann. Der Einfluss der Elektron-Elektron Wechselwirkung kann genau untersucht werden f��r den Fall von Doppelionisation mit ultrakurzen Laserpulsen. Da in diesem Fall beide Elektronen das Atom nahezu gleichzeitig verlassen, enth��llt eine Messung der Winkelverteilung beider Elektronen den Einflu�� der interelektronischen Coulomb-Wechselwirkung auf diesen Prozess. Um den Einfluss der Elektron-Elektron-Wechselwirkung auf den Doppelionisationsprozess zu quantifizieren, untersuchen wir den prototypischen Fall von Helium Doppelionisation mit zirkular polarisierten Laserfeldern., Photoionization, i.e. Einstein's photoelectric effect, often is one key ingredient to a broad variety of physical phenomena. Complementary to previous methods, attosecond metrology has provided the means to measure the time associated with the photoelectric effect. Neither is photoionization instantaneous nor is there a universal time associated with this process. Rather the exact emission time depends on the energy of the liberated electron and the details of the system it is leaving. The observed photoionization time delay (or advance) is imprinted on the quantum mechanical wave function as a variation of the phase shift. Being able to retrieve this scattering phase constitutes one major opportunity provided by attosecond metrology. Since the acquired quantum mechanical phase shift depends on the ionized system the measurement of photoionization phases can be used to probe and study matter. Even though the quantum mechanical phase carries a large amount of information it is, unfortunately, not directly accessible in experiment. Often, intricate detection schemes are necessary to retrieve phase information. Furthermore, in many cases theoretical calculations and analytical models are additionally needed to fully reconstruct the electronic dynamics from the measured phase information. Thus, theoretical calculations and experiments on simple atomic systems serve as backbone of our understanding and provide the playing ground for the exploration of more complex phenomena in, e.g., solids and topological materials. Within this thesis we numerically explore, in close collaboration with experimental groups, various measurement setups to investigate photoionization of atoms by ultrashort laser pulses using highly accurate ab initio simulations. We will mainly focus on the phases which can be obtained from the measurement of angular resolved electron spectra (angular distributions) for three prototypical ionization regimes and highlight which information can be gained by the different techniques. The simplest scenario we investigate is single ionization by high energetic ultrashort laser pulses. The phase acquired within this process can be determined with measurement protocols that require at least one additional photon absorption by the quasi-free electron after absorption of the high-energy photon. Within this thesis we will accurately determine the phase acquired within this free-free transition. As a second scenario, we investigate the quantum mechanical scattering phase near atomic Fano resonances, whose existence is a direct consequence of electron-electron correlation. We will determine in detail how this phase can be measured using attosecond pulses. Furthermore, we will show how strong few-cycle light fields can be used to projectively measure ultrafast electronic processes such as the build-up of the asymmetric Fano line shape. As a further application, we will show that angular distributions serve as ideal candidates to pin down electronic correlations for double ionization of atomic helium by circularly polarized light fields.

During multi-photon ionization of an atom it is well understood how the involved photons transfer their energy to the ion and the photoelectron. However, the transfer of the photon linear momentum is still not fully understood. Here, we present a time-resolved measurement of linear momentum transfer along the laser pulse propagation direction. We can show that the linear momentum transfer to the photoelectron depends on the ionization time within the laser cycle using the attoclock technique. We can mostly explain the measured linear momentum transfer within a classical model for a free electron in a laser field. However, corrections are required due to the parent-ion interaction and due to the initial momentum when the electron enters the continuum. The parent-ion interaction induces a negative attosecond time delay between the appearance in the continuum of the electron with minimal linear momentum transfer and the point in time with maximum ionization rate., Nature Communications, 10 (1), ISSN:2041-1723

The generation of attosecond pulse trains at free-electron lasers opens new opportunities in ultrafast science, as it gives access, for the first time, to reproducible, programmable, extreme ultraviolet (XUV) waveforms with high intensity. In this work, we present a detailed analysis of the theoretical model underlying the temporal characterization of the attosecond pulse trains recently generated at the free-electron laser FERMI. In particular, the validity of the approximations used for the correlated analysis of the photoelectron spectra generated in the two-color photoionization experiments are thoroughly discussed. The ranges of validity of the assumptions, in connection with the main experimental parameters, are derived.

A recent experiment [Villeneuve et al. Science 356, 1150 (2017)] has shown that two-color photoionization of neon by the combination of an attosecond XUV pulse train and a moderately strong, linearly polarized IR pulse can preferentially produce photoelectrons with orbital angular quantum number l = 3 (f-wave) and magnetic quantum number m = 0. This result was rationalized by the occurrence of different Stark shifts of m = 0 and |m| = 1 sub-levels in the IR laser field. Here we perform 3D time-dependent Schrodinger equation calculations with a neon effective potential to identify the mechanism for the selective excitation and ionization of m = 0 sub-levels. Calculations of the ionization and excitation yields as a function of the IR intensity and the XUV and IR photon energy reveal that a coupling between two dominant ionization channels involving 3p and 3d intermediate excitations is responsible for the observed m-level selectivity. We compare calculated and measured photoelectron velocity map images and ionization yields over a range of IR intensities and XUV and IR photon energies, and confirm that the m = 0 or |m| = 1 channel, and thus a single set of the quantum numbers, (Jion, l, m), can be selected by an appropriate choice of these parameters.

We propose a gauge-invariant formulation of the channel orbital-based time-dependent configuration interaction singles (TDCIS) method [Phys. Rev. A 74, 043420 (2006)], one of the powerful ab initio methods to investigate electron dynamics in atoms and molecules subject to an external laser field. In the present formulation, we derive the equations of motion (EOMs) in the velocity gauge using gauge-transformed orbitals, not fixed orbitals, that are equivalent to the conventional EOMs in the length gauge using fixed orbitals. The new velocity-gauge EOMs avoid the use of the length-gauge dipole operator, which diverges at large distance, and allows to exploit computational advantages of the velocity-gauge treatment over the length-gauge one, e.g, a faster convergence in simulations with intense and long-wavelength lasers, and the feasibility of exterior complex scaling as an absorbing boundary. The reformulated TDCIS method is applied to an exactly solvable model of one-dimensional helium atom in an intense laser field to numerically demonstrate the gauge invariance. We also discuss the consistent method for evaluating the time derivative of an observable, relevant e.g, in simulating high-harmonic generation., 8 pages, 4 figures

Recent experiments on measuring time-delays during tunnelling of cold atoms through an optically created potential barrier are reinvigorating the controversial debate regarding possible time-delays during light-induced tunnelling of an electron from an atom. Compelling theoretical and experimental arguments have been put forward to advocate opposite views, confirming or refuting the existence of finite tunnelling time delays. Yet, such a delay, whether present or not, is but a single quantity characterizing the tunnelling wavepacket; the underlying dynamics are richer. Here we propose to augment photo-electron detection in laser-induced tunnelling with detection of light emitted by the tunnelling electron -- the so-called Brunel radiation. Using a combination of single-color and two-color driving fields, we identify the all-optical signatures of the re-shaping of the tunnelling wavepacket as it emerges from the tunnelling barrier and moves away from the core. This reshaping includes not only an effective time-delay but also time-reversal asymmetry of the ionization process, which we describe theoretically and observe experimentally. We show how both delay and reshaping are mapped on the polarization properties of the Brunel radiation, with different harmonics behaving as different hands of a clock moving at different speeds. The all-optical detection paves the way to time-resolving optical tunnelling in condensed matter systems, e.g. tunnelling across bandgaps in solids, on the attosecond time-scale.

Attosecond pump-probe experiments performed in small molecules have allowed tracking charge dynamics in the natural time scale of electron motion. That this is also possible in biologically relevant molecules is still a matter of debate, because the large number of available nuclear degrees of freedom might destroy the coherent charge dynamics induced by the attosecond pulse. Here we investigate extreme ultraviolet-induced charge dynamics in the amino acid tryptophan. We find that, although nuclear motion and nonadiabatic effects introduce some decoherence in the moving electron wave packet, these do not significantly modify the coherence induced by the attosecond pulse during the early stages of the dynamics, at least for molecules in their equilibrium geometry. Our conclusions are based on elaborate theoretical calculations and the experimental observation of sub-4 fs dynamics, which can only be reasonably assigned to electronic motion. Hence, attosecond pump-probe spectroscopy appears as a promising approach to induce and image charge dynamics in complex molecules.

We demonstrate amplitude and carrier-envelope phase stabilization of a high energy IR optical parametric amplifier designed for Attosecond Science exploiting two high speed adaptive optical systems based on multi actuator adaptive lenses.

We generate angularly isolated beams of circularly polarized extreme ultraviolet light through the first implementation of non-collinear high harmonic generation with circularly polarized driving lasers. This non-collinear technique offers numerous advantages over previous methods, including the generation of higher photon energies, the separation of the harmonics from the pump beam, the production of both left and right circularly polarized harmonics at the same wavelength and the capability of separating the harmonics without using a spectrometer. To confirm the circular polarization of the beams and to demonstrate the practicality of this new light source, we measure the magnetic circular dichroism of a 20 nm iron film. Furthermore, we explain the mechanisms of non-collinear high harmonic generation using analytical descriptions in both the photon and wave models. Advanced numerical simulations indicate that this non-collinear mixing enables the generation of isolated attosecond pulses with circular polarization., This work was completed at JILA. D.H., J.E., T.F., K.D., H.C. and M.M. acknowledge support from the Department of Energy BES Award DE-FG02-99ER14982. M.M., H.K. and C.D. acknowledge support from the National Science Foundation’s Engineering Research Centre in Extreme Ultraviolet Science and Technology. C.D. acknowledges support from the Air Force Office of Scientific Research under MURI grant FA9550-10-1-0561. J.E. acknowledges support from the National Science Foundation Graduate Research Fellowship (DGE-1144083). C.H.-G. acknowledges support from a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–2013), under REA grant agreement no. 328334. C. H.-G. acknowledges support from Junta de Castilla y León (Project SA116U13) and MINECO (FIS2013-44174-P). A.J.-B. was supported by grants from the National Science Foundation (grants nos. PHY-1125844 and PHY-1068706). This work made use of the Janus supercomputer, which is supported by the National Science Foundation (award no. CNS-0821794) and the University of Colorado, Boulder. P.G. acknowledges support from the Deutsche Forschungsgemeinschaft (grant no. GR 4234/1–1). R.K. acknowledges the Swedish Research Council (VR) for financial support. A.B. acknowledges support from the Department of Energy, Office of Basic Sciences.

International audience; We propose a method for obtaining effective lifetimes of scattering electronic states for avoiding the artificially confinement of the wave function due to the use of incomplete basis sets in time-dependent electronic-structure calculations of atoms and molecules. In this method, using a fitting procedure, the lifetimes are extracted from the spatial asymptotic decay of the approximate scattering wave functions obtained with a given basis set. The method is based on a rigorous analysis of the complex-energy solutions of the Schrödinger equation. It gives lifetimes adapted to any given basis set without using any empirical parameters. The method can be considered as an ab initio version of the heuristic lifetime model of Klinkusch et al. [J. Chem. Phys. 131, 114304 (2009)]. The method is validated on the H and He atoms using Gaussian-type basis sets for calculation of high-harmonic-generation spectra.

Over the last few decades, the investigation of ultrafast phenomena occurring in atoms, molecules and solid-state systems under a strong-field regime of light-matter interaction has attracted great attention. The increasing request for a suitable optical technology is significantly boosting the development of powerful ultrafast laser sources. In this framework, Optical Parametric Amplification (OPA) is currently becoming a leading solution for applications in high-power ultra-broadband light burst generation. The main advantage provided by the OPA scheme consists of the possibility of exploring spectral ranges that are inaccessible by other laser technologies, as the InfraRed (IR) window. In this paper, we will give an overview on recent progress in the development of high-power few-optical-cycle parametric amplifiers in the near-IR and in the mid-IR spectral domain. In particular, the design of the most advanced OPA implementations is provided, containing a discussion on the key technical aspects. In addition, a review on their application to the study of strong-field ultrafast physical processes is reported.

Since the birth of quantum mechanics, scientists have been dreaming of imaging, and possibly controlling, the motion of electrons in atoms and molecules. These electrons are described in the quantum theory by wave functions. However, the wave function is an elusive quantity: It is a complex function with an amplitude and phase, but its square modulus is usually the only quantity directly measurable in experiments. In addition, visualizing the electron dynamics requires extremely high temporal resolution, on the order of attoseconds (1 as = 10−18 s). On page 1150 of this issue, Villeneuve et al. ( 1 ) report on an elegant experiment that demonstrates the possibility of fully reconstructing the electron wave function in the photoionization of neon and thus paving the way for coherent control of the process.

We report on the first streaking measurement of water-window attosecond pulses generated via high harmonic generation, driven by sub-2-cycle, CEP-stable, 1850 nm laser pulses. Both the central photon energy and the energy bandwidth far exceed what has been demonstrated thus far, warranting the investigation of the attosecond streaking technique for the soft X-ray regime and the limits of the FROGCRAB retrieval algorithm under such conditions. We also discuss the problem of attochirp compensation and issues regarding much lower photo-ionization cross sections compared with the XUV in addition to the fact that several shells of target gases are accessed simultaneously. Based on our investigation, we caution that the vastly different conditions in the soft X-ray regime warrant a diligent examination of the fidelity of the measurement and the retrieval procedure., Comment: 14 Pages, 12 figures

Results illustrating the nonlinear dynamics of ultrashort laser pulse filamentation in gases are presented, with particular emphasis on the filament properties useful for developing attosecond light sources. Two aspects of ultrashort pulse filaments are specifically discussed: (i) numerical simulation results on pulse self-compression by filamentation in a gas cell filled with noble gas. Measurements of high harmonics generated by the pulse extracted from the filament allows for the detection of intensity spikes and subcycle pulses generated within the filament. (ii) Simulation results on the spontaneous formation of conical wavepackets during filamentation in gases, which in turn can be used as efficient driving pulses for the generation of high harmonics and isolated attosecond pulses.

Attosecond Pulse Trains (APT) generated by high-harmonic generation (HHG) of high-intensity near-infrared (IR) laser pulses have proven valuable for studying the electronic dynamics of atomic and molecular species. However, the high intensities required for high-photon-energy, high-flux HHG usually limit the class of adequate laser systems to repetition rates below 10 kHz. Here, APT's generated from the 100 kHz, 160 W, 40 fs laser system (HR-1) currently under commissioning at the extreme light infrastructure attosecond light pulse source (ELI-ALPS) are reconstructed using the reconstruction of attosecond beating by interference of two-photon Transitions (RABBIT) technique. These experiments constitute the first attosecond time-resolved photoelectron spectroscopy measurements with attosecond pulses performed at 100 kHz repetition rate and one of the first experiments performed at ELI-ALPS in the framework of projects commissioning its newly installed technologies. These RABBIT measurements were taken with an additional IR field temporally locked to the extreme-ultraviolet APT, resulting in an atypical ω beating. We show that the phase of the 2ω beating recorded under these conditions is strictly identical to that observed in standard RABBIT measurements within second-order perturbation theory. This work highlights an experimental simplification for future experiments based on attosecond interferometry (or RABBIT), which is particularly useful when lasers with high average powers are used., Journal of Physics B: Atomic, Molecular and Optical Physics, 52 (23), ISSN:1361-6455, ISSN:0368-3508, ISSN:0953-4075, ISSN:0022-3700

Chemical Physics, 414, ISSN:0301-0104, ISSN:1873-4421

Spectroscopic measurements with increasingly higher time resolution are generally thought to require increasingly shorter laser pulses, as illustrated by the recent monitoring of the decay of core-excited krypton1 using attosecond photon pulses2, 3. However, an alternative approach to probing ultrafast dynamic processes might be provided by entanglement, which has improved the precision4, 5 of quantum optical measurements. Here we use this approach to observe the motion of a D2+ vibrational wave packet formed during the multiphoton ionization of D2 over several femtoseconds with a precision of about 200 attoseconds and 0.05 ångströms, by exploiting the correlation between the electronic and nuclear wave packets formed during the ionization event. An intense infrared laser field drives the electron wave packet, and electron recollision6, 7, 8, 9, 10, 11 probes the nuclear motion. Our results show that laser pulse duration need not limit the time resolution of a spectroscopic measurement, provided the process studied involves the formation of correlated wave packets, one of which can be controlled; spatial resolution is likewise not limited to the focal spot size or laser wavelength.

Brassinosteroids (BRs) are plant steroid hormones that control many aspects of plant growth and development. BRs are perceived at the cell-surface by the plasma membrane-localized receptor complex composed of the receptor kinase BRI1 and its co-receptor BAK1. Here we show that BRI1 is post-translationally modified by K63 polyubiquitin chains in vivo. Artificially ubiquitinated BRI1 is recognized at the trans-Golgi Network/Early Endosomes (TGN/EE) and rapidly routed for vacuolar degradation. Mass spectrometry analyses identified residue K866 as an in vivo ubiquitination target in BRI1 involved in the negative regulation of BRI1. Model prediction revealed several redundant ubiquitination sites required for the endosomal sorting and vacuolar targeting of BRI1. Using total internal reflection fluorescence microscopy (TIRF), we also uncovered a role for BRI1 ubiquitination in promoting internalization from the cell-surface. Finally, we demonstrate that the control of BRI1 protein dynamics by ubiquitination is a fundamental control mechanism for BR responses in plants. Altogether, our results identify K63-linked polyubiquitin chain formation as a dual targeting signal for BRI1 internalization and sorting along the endocytic pathway, and highlight its role in hormonally controlled plant development.

An intense light pulse can extract an electron wave packet from an atom near a field crest. This wave packet responds to the field, gaining and losing momentum as the field oscillates. During the process portions of the wave packet are inevitably driven back to the ion, and recombining with substantial momentum and thereby transferring the wave packet characteristics into a broad bandwidth burst of radiation that can extend continuously from the ultraviolet to the soft X-rays. Many characteristics of attosecond pulse are imposed by the time-dependent-field of a light pulse. By controlling the time-dependent-field, we control attosecond pulse generation. © 2013 IEEE., 2013 Conference on Lasers and Electro-Optics Europe and International Quantum Electronics Conference, CLEO/Europe-IQEC 2013, 12 May 2013 through 16 May 2013, Munich

Angstrom and attosecond control of free electron wave packets is one of the pinnacles of attosecond science. Orthogonally polarized two-color (OTC) laser fields allow to control the motion of field-ionizing electronic wave packets both in time and space[1]. In OTC pulses time and space are connected and thus an attosecond time scale is established in the polarization plane for both the emitted and the re-colliding wave packets[2,3]. © 2013 IEEE., 2013 Conference on Lasers and Electro-Optics Europe and International Quantum Electronics Conference, CLEO/Europe-IQEC 2013, 12 May 2013 through 16 May 2013, Munich

At the heart of high harmonic generation lies a combination of optical and collision physics entwined by a strong laser field. An electron, initially tunnel-ionized by the field, driven away then back in the continuum, finally recombines back to rest in its initial ground state via a radiative transition. The emitted attosecond (atto=10^-18) XUV light pulse carries all the information (polarization, amplitude and phase) about the photorecombination continuum-to-ground transition dipolar field. Photorecombination is related to the time-reversed photoionization process. In this perspective, high-harmonic spectroscopy extends well-established photoelectron spectroscopy, based on charged particle detection, to a fully coherent one, based on light characterization. The main achievement presented in this thesis is to use high harmonic generation to probe femtosecond (femto=10^-15) chemical dynamics for the first time. Thanks to the coherence imposed by the strong driving laser field, homodyne detection of attosecond pulses from excited molecules undergoing dynamics is achieved, the signal from unexcited molecules acting as the reference local oscillator. First, applying time-resolved high-harmonic spectroscopy to the photodissociation of a diatomic molecule, Br2 to Br + Br, allows us to follow the break of a chemical bond occurring in a few hundreds of femtoseconds. Second, extending it to a triatomic (NO2) lets us observe both the previously unseen (but predicted) early femtosecond conical intersection dynamics followed by the late picosecond statistical photodissociation taking place in the reaction NO2 to NO + O. Another important realization of this thesis is the development of a complementary technique to time-resolved high-harmonic spectroscopy called LAPIN, for Linked Attosecond Phase INterferometry. When combined together, time-resolved high-harmonic spectroscopy and LAPIN give access to the complex photorecombination dipole of aligned excited molecules. These achievements lay the basis for electron recollision tomographic imaging of a chemical reaction with unprecedented angstrom (1 angstrom= 0.1 nanometer) spatial resolution. Other contributions dedicated to the development of attosecond science and the generalization of high-harmonic spectroscopy as a novel, fully coherent molecular spectroscopy will also be presented in this thesis.

Ultrafast extreme-ultraviolet pulses are spectrally selected by a time-delay-compensated grating monochromator. The intrinsic very short duration of the pulses is obtained by exploiting the high-order harmonic generation process. The temporal characterization of the harmonic pulses is obtained using a cross-correlation method: pulses as short as 8 fs are measured at the output of the monochromator in the case of the 23rd harmonic. This value is in agreement with the expected duration of such pulses, indicating that the influence of the monochromator is negligible. The photon flux has been measured with a calibrated photodiode, pointing out the good efficiency of the monachromator, derived by the exploitation for the two gratings of the conical diffraction mounting.

All aspects of attosecond technology rely on electron wavepackets formed by ionization and controlled by strong laser fields. When the electron wavepacket is formed by tunnel ionization in linearly polarized light, attosecond electron or optical pulses can be produced, both of which will play significant rolls in attosecond spectroscopy. When the electron wavepacket is formed by an attosecond x-ray pulse, the x-ray pulse can be fully characterized by using a strong laser field. If an atomic, molecular or nuclear dynamic processes form correlated wavepackets, the decay dynamics can be measured with attosecond precision., XXIII International Conference on Photonic, Electronic, and Atomic Collisions, 23rd ICPEAC, July 23-29, 2003, Stockholm