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Photoionization microscopy is a technique that allows obtaining a macroscopic image of an electronic wave function which is usually confined into an atom. The experiment consists in ionizing an atom just above the threshold in a static electric field. The classical study of the movement of an electron in a combined coulombian and electric field shows that a infinite number of trajectories lead to a single point. In a quantum point of view, the partial wave functions which follow these trajectories interfere, and this phenomena is visualized on a position sensitive detector. This thesis proposes a classical, semiclassical and quantum study of photoionization microscopy. Our simulations of wave packet propagation use a Slit-Operator algorithm, and we propose a systematic study of Hydrogen. We show that it is possible to visualize by photoionization microscopy resonant Stark state in the continuum of Hydrogen, where the dark fringes of the interferences are linked with the nodes of the wave function. Furthermore, we describe the effect of the electonic core of non hydrogenic atoms on the microscopy of these states., La microscopie de photoïonisation est une technique qui permet d'obtenir une image macroscopique de la fonction d'onde électronique habituellement confinée autour d'un atome. L'expérience consiste à photoïoniser près du seuil un atome en présence d'un champ électrique statique. Le traitement classique du mouvement d'un électron se déplaçant dans un champ coulombien en présence d'un champ électrique statique, montre qu'une multitude de trajectoires électroniques mènent à un point de l'espace classiquement accessible. Quantiquement, les ondes partielles associées à ces trajectoires interfèrent, et ce phénomène est directement mesurable au moyen d'un détecteur sensible en position. Cette thèse propose une étude classique, semi-classique et quantique de la microscopie de photoïonisation. L'originalité de ce travail est le développement de simulations par propagation de paquets d'ondes grâce un algorithme de type Split-Operator. Nous proposons une étude systématique de l'Hydrogène et montrons qu'il est possible d'observer à l'aide d'un microscope de photoïonisation, la fonction d'onde des états Stark résonants du continuum d'ionisation de l'Hydrogène. Enfin nous décrivons l'effet du cœur électronique sur la microscopie d'atomes non-hydrogénoïdes.

Several theoretical models are used to explain the origin of the recently observed (unexpected) spectral progression in the Coulomb explosion spectra of the hydrogen molecule photoionized by an intense ultrashort laser pulse. In the first ionization step the molecule loses its first electron and then the $\mathrm{H}_{2}{}^{+}$ molecular ion dissociates. Next, at the intermediate stage of the dissociation process, a localized electron state is created from which the second ionization occurs at each laser half-cycle. It is shown that interference between a net-two-photon and a one-photon transition introduces a dynamic structure into the nuclear wave packet corresponding to this localized electron state which leads to the regular spectral progressions seen in the experiment. We confirm these spectral progressions using numerical simulations based on a time-dependent Schr\"odinger equation describing the exact three-body dynamics of $\mathrm{H}_{2}{}^{+}$ in one dimension.

Im ersten Teil der Dissertation werden diverse Typen von Wasserstoff-Brücken, wie die O-H···O, die N-H···O und die N-H···Pi Brücke in isolierten Heterodimeren aus Indol, Phenol und Naphthol mit Wasser und Benzol, untersucht. Mit Mass Analyzed Threshold Ionization (MATI), einer kombinierten Technik aus Flugzeit-Massenspektrometrie und resonanter Mehr-Photonen-Ionisation (REMPI) im gekühlten Überschall-Molekularstrahl, gelang die Aufzeichnung von kationischen Spektren dieser Systeme und damit u.a. die präzise Ermittlung von intermolekularen Bindungsenergien. Die Auswertung der Ergebnisse an Naphthol wurde durch ab-initio Rechnungen unterstützt. Der zweite Teil der Arbeit behandelt den intramolekularen, dissoziativen Elektronentransfer (DET) in, für diesen Zweck maßgeschneiderten, Ferrocenderivaten in der Gasphase. Der DET erfolgt hierbei von einer Ferrocen-Einheit zu einer kovalent gebundenen, lokal photoionisierten Chromophor-Gruppe mit höherem Ionisationspotential. In the first part of the dissertation various types of hydrogen bonds, such as the O-H···O, the N-H···O and the N-H···Pi bridge in isolated heterodimeres of indole, phenol and naphthol with water and benzene, are analyzed. With Mass Analyzed Threshold Ionization (MATI), a combined technique of time-of-flight mass spectrometry and resonant multi-photon ionization (REMPI) in a cooled supersonic molecular jet, we successfully recorded cationic spectra of these systems and hence, among other things, precisely determined intermolecular binding energies. The analysis of the naphthol results was supported by ab-initio calculations. The second part of the dissertation deals with intramolecular, dissociative elektron-transfer (DET) in tailor-made ferrocene derivatives in the gas phase. Here, the DET takes place from a ferrocene unit to a covalent bound, locally photoionized chromophor unit with higher ionization potential.

A combined photoconductivity, absorption, and thermoluminescence study was performed to understand the absence of luminescence from Ce3+ in LaAlO3:Ce3+. It is demonstrated that the absence of luminescence is the result of Ce3+ ionization from the 5d excited states, which are all located in the conduction band. Ce3+ ionization is accompanied by the formation of several broad absorption and photoconductivity bands, which are assigned to electron traps. A time and temperature dependent optical investigation of these traps reveals the conditions under which electrons are transferred from Ce3+ to traps and vice versa, from traps back to Ce4+. The observed difference in energy needed to thermally or optically release electrons from traps is qualitatively explained in terms of the location of the ground and excited states of the electron traps with respect to the conduction band.

Two-color above threshold ionization of helium and xenon has been used to analyze the synchronization between individual pulses of the femtosecond extreme ultraviolet (XUV) free electron laser in Hamburg and an independent intense 120 fs mode-locked Ti:sapphire laser. Characteristic sidebands appear in the photoelectron spectra when the two pulses overlap spatially and temporally. The cross-correlation curve points to a 250 fs rms jitter between the two sources at the experiment. A more precise determination of the temporal fluctuation between the XUV and infrared pulses is obtained through the analysis of the single-shot sideband intensities.

This thesis deals with long-range interactions between cold Rydberg atoms. It describesresults of the first realization of the dipole blockade, characterized in an ensemble of atomsby a strong limitation of the excitation to Rydberg states, due to the shift of the energy levelslinked to the dipole-dipole interactions between Rydberg atoms. The importance of thedipole blockade has been underlined in quantum information with the possible realization oftwo qubits conditionnal quantum gates. The evidence for the dipole blockade is brought usingtwo different configurations: in the presence of a static electric field and using Förster resonances.Getting the dipole blockade requires a very good control of experimental conditions :compensation for stray electric fields, control of ionization sources, control of the intensity ofexcitation to stay below the power saturation threshold. Complementary experiments havebeen carried out, such as the study of dipolar forces between Rydberg atoms responsible forPenning ionization, which can be at the origin of the formation of an ultra-cold plasma. Anotherstudy concerns the excitation transfert at Föster resonance, whose coherence propertieshave been studied by an original spectroscopic technique close to spectral hole-burning.; Cette thèse traite des interactions à très longue portée entre atomes de Rydberg froids.Elle présente les résultats de la première réalisation du blocage dipolaire, qui se traduit parune limitation de l'excitation d'un ensemble d'atomes vers un état de Rydberg en raison dudéplacement des niveaux d'énergie lié aux interactions dipôle-dipôle entre atomes. L'importancede l'implication du blocage dipolaire a été soulignée pour l'information quantique avecla possible réalisation de portes quantiques conditionnelles à deux qubits. La mise en évidencedu blocage dipolaire est présentée dans le cadre de deux configurations expérimentales différentes: en présence d'un champ électrique et à résonance de Förster. Parvenir à la maîtrise dublocage dipolaire de l'excitation vers des états de Rydberg a demandé un contrôle importantdes conditions expérimentales : compensation des champs parasites, contrôle des sources d'ionisationd'atomes, contrôle de l'intensité laser d'excitation pour se placer en-dessous du seuilde saturation par puissance. Des études complémentaires ont été réalisées, telle que celle desforces dipolaires entre atomes de Rydberg responsables d'un phénomène d'ionisation Penning,aussi à l'origine de la formation d'un plasma ultra-froid. Une autre étude concerne le processusdu transfert d'excitation à résonance de Förster, dont les propriétés de cohérence ont étéanalysés par une technique spectroscopique originale dite de "creusement spectral".

This thesis deals with long-range interactions between cold Rydberg atoms. It describesresults of the first realization of the dipole blockade, characterized in an ensemble of atomsby a strong limitation of the excitation to Rydberg states, due to the shift of the energy levelslinked to the dipole-dipole interactions between Rydberg atoms. The importance of thedipole blockade has been underlined in quantum information with the possible realization oftwo qubits conditionnal quantum gates. The evidence for the dipole blockade is brought usingtwo different configurations: in the presence of a static electric field and using Förster resonances.Getting the dipole blockade requires a very good control of experimental conditions :compensation for stray electric fields, control of ionization sources, control of the intensity ofexcitation to stay below the power saturation threshold. Complementary experiments havebeen carried out, such as the study of dipolar forces between Rydberg atoms responsible forPenning ionization, which can be at the origin of the formation of an ultra-cold plasma. Anotherstudy concerns the excitation transfert at Föster resonance, whose coherence propertieshave been studied by an original spectroscopic technique close to spectral hole-burning.; Cette thèse traite des interactions à très longue portée entre atomes de Rydberg froids.Elle présente les résultats de la première réalisation du blocage dipolaire, qui se traduit parune limitation de l'excitation d'un ensemble d'atomes vers un état de Rydberg en raison dudéplacement des niveaux d'énergie lié aux interactions dipôle-dipôle entre atomes. L'importancede l'implication du blocage dipolaire a été soulignée pour l'information quantique avecla possible réalisation de portes quantiques conditionnelles à deux qubits. La mise en évidencedu blocage dipolaire est présentée dans le cadre de deux configurations expérimentales différentes: en présence d'un champ électrique et à résonance de Förster. Parvenir à la maîtrise dublocage dipolaire de l'excitation vers des états de Rydberg a demandé un contrôle importantdes conditions expérimentales : compensation des champs parasites, contrôle des sources d'ionisationd'atomes, contrôle de l'intensité laser d'excitation pour se placer en-dessous du seuilde saturation par puissance. Des études complémentaires ont été réalisées, telle que celle desforces dipolaires entre atomes de Rydberg responsables d'un phénomène d'ionisation Penning,aussi à l'origine de la formation d'un plasma ultra-froid. Une autre étude concerne le processusdu transfert d'excitation à résonance de Förster, dont les propriétés de cohérence ont étéanalysés par une technique spectroscopique originale dite de "creusement spectral".

The authors report time resolved photoelectron spectra of the 1B2 (1Σu+) state of CS2 at pump wavelengths in the region of 200 nm. In contrast to previous studies, the authors find that the predissociation dynamics is not well described by a single exponential decay. Biexponential modeling of the authors' data reveals a rapid decay pathway (tau

Streamer discharges play a central role in electric breakdown of matter in pulsed electric fields, both in nature and in technology. Reliable and fast computations of the minimal model for negative streamers in simple gases like nitrogen have recently been developed. However, photoionization was not included; it is important in air and poses a major numerical challenge. We here introduce a fast and reliable method to include photoionization into our numerical scheme with adaptice grids, and we discuss its importance for negative streamers. In particular, we identify different propagation regimes where photoionization does or does not play a role., 4 pages; 3 figures; Submitted to Applied Physics Letters

Streamer discharges play a central role in electric breakdown of matter in pulsed electric fields, both in nature and in technology. Reliable and fast computations of the minimal model for negative streamers in simple gases like nitrogen have recently been developed. However, photoionization was not included; it is important in air and poses a major numerical challenge. We here introduce a fast and reliable method to include photoionization into our numerical scheme with adaptice grids, and we discuss its importance for negative streamers. In particular, we identify different propagation regimes where photoionization does or does not play a role.

In dieser Arbeit wurde das Dissoziationsverhalten kleiner organischer Kohlenwasserstoffradikale untersucht. Zum einen wurde die dissoziative Photoionisation des Allyl-, Propargyl- und Ethylradikals durch die Verbindung einer herkömmlichen, gepulsten Pyrolysequelle mit Synchrotronstrahlung untersucht. Zum anderen wurden von verschiedenen Propyl- und Butyl-Radikalisomeren die Raten des Wasserstoffverlustes bei Anregung mit 239 nm gemessen. Es konnte gezeigt werden, dass die Kombination einer gepulsten Radikalquelle mit Synchrotronstrahlung technisch umsetzbar ist. Der Vorteil dieser Methode liegt darin, dass es möglich ist, die Radikale über einen kontinuierlichen Energiebereich von mehreren eV innerhalb kurzer Zeit zu betrachten, was mit einem normalen Labor-Aufbau mit Farbstofflasern nicht möglich ist. So konnten anhand von Photoelektronenspektren und Photoionisationseffizienzkurven die Ionisierungsenergie des Allyl-, des Propargyl- und des Ethyl-Radikals in guter Übereinstimmung zu älteren Literaturdaten bestätigt werden. Anhand von Messungen zur dissoziativen Photoionisation konnte gezeigt werden, dass das Allyl und das Ethyl wie angenommen ein Wasserstoff-Molekül abspalten. Beim Allyl gibt es hierbei zwei mögliche Dissoziationsprodukte: das Cyclopropyl- und das Propargyl-Kation. Einfache RRKM-Rechnungen deuten darauf hin, dass bei geringen Überschussenergien das Cyclopropenyl-, bei größeren hingegen bevorzugt das Propargyl-Kation gebildet wird. Das Ethyl-System besitzt einen nicht-klassischen, überbrückten ionischen Zustand. Die dissoziative Photoionisation führt zum Vinyl-Kation und setzt bei deutlich höheren Energien ein, als von der Theorie vorhergesagt. Alle Erklärungsversuche sind unbefriedigend, daher sollten hier weitere experimentelle und theoretische Untersuchungen angeschlossen werden. Die dissoziative Photoionisation des Propargyl-Radikals besitzt, theoretischen Berechnungen zufolge, zwei konkurrierende Zerfallskanäle. Wie die Experimente jedoch zeigten, erfolgt nur die Abspaltung von H unter Bildung des Cyclopropenyliden-Kations im vorhergesagten Energiebereich. Der Konkurrenzprozess, Bildung von Propinyliden und H$_2$, setzt erst bei deutlich höheren Photonenenergien ein. Daneben konnte erstmals der Dreikörperzerfall des Propargylbromids und des Ethyliodids zum Propinyliden- respektive Vinyl-Kation beobachtet und die Auftrittsenergien bestimmt werden. Diese stimmen gut mit den thermochemischen Vorhersagen überein. In Zukunft ist geplant, mit ZEKE-Spektroskopie die Ionisierungsenergien der untersuchten Radikale exakt zu bestimmen. Insbesondere beim Ethyl-Radikal mit seinem nicht-klassischen Verhalten muss noch viel geklärt werden. Des Weiteren ist geplant, weitere Experimente am Synchrotron durchzuführen. Denkbar wäre hier, Ionen in definierten, schwingungsangeregten Zuständen zu erzeugen und an ihnen Ionen-Molekül-Reaktionen durchzuführen. Ausgehend von den jeweiligen Azoalkanen wurden die Wasserstoff-Disso"-zia"-tions"-raten der \n-Propyl-, \n-Butyl- und \sec-Butyl-Radikale bei einer Anregungswellenlänge von 239 nm gemessen. Ähnlich wie dies bereits bei früheren Messungen am \tert-Butyl- und Ethyl-Radikal der Fall war, waren diese Raten um 2--3 Größenordnungen schneller als von einfachen RRKM-Rechnungen vorhergesagt. %Eine mögliche Erklärung wäre ein konkurrierender %Deaktivierungskanal, z.B.\ der C--C-Bindungsbruch im Radikal. Da %jedoch der Wasserstoffverlust von einem der Spaltprodukte nicht in %einem Einphotonenprozess zu bewerkstelligen ist, muss dieser Kanal %ausgeschlossen werden. Ein anderer, noch sehr spekulativer %Erklärungsversuch geht von einem langlebigen 3p-Zustand, der ein %Minimum in der Geometrie mit einer verlängerten C--C-Bindung %darstellt, aus. Koppelt dieser Zustand mit dem darunterliegenden %3s-Zustand, kann es zur Deaktivierung kommen, jedoch nur, wenn die %Überschussenergie ausreichend groß ist. Dies würde das Verhalten der %Dissoziationsrate bei den Propyl- und Butyl-Radikalisomeren sowie dem %Ethylradikal erklären. Es wurde eine sehr spekulative Erklärung vorgestellt, die dieses Verhalten erklären könnte. Die vorliegenden Daten können diese Theorie aber weder bestätigen noch widerlegen. Es sieht jedoch so aus, als ob diese Diskrepanz zwischen Theorie und Experiment ein allgemeines Phänomen in Alkylradikalen darstellt. Um diese Theorie weiter zu erhärten, kann man einerseits noch weitere Alkylradikale untersuchen, ob sie ähnliche Differenzen zwischen Experiment und Theorie zeigen. Andererseits sollten auch bei den Propyl- und Butylradikalen Experimente mit unterschiedlichen Anregungswellenlängen durchgeführt werden um zu klären, ob sie sich analog zum \tert-Butyl- und Ethylradikal verhalten., In this work the dissociation behaviour of small organic hydrocarbon radicals was investigated. On the one hand the dissociative photoionisation of the allyl, propargyl and ethyl radicals was investigated by the combination of a conventional, pulsed radical source with synchrotron radiation. On the other hand the rates of hydrogen loss from various propyl and butyl isomers were measured with an excitation wavelength of 239 nm. It could be shown that the combination of a pulsed radical source with synchrotron radiation is technically possible. The advantage of this method is the possibility to investigate radicals over a continuous energy range of several eV within a short duration. This is not possible with a normal laboratory setup. By means of the photoelectron spectra and photoion efficiency curves the ionisation potentials of the allyl, propargyl and ethyl radicals could be measured in good agreement with older literature data. It could be shown that the allyl and the ethyl lose a hydrogen molecule -- as suspected -- upon dissociative photoionisation. For the allyl there are two possible dissociation products: the cyclopropenyl and the propargyl cations. Simple RRKM calculations indicate that at low excess energies the cyclopropenyl, and at higher excess energies the propargyl cation is formed preferentially. The ethyl system has a non-classical bridged ionic state. The dissociative photoionisation leads to the vinyl cation and starts at much higher energies than predicted by theory. All explanation attempts are unsatisfying. Therefore further experimental and theoretical studies should consecute. The dissociative photoionisation of the propargyl radical has, according to theoretical studies, two competing decay channels. But the experiments showed that only the loss of H and the formation of the cyclopropenylidene cation occurs in the predicted energy range. The competing channel, loss of H$_2$ and formation of propenylidene, starts at much higher photon energies. Besides this the three body decay of the propargylbromide and the ethyl\-iodide to the propinylidene and the vinyl cations could be observed. The appearence energies could also be measured. They are in good agreement with thermochemical predictions. For the future it is planned to determine the ionisation potentials of the observed radicals exactly with ZEKE spectroscopy. Especially the ethyl radical with its non-classical behaviour needs further examination. Furthermore it is planned to do additional experiments at the synchrotron. It is thinkable to produce ions in well-defined, vibrationally excited states to carry out ion-molecule reactions with them. The hydrogen dissociation rates of the \n-propyl, \n-butyl and \sec-butyl radicals were measured with an excitation wavelength of 239 nm. Similar to earlier measurements with the \tert-butyl and ethyl radicals the observed rates were \mbox{faster} in the order of 2--3 magnitudes than predicted by simple RRKM calculations. A very speculative explanation was presented that could explain this behaviour. The available data can neither confirm nor disprove this theory. It seems that this discrepancy between experiment and theory is a common phenomenon in alkyl radicals. To substantiate this theory more alkyl radicals could be examined for this difference between experiment and theory. But also for the propyl and butyl radical isomers experiments with different excitation wavelengths should be carried out.

International audience; The reaction between acetylene ion and ethylene is revisited to investigate possible dynamical effects in one of the reactions, which are considered as benchmark systems for statistical models. Reactant ions are produced by photoionisation with synchrotron radiation and are selected in internal energy by a coincidence method between threshold photoelectrons and photoions. Measured absolute reaction cross-sections decrease with collision energy (0.1-1 eV CM), but increase with acetylene ion vibrational energy for the three exothermic channels giving C4H5+, C3H3+ and C2H4+ ionic products. Even though RRKM calculations are shown to fit rather well experimental results for both the bimolecular (C2H2+ + C2H4) and unimolecular (C4H6+) systems, some experimental results clearly indicate dynamical effects in this reaction for the C2H4+ production channel, in particular the strongly backward peaked C2H4+ angular distribution. We propose an alternative model combining reaction control both by dynamics and by statistics: the first step in the reactant approach is the capture between reactants, followed by a charge transfer process controlled by dynamics. In a second step, either the two reactants separate leading to C2H4+ ionic products, or there is a rearrangement into 1,3-butadiene (C4H6+) ion which dissociates statistically into C4H5+ + H and C3H3+ + CH3 products. Charge transfer is shown to be possible at intermediate intermolecular distances, slightly shorter than the capture distance, provided that one takes into account the charge-quadrupole interaction in addition to the ion-induced dipole interaction. This work clearly shows that a good fit of experimental data by RRKM calculations does not prove that the reaction elementary mechanism is controlled by statistics.

We describe how correlations between electrons can be used to trace the dynamics of correlated two-electron ionization with attosecond precision, without using attosecond pulses. The approach is illustrated using the example of Auger or Coster-Kronig decay triggered by photoionization with an extreme ultraviolet pulse. It requires correlated measurements of angle-resolved energy spectra of both the photo- and Auger electrons in the presence of a laser pulse. To reconstruct the dynamics, we use not only classical time and energy correlation, but also entanglement between the two electrons.

Attosecond pulses are produced when an intense infrared laser pulse induces a dipole interaction between a sublaser cycle recollision electron wave packet and the remaining coherently related bound-state population. By solving the time-dependent Schrödinger equation we show that, if the recollision electron is extracted from one or more electronic states that contribute to the bound-state wave packet, then the spectrum of the attosecond pulse is modulated depending on the relative motion of the continuum and bound wave packets. When the internal electron and recollision electron wave packet counterpropagate, the radiation intensity is lower. We show that we can fully characterize the attosecond bound-state wave packet dynamics. We demonstrate that electron motion from a two-level molecule with an energy difference of 14 eV, corresponding to a period of 290 asec, can be resolved.

A flame-sampling molecular-beam photoionization mass spectrometer, recently designed and constructed for use with a synchrotron-radiation light source, provides significant improvements over previous molecular-beam mass spectrometers that have employed either electron-impact ionization or vacuum ultraviolet laser photoionization. These include superior signal-to-noise ratio, soft ionization, and photon energies easily and precisely tunable [E∕ΔE(FWHM)≈250–400] over the 7.8–17-eV range required for quantitative measurements of the concentrations and isomeric compositions of flame species. Mass resolution of the time-of-flight mass spectrometer is m∕Δm=400 and sensitivity reaches ppm levels. The design of the instrument and its advantages for studies of flame chemistry are discussed.

The studies presented in this thesis deal with resonant and non-resonant excitation of free variable size clusters using synchrotron radiation in the soft X-ray regime. The post collision interaction (PCI) effect is investigated in free variable size krypton and argon clusters near the Kr 3d and Ar 2p ionization energies. The core ionization energies of surface and bulk sites in variable size clusters can be clearly distinguished. This is mostly due to the polarization screening. It is found that the asymmetry, which is a consequence of PCI, is characteristically smaller for clusters than for isolated atoms. Moreover, there is less asymmetry for bulk sites than for surface sites in variable size rare gas clusters. We assign the results in terms of mechanisms that are based on quantum mechanical models of post collision interaction. Complementary experiments on the photoionization of free van der Waals clusters are performed by using zero kinetic energy (ZEKE) photoelectron spectroscopy in the Ar 2p-, Kr 3d-, Ne 1s-, and N2-regimes. The experimental approach is also suitable to detect cluster size dependent changes in electronic structure. This also allows us to study post collision interaction in variable size clusters. The parameters of the PCI profiles deduced for ZEKE experiments indicate that there are no significant changes in core ionization dynamics compared to near-threshold experiments. Results from model calculations in Kr 3d ionization energy indicate that different geometric sites can be clearly distinguished from each other by their substantial shift in Kr 3d ionization energy, though the dimer shows almost the same Kr 3d ionization energy as the free atom. A comparison with the experimental results indicates that there is resemblance with the model calculations, even though close-lying ionization energies are blended and require deconvolutions of the experimental spectra. It is evident from the present work that one can observe distinct shifts in core ionization energies in van der Waals clusters that are formed in wide size distributions of a jet expansion. The emission of ultraviolet fluorescence radiation from variable size argon clusters is investigated with high spectral resolution in the Ar 2p-excitation regime. The fluorescence excitation spectra reveal strong fluorescence intensity in the Ar 2p-continuum, but no evidence for the occurrence of discrete low-lying core-exciton states in the near-edge regime. This finding is different from the absorption and photoionization cross sections of argon clusters and the solid. The dispersed fluorescence shows a broad molecular band centered near 280 nm. The present results are consistent with the formation of singly charged, excited moieties within the clusters, which are assigned as sources of the radiative relaxation in the 280 nm regime. A fast energy transfer process (interatomic Coulombic decay, ICD) is assigned to be primarily the origin of these singly charged, excited cations besides intra-cluster electron impact ionization by Auger electrons. Our findings give possibly the first experimental evidence for ICD in the core level regime. Free, variable size nitrogen clusters are investigated in the N 1s excitation regime in comparison with the free molecule and solid nitrogen. The conversion of Rydberg states into core excitons, surface and bulk, was studied. The experimental results are simulated by ab initio calculations using (N2)13 as a reasonable prototype cluster structure that allows us to simulate both surface and bulk properties in comparison with the isolated molecule. The present results clearly show that there are specific properties, such as molecular orientation, in molecular van der Waals clusters, which do not exist in atomic van der Waals clusters. It is shown that inner and outer surface sites give rise to distinct energy shifts of the low lying surface core excitons., In der vorliegenden Dissertation wurden Experimente zur resonanten und nicht-resonanten Anregung von Clustern variabler Größe durchgeführt. Hierzu kam Synchrotronstrahlung im weichen Röntgenbereich zum Einsatz. Der "Post-Collision Interaction"-Effekt (PCI) wurde im Detail am Beispiel von Krypton und Argon-Clustern im Bereich der Kr 3d- und Ar 2p-Anregung studiert. Es lassen sich die Ionisierungsenergien von Atomen, die an der Oberfläche bzw. im Volumen gebunden sind, klar unterscheiden. Dies ist aufgrund der unterschiedlichen Polarisationsabschirmung möglich, die zu einer Verschiebung der Innerschalen-Ionisierungsenergien führt. Die Linienformen der Photoelektronenbanden werden asymmetrisch, wenn die Anregungsenergie geringfügig über der Ionisierungsenergie liegt. Dies lässt sich auf den PCI-Effekt zurückführen. Es wird beobachtet, dass die Asymmetrie vom isolierten Atom über Oberflächenatome zu den im Volumen gebundenen Atomen abnimmt. Diese Veränderung der Linienformen wird mit Hilfe von Mechanismen, die auf Grundlage von quantenmechanischen Modellen basieren, interpretiert. Komplementäre Experimente wurden an Argon- und Neon-Clustern zur Nullvolt-Photoelektronen-Spektroskopie (ZEKE) durchgeführt (Anregung der Ar 2p-Kante, Kr 3d-Kante, N2 1s und Ne 1s-Kante). Auch mit diesem Ansatz lassen sich größenabhängige Veränderungen der elektronischen Struktur in Clustern sowie die Bedeutung des PCI Effektes bestimmen. Ein Vergleich dieser Resultate mit der Anregung, die nahe der Ionisationsschwelle liegt, zeigt, dass es zu keiner signifikanten Veränderung der Ionisationsdynamik als Funktion der Anregungsenergie kommt. Berechnungen zur Ioniserungsenergien von Krypton-Clustern im Bereich der Kr 3d-Anregung zeigen, dass sich einzelne geometrische Orte klar in ihrer Ionisierungsenergie unterscheiden. Das Krypton-Dimer zeigt allerdings fast dieselbe 3d-Ioniserungsenergie wie das freie Atom. Der Vergleich mit den experimentellen Resultaten zeigt, dass eine gute übereinstimmung zwischen Modell und Experiment besteht. Allerdings müssen die experimentellen Spektren entfaltet werden, da die relativen Verschiebungen der Ionisierungsenergien zu gering sind und die Rumpflochlebensdauer zu einer Verbreiterung der Banden führt. Die Resultate belegen, dass sich ausgezeichnete Werte für Rumpfniveau-Ionisierungsenergien bestimmen lassen, obwohl die Cluster in breiten Größenverteilungen vorliegen. Dies lässt sich durch die ortsspezifische Photoionisation erklären. Die Emission von Fluoreszenzstrahlung im ultravioletten Spektralbereich nach Rumpfniveauanregung wurde im Fall von 2p-angeregten Argon-Clustern untersucht. Die hochaufgel östen Spektren zeigen hohe Intensität im 2p-Kontinuum, jedoch keinen Hinweis auf signifikante Beiträge im Bereich der Rumpfniveau-Excitonen. Dieses Ergebnis unterscheidet sich vom Absoprtions- und Photoionisationsquerschnitt von Argon-Clustern sowie festem Argon. Die dispergierte Fluoreszenz liefert eine intensive Bande bei 280 nm. Dieses Resultat lässt sich mit der Fluoreszenz von einfach geladenen, angeregten Argon-Clustern erklären. Die Bildung von einfach geladenen Ionen nach primärer Doppelionisation im Ar 2p-Kontinuum wird durch einen schnellen Energietransfer-Prozess (Interatomic Coulombic Decay, ICD) erklärt. Er läuft nach der Rumpfniveauanregung ab und liefert, neben der Elektronenstoßionisation durch schnelle Auger-Elektronen, einfach geladene Clusterfragmente, die nachfolgend strahlend relaxieren. Dieses Ergebnis ist als erster Hinweis darauf zu werten, dass der ICD-Prozess auch im Bereich der Innerschalenanregung auftritt. Freie Stickstoff-Cluster variabler Größe wurden im Bereich der N 1s-Anregung untersucht. Hier stand die Umwandlung der Rydberg-Zustände in die entsprechenden Oberflächen- und Volumen-Excitonen in Fokus der Studien. Die Resultate wurden mit denen zu freiem und kondensiertem Stickstoff verglichen. Die experimentellen Resultate lassen auch einen Vergleich mit ab initio Rechnungen zu, wofür (N2)13 als Prototyp-Cluster genutzt wurde, da hier sowohl oberflähen - als auch volumengebundene Moleküle auftreten. Diese Resultate zeigen signifikante Unterschiede im Vergleich zu atomaren Clustern. Es zeigt sich, dass die molekulare Orientierung die Lage der Excitonenbanden beeinflusßt. Ebenso treten signifikante Energieverschiebungen relativ zum isolierten Molekül auf, die sich durch Absorption von Zentren erklären lassen, die entweder auf der inneren bzw. äußeren Oberfläche der Cluster gebunden sind.

Im Rahmen der vorliegenden Arbeit werden Experimente zur Photoneninduzierten Fluoreszenzspektroskopie (PIFS) an Argonatomen beschrieben. Die Experimente wurden am Berliner Elektronenspeicherring für Synchrotronstrahlung (BESSY) durchgeführt. Dabei lag das Hauptaugenmerk auf dem Schwellenbereich der Ar+* - Satelliten. Die Messungen von absoluten Wirkungsquerschnitten zur Photoionisation eines Ar 3p - Elektrons bei gleichzeitiger Anregung eines zweiten 3p Elektrons in das 4s - Orbital dienen im Vergleich mit theoretischen Beschreibungsmethoden dazu, die Bedeutung der Elektronenkorrelationen und der Vielteilchendynamik aufzuzeigen. Die 3p4(3P) - Core Konfiguration bildet mit dem angeregten 4s - Elektron 4P und 2P - Satellitenzustände als niedrigste angeregte Ionenzustände. Die Auswahl dieser Zustände und des entsprechenden Primärenergiebereichs orientiert sich an der Forderung, möglichst interpretierbare und interessante Bereiche und Fragestellungen der komplexen Mechanismen der photoneninduzierten Satellitenbesetzung zu isolieren. In dieser Arbeit werden sowohl Quartett- als auch Dublettzustände beobachtet und deren Anregungsquerschnitte bei Photonenenergien zwischen 32,475 eV und 33,050 eV gemessen. Die hier durchgeführten Untersuchungen von Zweielektronenprozessen, welche im Vergleich zu den Einelektronenprozessen wesentlich schwächerer Natur sind, und die dazu notwendige Methode der Fluoreszenzspektroskopie im Spektralbereich des Vakuumultravioletten sind auf der experimentellen Seite mit dem Problem geringer Intensitäten behaftet. Diese sind durch die Verwendung von Undulatorbeamlines und langen Integrationszeiten zu kompensieren. Die durchgeführten Experimente lösen in diesem Energiebereich Resonanzen in regelmäßiger Abfolge erstmals auf. Sie zeigen, dass die Quartettzustände überwiegend durch die Autoionisation zweier identifizierter Rydbergserien besetzt werden. Diese sind als (3P) 4s 2P3/2,1/2 np - Serien identifiziert. Im Anregungsenergiebereich zwischen 32,899 eV und 33,019 eV, also zwischen der energetischen Schwelle des Ar+ 3p4 (3P) 4s 2P3/2 und des Ar+ 3p4 (3P) 4s 2P1/2 Zustands, wird für den 2P3/2 Zustand eine Überlagerung von direkter und resonanter Besetzung beobachtet. Die Rydbergserien werden simultan in Absorption beobachtet. Diese Daten lassen sich mit Literaturwerten bei durchaus verbesserter eigener Primärenergieauflösung vergleichen. Durch die Messung von absoluten Wirkungsquerschnitten der totalen Absorption und aller offenen Zerfallskanäle hat man eine vollständige experimentelle Datenbasis für einen Test von Berechnungen eines überschaubaren Anregungs- und Zerfallsszenarios erhalten. Eine der Stärken der PIFS besteht in der Möglichkeit der Bestimmung der relativen Verteilung der drei erlaubten Elektronenpartialwellen. Diese Verteilung ist aus der Messung der Größen A20 (Alignmentparameter) und O10 (Orientierungsparameter) der besetzten Ionenzustände ableitbar. Unter Verwendung eines Wollastonprismas wurde eine resonanzartige Variation von A20 der Ar+ 3s23p4 (3P) 4p 2P3/2, 2D3/2 - Zustände im Primärenergiebereich um 36,6 eV aus einer Vorläuferarbeit bei BESSY I mit der gesteigerten Primärauflösung von BESSY II untersucht. Die Verwendung eines Wollastonprismas zur Polarisationsanalyse der Fluoreszenzstrahlung ist allerdings auf den sichtbaren Spektralbereich beschränkt und lässt sich deswegen nicht auf die oben erwähnten Messungen der Ar+ 3p4 (3P) 4s - Zustände übertragen. Für die Ar+ 3s23p4 (3P) 4s -, sowie für höher angeregte Satelliten existieren im Primärenergiebereich zwischen 32,8 und 36 eV bisher nicht publizierte relative Wirkungsquerschnitts- und auch Alignmentmessungen, welche im Rahmen früherer Forschungsprojekte bei BESSY I ermittelt wurden. Diese sind im Anhang der Arbeit zusammengefasst, um einen Leitfaden für nachfolgende Untersuchungen mit gesteigerter Primärauflösung bereit zu halten. Der Beschreibung des experimentellen Aufbaus wird ein kurzer Überblick der anwendbaren theoretischen Beschreibungsmethoden der untersuchten Wechselwirkungsprozesse vorangestellt., This work presents experiments on photon induced Fluorescence Spectroscopy of the Argon atom. The experiments were carried out at the Berlin Electron Storage Ring BESSY II. The threshold range of the Ar+* - Satellites was in the main focus. The measurements of absolute cross sections of the photoionization of an Ar 3p - electron, with simultaneous excitation of a second 3p electron into the 4s level, documents the existence of electron correlations and underlines their importance for the test of many body calculations. The 3p4(3P) - core configuration forms with the 4s electron 4P and 2P - finestructure components as satellites of lowest excitation energy. The measurement of these levels is necessary to isolate interesting and interpretable aspects out of the manifold complex of the photon induced satellite production. This work deals with emission cross sections between 32.475 eV and 33.050 eV. The measurement of the very weak two electron processes and the required method of fluorescence spectroscopy in the range of the vacuum ultraviolet leads to very low signal rates. These have to be compensated by using undulator beamlines and long integration times. For the first time, the performed experiments resolve resonances at regular intervals in the whole energy range. The energy dependence of the cross section for populating quartet states is dominated by the autoionization of doubly excited Rydbergstates. Two series have been identified as (3P) 4s 2P3/2 np - and (3P) 4s 2P1/2 np - series. Only in the range between 32.899 and 32.019 eV of the exciting photons, this means between the energetic threshold of the Ar+ 3p4 (3P) 4s 2P3/2 and the Ar+ 3p4 (3P) 4s 2P1/2 state, a superposition of direct and resonant population for the 3p4 (3P) 4s 2P3/2 state is observed. While performing the fluorescence measurements the total absorption was measured with increased primary resolution in order to compare it with earlier published data. By measuring absolute cross sections of the total ion yield and of the open radiative decay channels a complete experimental data basis for a test of calculations of a manageable excitation- and decayscenario is obtained. Basically, the PIFS - Method allows to measure the relative contribution of the three allowed electron partial waves. These contributions can be derived from a measurement of the alignment parameter A20 and orientation parameter O10 of the populated ionic states. A resonant-like variation of the A20 parameter in the primary energy range of about 36.6 eV was remeasured with the high primary resolution of BESSY II using a Wollastonprism. This variation was observed with lower resolution at BESSY I. The use of a Wollastonprism is restricted to the visible spectral range, so this method cannot be transferred to the investigations on the Ar+ 3p4 (3P) 4s - states mentioned above. So far unpublished measurements, done earlier at BESSY I, of relative cross sections and alignment parameters of the Ar+ 3s23p4 (3P) 4s - Satellite and of Satellites with higher excitation energy, in the range between 32.8 and 36 eV of the exciting photons, are summarized in the appendix. Before describing the experimental setup a short overview of the theoretical treatment of the investigated interaction processes is given.

The duration-time concept, vastly successful for interpreting the frequency dependence of resonant radiative and nonradiative x-ray scattering spectra, is tested for fine-scale features that can be obtained with state of the art high-resolution spectroscopy. For that purpose resonant photoelectron (RPE) spectra of the first three outermost singly ionized valence states X 2, A 2u, and B 2, are measured for selective excitations to different vibrational levels (up to n = 13) of the N 1s* photoabsorption resonance in N2 and for negative photon frequency detuning relative to the adiabatic 0-0 transition of this resonance. It is found that different parts of the RPE spectrum converge to the spectral profile of direct photoionization (fast scattering) for different detunings, and that the RPE profiles are asymmetrical as a function of frequency detuning. The observed asymmetry contradicts the picture based on the simplified notation of a common scattering duration time, but is shown to agree with the here elaborated concept of partial and mean duration times. Results of the measurements and the simulations show that the duration time of the scattering process varies for different final electronic and different final vibrational states. This owes to two physical reasons: one is the competition between the fast "vertical" and the slow "resonant" scattering channels and the other is the slowing down of the scattering process near the zeros of the real part of the scattering amplitude.

The present study describes the characterization of energy and structure of HBr2+ in its low-lying electronic states, achieved through an extension of a new empirical method [Chem. Phys. Lett. 379, 139 (2003)] recently introduced to evaluate the interatomic interaction in the HX2+ (X=F,Cl,Br,I) molecular dications. The method is based on identification of the main components of the interaction and their evaluation through some simple correlation formulas. Potential energy curves, given in a simple, natural, and analytical form, made possible the calculations of some important properties, such as double-photoionization energy thresholds, vibrational spacing, average lifetime, and Franck-Condon factors. The predictions, compared with data available in the literature, are of great interest for the analysis and interpretation of some new experimental results. (C) 2004 American Institute of Physics.

The mismatch of the lattice constants and of the thermal expansion coefficients between gallium nitride (GaN) and its substrates like sapphire, silicon and silicon carbide is responsible for a high concentration of defects in AlGaN/GaN layers. The influence of these electrically active centres (traps) and their respective trapping effects are investigated using four different measurement methods. With photoionisation spectroscopy on layers grown on sapphire and silicon substrates two resp. three different traps responsible for the current collapse were found. Current collapse is assigned to traps in the GaN buffer layer. Using backgating current deep level transient spectroscopy the activation energies of a minority and a majority trap in the GaN buffer on silicon substrate were determined. For pulse measurements the drain current is measured at pulsed gate voltages. These measurements are used to investigate the influence of the "gate lag", i.e. trapping in surface states. This effect was suppressed largely by passivating the surface with a silicon nitride layer. Finally with admittance spectroscopy the activation energy of a GaN buffer anti-site, which is caused by sputtering during the processing of transistors, could be determined. Thus it was shown that surface states of AlGaN as well as defects in GaN, i.e. lattice defects and impurities, are responsible for trapping effects in AlGaN/GaN high electron mobility transistors. These traps could be confirmed in literature.

Contains fulltext : 60341.pdf (Publisher’s version ) (Open Access) We have studied dissociative multiphoton ionization of NO2 by time-resolved velocity map imaging in a two-color pump-probe experiment using the 400 and 266 nm harmonics of a regeneratively amplified titanium-sapphire laser. We observe that most of the ion signal appears as NO+ with similar to0.28 eV peak kinetic energy. Approximately 600 fs period oscillations indicative of wave packet motion are also observed in the NO+ decay. We attribute the signal to two competitive mechanisms. The first involving three-photon 400 nm absorption followed by dissociative ionization of the pumped state by a subsequent 266 nm photon. The second involving one-photon 400 nm absorption to the B-2(2) state of NO2 followed by two-photon dissociative ionization at 266 nm. This interpretation is derived from the observation that the total NO+ ion signal exhibits biexponential decay, 0.72 exp(-t/90+/-10)+0.28 exp(-t/4000+/-400), where t is the 266 nm delay in femtoseconds. The fast decay of the majority of the NO+ signal suggests a direct dissociation via the bending mode of the pumped state. (C) 2004 American Institute of Physics.

We present an inversion method called pBasex aimed at reconstructing the original Newton sphere of expanding charged particles from its two-dimensional projection by fitting a set of basis functions with a known inverse Abel integral. The basis functions have been adapted to the polar symmetry of the photoionization process to optimize the energy and angular resolution while minimizing the CPU time and the response to the cartesian noise that could be given by the detection system. The method presented here only applies to systems with a unique axis of symmetry although it can be adapted to overcome this restriction. It has been tested on both simulated and experimental noisy images and compared to the Fourier-Hankel algorithm and the original Cartesian basis set used by [Dribinski et al.Rev. Sci. Instrum. 73, 2634 (2002)], and appears to give a better performance where odd Legendre polynomials are involved, while in the images where only even terms are present the method has been shown to be faster and simpler without compromising its accuracy.

Electron delocalization processes of optically excited states of Ce3+ impurities in Lu2SiO5 were investigated by means of a temperature and spectrally resolved photoconductivity study. By monitoring separately the strength of the photocurrent resulting from excitation into each of the Ce3+?5d absorption bands, over a broad temperature region, three different delocalization processes, namely direct photoionization, thermal ionization, and tunneling, have been identified. The relative probabilities and temperature dependencies of each of these processes are discussed. The observed exponential temperature increase in the photocurrent, which spans six orders of magnitude, allows for the exact placement of the lowest energy 5d levels of the Ce3+ ions within the band gap. For Lu2SiO5:Ce3+, the lowest 5d state is determined to be 0.45 eV below the conduction band edge.

The photoionization and photoelectron spectroscopy of He nanodroplets (10(4) atoms) has been studied by photoelectron imaging with photon energies from 22.5-24.5 eV. Total electron yield measurements reveal broad features, whose onset is approximately 1.5 eV below the ionization potential of atomic He. The photoelectron spectra are dominated by very low energy electrons, withE(k)less than 0.6 meV. These results are attributed to the formation and autoionization of highly vibrationally excited He(*)(n) Rydberg states within the cluster, followed by strong final state interactions between the photoelectron and the droplet.

We describe a recently designed and constructed system based on a 1 m normal incidence vacuum monochromator with corrected (toroidal) optics that produces a wavelength tuneable and collimated vacuum-ultraviolet (VUV) (lambda = 30 - 100 nm) beam. The VUV continuum source is a laser-generated gold plasma. The primary function of the system is the measurement of time resolved "images'' or spatial distributions of photoabsorption/photoionization in expanding laser plasma plumes. This is achieved by passing the beam through the sample of interest (in our case a second synchronised plasma) and recording the "footprint'' of the attenuated beam on a charge coupled device. Using this VUV photoabsorption imaging or "shadowgraphy'' technique we track and extract column density distributions in expanding plasma plumes. We can also measure the plume front velocity. We have characterized the system, particularly in relation to spectral and spatial resolution and the experimental results meet very well the expectations from ray tracing done at the design phase. We present first photoabsorption images and column density distributions of laser produced Ca plumes from the system.

Die Massenspektrometrie gehört zu den universellen und leistungsfähigen analytischen Hilfsmitteln in der organischen Chemie. So ermöglichen beispielsweise Sektorfeld – Massenspektrometer Messungen an organischen Molekülen sowie den daraus entstehenden Fragmenten. Die Flugzeit der Ionen in diesem Massenspektrometer begrenzt dabei jedoch das zeitliche Auflösungsvermögen. Mit der Kombination aus der von W. Paul entwickelten Quadrupolionenfalle sowie einem Flugzeitmassenspektrometer konnte der Beobachtungszeitraum der Ionen deutliche vergrößert werden. Bisher kamen bei zeitabhängigen Untersuchungen ausschließlich Stoßgase zum Einsatz. Die Edelgase dienten als Puffer, um die Speichereigenschaften zu optimieren. Die auf diese Weise erhaltenen Daten ließen allerdings keine Schlüsse auf unimolekulare Reaktionen zu, da die Ergebnisse durch Stöße der gespeicherten Ionen mit dem Puffergas verfälscht worden waren. Im Rahmen der vorliegenden Arbeit wurde eine neue Methode entwickelt, die weiterführende experimentelle Untersuchungen erlaubt. Dabei wurde erstmalig eine externe Ionenquelle mit einer Quadrupolionenfalle kombiniert, um extern erzeugte Ionen ohne Verwendung von Puffergas über einen längeren Zeitraum wechselwirkungsfrei zu speichern. Untersucht wurden der Zerfallsmechanismus von n – Propylbenzolradikalkationen sowie die Photionisationseffizienz von n – Propylbenzol und p – Xylol.

Summary form only given. Correlation offers a new route to time resolved measurements. Exploiting correlation we measure H/sub 2//sup +/ double ionization dynamics with sub-femtosecond precision and we measure vibrational wave packet dynamics in D/sub 2//sup +/ with ∼0.03- Angstrom, ∼200-Attosecond precision.

International audience; Luminescence, decay, and thermal quenching under photonic excitation of CaGa2S4:Eu2+ mono and polycrystals have been measured. For excitation into the lower 5d states, thermal quenching starts above 350 K, which makes this material appropriate for use as a yellow phosphor under excitation by blue-emitting diodes. Thermoluminescence after 337.1 nm irradiation is explained by photoionization of Eu2+ ions.

In this paper we demonstrate that the vector correlation approach for the study of dissociative photoionization (DPI) of linear molecules enables us to achieve a complete description of molecular photoionization by performing a single experiment using only one state of circularly, or elliptically, polarized light. This is illustrated by the derivation of the complex dipole matrix elements for the benchmark DPI reaction of the NO molecule, where (4)–1 inner-valence ionization is induced by left-handed circularly polarized synchrotron radiation at h= 23.65 eV. The importance of electronic correlation for this process is emphasized by comparing the experimental results with multichannel Schwinger configuration interaction calculations. The energy dependence of the transition matrix elements and that of the electronic correlation in the 25–40 eV energy range are illustrated by the calculations and compared with the present results and recent experimental studies at 40.8 eV.

We report the first use of synchrotron radiation, continuously tunable from 8 to 15 eV, for flame-sampling photoionization mass spectrometry (PIMS). Synchrotron radiation offers important advantages over the use of pulsed vacuum ultraviolet lasers for PIMS; these include superior signal-to-noise, soft ionization, and access to photon energies outside the limited tuning ranges of current VUV laser sources. Near-threshold photoionization efficiency measurements were used to determine the absolute concentrations of the allene and propyne isomers of C3H4 in low-pressure laminar ethylene–oxygen and benzene–oxygen flames. Similar measurements of the isomeric composition of C2H4O species in a fuel-rich ethylene–oxygen flame revealed the presence of substantial concentrations of ethenol (vinyl alcohol) and acetaldehyde. Ethenol has not been previously detected in hydrocarbon flames. Absolute photoionization cross sections were measured for ethylene, allene, propyne, and acetaldehyde, using propene as a calibration standard. PIE curves are presented for several additional reaction intermediates prominent in hydrocarbon flames.