Search

Self-amplified spontaneous-emission (SASE) free-electron lasers (FELs) deliver ultrashort pulses with femtosecond durations. Due to the fluctuating nature of the radiation properties of SASE FELs, characterizing FEL pulses on a single-shot basis is necessary. Therefore, we use terahertz streaking to characterize the temporal properties of ultrashort extreme ultraviolet pulses from the free-electron laser in Hamburg (FLASH). In this study, pulse duration as well as pulse energy are measured in a wavelength range from 8 to 34 nm as functions of undulators contributing to the lasing process. The results are compared to one-dimensional and three-dimensional, time-dependent FEL simulations.

Journal of physics / B 53(18), 184004 (2020). doi:10.1088/1361-6455/ab9c38, Ultra-short extreme ultraviolet pulses from the free-electron laser FLASH are characterized using terahertz-field driven streaking. Measurements at different ultra-short extreme ultraviolet wavelengths and pulse durations as well as numerical simulations were performed to explore the application range and accuracy of the method. For the simulation of streaking, a standard classical approach is used which is compared to quantum mechanical theory, based on strong field approximation. Various factors limiting the temporal resolution of the presented terahertz streaking setup are investigated and discussed. Special attention is paid to the cases of very short (∼10 fs) and long (up to∼350 fs) pulses., Published by IOP Publ., Bristol

The possibility to produce ultrashort light pulses with pulse durations in the femtosecond and attosecond time scale (currently even down to a pulse length of 43as [1]) has triggered a worldwide eort of research on time-resolved measurements of electronic dynamics [2, 3, 4] in atoms, molecules [5] and, since very recently, even in solids [6]. For molecules, restructuring of the electronic system induces nuclear dynamics such as the breakage of a chemical bond, which is the primary process of any chemical reaction. In order to study processes like photoionization or photodissociation, measurement techniques needed to be developed, giving us more insight into those processes. One such technique is Photoelectron-Photoion Coincidence Spectroscopy (PEPICO) where the energies of electrons and the mass-to-charge ratio of ions emerging from the interaction of a molecule with an ultrashort light pulse are measured in coincidence. This approach adopted in a time-resolved manner to a pump-probe scheme has already shown great results like direct observation of photochemical activation energy in acetone [7], discovering electronic shake-up in molecular fragmentation processes [8] or the detection of Feshbach resonances [9]. The motivation for this diploma thesis is the eort to implement and test the possibility for PEPICO measurements in an attosecond XUV beamline equipped with a single time-of- ight mass spectrometer. The implementation was enabled by developing a fast high-voltage electrical switch, which permits switching from 0 to 2kV within a sub-100 nanosecond duration synchronously with the kHz repetition rate of the laser pulse used for PEPICO measurements, and by developing a suitable software for triggering, data read-out and data visualization during the experiment.

International audience; The emission of high-order harmonics in the extreme ultraviolet range from the interaction of a short, intense laser pulse with a grating target is investigated experimentally. When resonantly exciting a surface plasmon, both the intensity and the highest order observed for the harmonic emission along the grating surface increase with respect to a flat target. Harmonics are obtained when a suitable density gradient is preformed at the target surface, demonstrating the possibility to manipulate the grating profile on a nanometric scale without preventing the surface plasmon excitation. In support of this, the harmonic emission is spatiotemporally correlated to the acceleration of multi-MeV electron bunches along the grating surface. Particle-in-cell simulations reproduce the experimental results and give insight on the mechanism of high harmonic generation in the presence of surface plasmons.

Electron correlation in a molecule is one of the main difficulties of the N-bodies problem. One mean to enhance multielectronic effects is to use extreme ultraviolet light (XUV) in order to ionize inner-valence electrons of complex polyatomic systems. Thus, the produced cationic states result from a higher order photo-excitation processes (such as “2-hole, 1particle”) and their dynamics lead to considerations out of the frame of the Born-Oppenheimer approximation. Recent developments in ultrafast science concerning the XUV ultrashort pulses sources, produced by high harmonic generation (HHG), allow studying these mechanisms from the hundreds of femtoseconds (1 fs = 10-15 s) timescale up to the attosecond (1 as = 10-18 s) timescale.During this thesis I have firstly studied the static response of carboneous and biological molecules to a multi-photonic infrared (IR) femtosecond excitation thanks to a velocity map imaging spectrometer (VMIS). Then, through a multi-scale approach, I have investigated, in these complex systems, the dynamics induced by XUV femtosecond and attosecond pulses. I have especially studied, in Polycyclic Aromatic Hydrocarbons (PAHs), the evolution of highly excited cationic states and the effect of the molecular potential during the photoionization process, thanks to a XUV-pump IR-probe spectroscopy scheme coupled to a VMIS. Finally, I have examined the role of the ultrafast charge dynamics induced by XUV photo-ionization on fragmentation mechanisms in the caffeine biomolecule. The observed processes are entire part of a multi-scale approach of the ultrafast molecular physics and allow a better understanding of the implication of multielectronic effects and non-adiabatic couplings in complex polyatomic systems; La corrélation électronique dans une molécule est une des difficultés principales du problème à N corps. Un moyen d'exalter des effets multiélectroniques est l'utilisation de rayonnements de l'extrême ultra-violet (UVX) pour photo-ioniser des électrons de valences internes de systèmes poly-atomiques complexes. Les états cationiques ainsi créés résultent d'excitations d'ordre supérieur (de type « 2-hole 1-particle ») et leur dynamiques subséquentes mènent à des considérations en dehors du cadre de l'approximation de Born-Oppenheimer. Les développements récents en matière de sources d'impulsions UVX ultracourtes, notamment produites par génération d'harmoniques d'ordres élevés (HHG), permettent d'étudier ces mécanismes sur des échelles de temps de temps allant de quelques centaines de femtoseconde (1 fs = 10-15 s) jusqu'à l'attoseconde (1 as = 10-18 s).Lors de cette thèse, j'ai premièrement étudié la réponse statique de molécules carbonées ou biologiques à une excitation femtoseconde infrarouge (IR) multi-photonique à l'aide d'un spectromètre imageant les vecteurs vitesses des photoélectrons (VMIS). Ensuite, à travers une approche multi-échelle, j'ai exploré, dans ces systèmes complexes, les dynamiques induites par impulsions femtosecondes et attosecondes UVX. En particulier, j'ai étudié, dans les Hydrocarbures Aromatiques Polycycliques (HAPs), l'évolution des états cationiques hautement excités ainsi que l'effet du potentiel moléculaire lors du processus de photo-ionisation, grâce à un schéma de spectroscopie UVX-pompe IR-sonde couplé à un VMIS. Enfin, j'ai examiné le rôle de la dynamique ultrarapide des charges induites par une photo-ionisation UVX en rapport avec la fragmentation de la biomolécule de caféine.Les processus observés s'intègrent à une approche multi-échelle de la physique moléculaire ultra-rapide et permettent de mieux saisir l'implication des effets multiélectroniques et des couplages non-adiabatiques dans les systèmes polyatomiques complexes

Electron correlation in a molecule is one of the main difficulties of the N-bodies problem. One mean to enhance multielectronic effects is to use extreme ultraviolet light (XUV) in order to ionize inner-valence electrons of complex polyatomic systems. Thus, the produced cationic states result from a higher order photo-excitation processes (such as “2-hole, 1particle”) and their dynamics lead to considerations out of the frame of the Born-Oppenheimer approximation. Recent developments in ultrafast science concerning the XUV ultrashort pulses sources, produced by high harmonic generation (HHG), allow studying these mechanisms from the hundreds of femtoseconds (1 fs = 10-15 s) timescale up to the attosecond (1 as = 10-18 s) timescale.During this thesis I have firstly studied the static response of carboneous and biological molecules to a multi-photonic infrared (IR) femtosecond excitation thanks to a velocity map imaging spectrometer (VMIS). Then, through a multi-scale approach, I have investigated, in these complex systems, the dynamics induced by XUV femtosecond and attosecond pulses. I have especially studied, in Polycyclic Aromatic Hydrocarbons (PAHs), the evolution of highly excited cationic states and the effect of the molecular potential during the photoionization process, thanks to a XUV-pump IR-probe spectroscopy scheme coupled to a VMIS. Finally, I have examined the role of the ultrafast charge dynamics induced by XUV photo-ionization on fragmentation mechanisms in the caffeine biomolecule. The observed processes are entire part of a multi-scale approach of the ultrafast molecular physics and allow a better understanding of the implication of multielectronic effects and non-adiabatic couplings in complex polyatomic systems, La corrélation électronique dans une molécule est une des difficultés principales du problème à N corps. Un moyen d'exalter des effets multiélectroniques est l'utilisation de rayonnements de l'extrême ultra-violet (UVX) pour photo-ioniser des électrons de valences internes de systèmes poly-atomiques complexes. Les états cationiques ainsi créés résultent d'excitations d'ordre supérieur (de type « 2-hole 1-particle ») et leur dynamiques subséquentes mènent à des considérations en dehors du cadre de l'approximation de Born-Oppenheimer. Les développements récents en matière de sources d'impulsions UVX ultracourtes, notamment produites par génération d'harmoniques d'ordres élevés (HHG), permettent d'étudier ces mécanismes sur des échelles de temps de temps allant de quelques centaines de femtoseconde (1 fs = 10-15 s) jusqu'à l'attoseconde (1 as = 10-18 s).Lors de cette thèse, j'ai premièrement étudié la réponse statique de molécules carbonées ou biologiques à une excitation femtoseconde infrarouge (IR) multi-photonique à l'aide d'un spectromètre imageant les vecteurs vitesses des photoélectrons (VMIS). Ensuite, à travers une approche multi-échelle, j'ai exploré, dans ces systèmes complexes, les dynamiques induites par impulsions femtosecondes et attosecondes UVX. En particulier, j'ai étudié, dans les Hydrocarbures Aromatiques Polycycliques (HAPs), l'évolution des états cationiques hautement excités ainsi que l'effet du potentiel moléculaire lors du processus de photo-ionisation, grâce à un schéma de spectroscopie UVX-pompe IR-sonde couplé à un VMIS. Enfin, j'ai examiné le rôle de la dynamique ultrarapide des charges induites par une photo-ionisation UVX en rapport avec la fragmentation de la biomolécule de caféine.Les processus observés s'intègrent à une approche multi-échelle de la physique moléculaire ultra-rapide et permettent de mieux saisir l'implication des effets multiélectroniques et des couplages non-adiabatiques dans les systèmes polyatomiques complexes