No description available.

This thesis presents results on the description of plasma waves in terms of wavepackets. The wave field is decomposed into a distribution of wavepackets in a space of position, wavevector, time, and frequency. A complex structure joining each pair of Fourier conjugate variables into a single complex coordinate allows the efficient derivation of equations of motion for the phase space distribution by exploiting its analytic properties. The Wick symbol calculus, a mathematical tool generalizing many convenient properties of the Fourier transform to a local setting, is used to derive new exact phase space equations which maintain full information on the phase of the waves and include effects nonlocal in phase space such as harmonic generation. A general purpose asymptotic expansion of the Wick symbol product formula is used to treat dispersion, refraction, photon acceleration, and ponderomotive forces. Examples studied include the nonlinear Schrödinger equation, mode conversion, and the Vlasov equation. The structure of partially coherent wave fields is understood in terms of zeros in the phase space distribution caused by dislocations in its complex phase which are shown to be correlated with the field entropy. Simulations of plasma heating by crossing electron beams are understood by representing the resulting plasma waves in phase space. The local coherence properties of the beam driven Langmuir waves are studied numerically.

High angular momentum Rydberg wave packets are studied. Application of carefully tailored electric fields to low angular momentum, high- n ( n ∼ 300) Rydberg atoms creates coherent superpositions of Stark states with near extreme values of angular momentum, [cursive l]. Wave packet components orbit the parent nucleus at rates that depend on their energy, leading to periods of localization and delocalization as the components come into and go out of phase with each other. Monitoring survival probability signals in the presence of position dependent probing leads to observation of characteristic oscillations based on the composition of the wave packet. The discrete nature of electron energy levels is observed through the measurement of quantum revivals in the wave packet localization signal. Time-domain spectroscopy of these signals allows determination of both the population and phase of individual superposition components. Precise manipulation of wave packets is achieved through further application of pulsed electric fields. Decoherence effects due to background gas collisions and electrical noise are also detailed. Quantized classical trajectory Monte-Carlo simulations are introduced and agree remarkably well with experimental results.

No description available.

X-rays have been widely exploited to unravel the structure of matter since their discovery in 1895. Nowadays, with the emergence of new X-ray sources with higher intensity and very short pulse duration, notably X-ray Free Electron Lasers, the number of experiments that may be considered in the X-ray regime has increased dramatically, making the characterization of gas phase atoms and molecules in space and time possible. This thesis explores in the theoretical analysis and calculation of X-ray scattering atoms and molecules, far beyond the independent atom model. Amethod to calculate inelastic X-ray scattering from atoms and molecules is presented. The method utilizes electronic wavefunctions calculated using ab-initio electronic structure methods. Wavefunctions expressed in Gaussian type orbitals allow for efficient calculations based on analytical Fourier transforms of the electron density and overlap integrals. The method is validated by extensive calculations of inelastic cross-sections in H, He+, He, Ne, C, Na and N2. The calculated cross-sections are compared to cross-sections from inelastic X-ray scattering experiments, electron energy-loss spectroscopy, and theoretical reference values. We then begin to account for the effect of nuclear motion, in the first instance by predicting elastic X-ray scattering from state-selected molecules. We find strong signatures corresponding to the specific vibrational and rotational state of (polyatomic) molecules. The ultimate goal of this thesis is to study atomic and molecular wavepackets using time-resolved X-ray scattering. We present a theoretical framework based on quantum electrodynamics and explore various elastic and inelastic limits of the scattering expressions. We then explore X-ray scattering from electronic wavepackets, following on from work by other groups, and finally examine the time-resolved X-ray scattering from non-adiabatic electronic-nuclear wavepackets in the H2 molecule, demonstrating the importance of accounting for the inelastic effects.

<p>The main focus of this thesis is the investigation of the dissociation dynamics of aryl halides using femtosecond pump-probe spectroscopy. In the monohalogenated aryl halides, iodo-, bromo- and chlorobenzene, the rate of dissociation following excitation at 266 nm in the gas phase increased with increasing mass of the halogen atom. This process was assigned to predissociation of the initially excited singlet (π, π*) state via a repulsive triplet (n, σ*) state due to spin-orbit interaction. In addition to the predissociative mechanism, a direct dissociation channel was observed in iodobenzene. The rate of the predissociation in bromobenzene was found to be faster in the condensed phase than in the gas phase, which can be explained by solvent-induced symmetry perturbations. <i>Ab initio</i> calculations of the potential energy surfaces of the ground state and several low lying excited states in bromobenzene have been performed in order to verify the suggested mechanism. Substituting one of the hydrogen atoms in bromobenzene affected the predissociation rate significantly. In o-, m- and p-dibromobenzene the predissociation rate increased with decreasing distance between the bromine atoms in accordance with an increased spin-orbit interaction introduced by the bromine substituent. The fastest predissociation rate was observed in 1,3,5-tribromobenzene. With chlorine and fluorine substitution, inductive and conjugative effects were found to be of importance. In the o- and m-isomers of the dihalogenated aryl halides, an additional faster dissociation channel was observed. Guided by <i>ab initio</i> calculations of the potential energy surfaces in the dibromobenzene isomers, we ascribed the fast dissociation pathway to predissociation of an initially excited triplet state. Upon methyl group substitution in bromobenzene, the decreased lifetime of the initially excited state was attributed to an incresaed density of coupled states.</p><p>Another system which has been studied in the condensed phase is diiodomethane. Using Car-Parrinello molecular dynamics simulations we observed a prompt dissociation and subsequent recombination to the isomer, iso-diiodomethane, in acetonitrile solution.</p><p>Vibrational wavepacket dynamics in the C (¹Σ⁺) state of NaK were studied using a direct ionization probing scheme. A simple analytical expression for the pump-probe signal was developed in order to see what factors that govern direct ionization of the vibrational wavepacket. Our experimental data was consistent with a photoionization transition dipole moment that varies with internuclear distance.</p>

The main focus of this thesis is the investigation of the dissociation dynamics of aryl halides using femtosecond pump-probe spectroscopy. In the monohalogenated aryl halides, iodo-, bromo- and chlorobenzene, the rate of dissociation following excitation at 266 nm in the gas phase increased with increasing mass of the halogen atom. This process was assigned to predissociation of the initially excited singlet (π, π*) state via a repulsive triplet (n, σ*) state due to spin-orbit interaction. In addition to the predissociative mechanism, a direct dissociation channel was observed in iodobenzene. The rate of the predissociation in bromobenzene was found to be faster in the condensed phase than in the gas phase, which can be explained by solvent-induced symmetry perturbations. Ab initio calculations of the potential energy surfaces of the ground state and several low lying excited states in bromobenzene have been performed in order to verify the suggested mechanism. Substituting one of the hydrogen atoms in bromobenzene affected the predissociation rate significantly. In o-, m- and p-dibromobenzene the predissociation rate increased with decreasing distance between the bromine atoms in accordance with an increased spin-orbit interaction introduced by the bromine substituent. The fastest predissociation rate was observed in 1,3,5-tribromobenzene. With chlorine and fluorine substitution, inductive and conjugative effects were found to be of importance. In the o- and m-isomers of the dihalogenated aryl halides, an additional faster dissociation channel was observed. Guided by ab initio calculations of the potential energy surfaces in the dibromobenzene isomers, we ascribed the fast dissociation pathway to predissociation of an initially excited triplet state. Upon methyl group substitution in bromobenzene, the decreased lifetime of the initially excited state was attributed to an incresaed density of coupled states. Another system which has been studied in the condensed phase is diiodomethane. Using Car-Parrinello molecular dynamics simulations we observed a prompt dissociation and subsequent recombination to the isomer, iso-diiodomethane, in acetonitrile solution. Vibrational wavepacket dynamics in the C (1Σ+) state of NaK were studied using a direct ionization probing scheme. A simple analytical expression for the pump-probe signal was developed in order to see what factors that govern direct ionization of the vibrational wavepacket. Our experimental data was consistent with a photoionization transition dipole moment that varies with internuclear distance.

Die Multiexzitonen-Theorie des Anregungsenergie-Transfers in Farbstoff-Protein-Komplexen und biologischen Antennensystemen wird um den Prozess der Exziton-Exziton-Vernichtung erweitert. Um eine mikroskopische Beschreibung zu erzielen, wird eine Herangehensweise benutzt, die auf der Internen Konversion der Anregungsenergie innerhalb der Farbstoffmoleküle basiert. Diese Interne Konversion führt zu nicht strahlenden Übergängen von höheren zu niedrigeren Exziton-Mannigfaltigkeiten. Neben der Einbeziehung der Exziton-Exziton-Vernichtung beinhaltet die hier verwendete Multiexziton-Dichtematrixtheorie auch die Kopplung zu niedrig-energetischen Schwingungs-Freiheitsgraden und dem elektrischen Feld. Für den Übergang von der Zwei- zu der Einexzitonen -Mannigfaltigkeit werden exakte und genäherte Ausdrücke hergeleitet. Die erste Anwendung der erweiterten Multiexziton-Dichtematrixtheorie ist die Berechnung von ultra-schnellen differentiellen Absoptionsspektren. Um den Prozess der Exziton-Exziton-Vernichtung in intensitätsabhängigen differenziellen Absorptionsspektren näher zu untersuchen, wird diese Herangehensweise auf den B850 Ring des LH2 von rhodobacter sphaeroides angewendet. Die Bedeutung der Exziton-Exziton-Vernichtung und der Einfluss von statischer Unordnung werden detailiert diskutiert. Die Simulationen der differentiellen Absorptionsspektren mit statischer Unordnung und Orientierungsmittelung zeigen gute Übereinstimmung mit experimentellen Beobachtngen. Durch die Veröffentlichung der Strukturdaten des Photosystem I (PS1) von Synechococcus elongatus wurde es zum ersten Mal möglich, ein Exziton-Modell für die 96 Chorophyllmoleküle einzuführen, die in die Proteinmatrix dieses Antennensystems eingebettet sind. Das Ziel dabei ist, sowohl die linearen Spektren in einem weiten Temperaturbereich, als auch die zeitaufgelöste Fluoreszenz zu reproduzieren. Die Kopplungen und die Dipolmomente der Chlorophyllmoleküle wurden den Strukturdaten entnommen. Da die Energien der einzelnen Farbstoffe stark von deren unmittelbarer Umgebung abhängt, werden diese bestimmt, indem simulierte Absorption, Lineardichroismus und Zirkulardichroismus bei niedrigen Temperaturen den experimentellen Spektren angepasst werden. Nachdem einige Chlorophyllmoleküle den Zuständen mit den niedrigsten Energien zugeordnet wurden, werden die Energien mit Hilfe eines evolutionären Algorithmus angepasst. Die Qualität des PS1 Modells wird durch die Berechnung der zeitabhängigen Fluoreszenz untermauert (mit zusätzlicher inhomogener Linienbreite), die Simulationen stimmen gut mit aktuellen experimentellen Resultaten überein. Die oben erwähnten Exziton-Modelle beschreiben die jeweiligen Experimente erfolgreich. Der nächste Schritt ist, diese Modelle zu nutzen, um einen neuen Typ von Experiment vorzuschlagen, das Exciton-Steuerungs-Experiment. Auf dem Exciton-Modell des FMO Komplexes von Prosthecochloris aestuarii und dem oben erwähnten PS1 Modell von Synechococcus elongatus aufbauend wird die Bildung von exzitonischen Wellenpaketen durch Laser-Anregung studiert. Diese stellen eine kohärente Überlagerung exzitonischer Zustände dar, ähnlich der bei Schwingungs-Wellenpaketen. Um die spezielle Form des Femtosekunden-Laserpulses zu bestimmen, der zu einer räumlichen Lokalisierung der Anregungsenergie führt, wird die Theorie der optimalen Steuerung verwendet. Die Möglichkeit, solch einen lokalisierten Zielzustand zu erreichen, wird aufgezeigt, auch unter dem Einfluss von energetischer Unordnung und Exziton-Exziton Vernichtung. Ferner wird gezeigt, dass die Effizienz der Lokalisierung und die Länge des optimalen Pulses stark von der Temperatur abhängen. / The multi-exciton description of excitation energy transfer in chromophore complexes and biological light harvesting antenna systems is extended to include the exciton-exciton annihilation processes. To achieve a complete microscopic description the approach is based on intra--chromophore internal conversion processes which leads to non-radiative transitions from higher to lower lying exciton manifolds. Besides an inclusion of exciton-exciton annihilation the used multi-exciton density matrix theory also accounts for a coupling to low-frequency vibrational modes and the radiation field. Concentrating on transitions from the two- to the single-exciton manifold exact and approximate expressions for the annihilation rate are derived. A first application of the introduced extended multi-exciton density matrix theory is given by the computation of ultrafast transient absorption spectra. To elucidate the process of exciton-exciton annihilation in intensity dependent transient absorption data the approach is applied to the B850 ring of the LH2 found in rhodobacter sphaeroides. The signatures of exciton-exciton annihilation as well as the influence of static disorder are discussed in detail. The simulations of transient absorption including static disorder and orientational averaging are in good agreement with experimental data. The recently published structure of the Photosystem I (PS1) of Synechococcus elongatus made it for the first time possible to introduce an excitonic model for the 96 chlorophylls embedded in the protein matrix of that core-antenna system, as presented in this work. The challenge has been to reproduce linear frequency domain spectra in a wide temperature range as well as the time resolved fluorescence. The couplings and the dipole-moments of the chlorophylls are extracted from the x-ray crystal structure. Since the position of the energetic levels of the chlorophylls depend on the respective surrounding their determination is achieved by fitting low temperature absorption, linear dichroism and circular dichroism at the same time. After assigning some chromophores to the red-most states, an evolutionary algorithm is used to get the best fit. The quality of the resulting PS1 model (additionally accounting for inhomogeneous line broadening) is confirmed in calculating time dependent fluorescence spectra which show a good agreement with recent experimental results. The outlined method is also applicable to other photosynthetic antenna systems. The above described exciton models successfully explain the respective measurements. In a second step, they will be used to propose a new type of experiment, the exciton control experiment. Based on an exciton model for the FMO complex of Prosthecochloris aestuarii and the proposed PS1 model of Synechococcus elongatus one studies the laser pulse formation of excitonic wavepackets, i.e. a coherent superposition of excitonic states similar to vibrational wavepackets. Optimal Control theory is used to calculate the shape of femtosecond laser pulses that leads to a spatial localization of excitation energy. The possibility to populate such a localized target state is demonstrated, even in the presence of disorder or exciton-exciton annihilation, and it is shown that the efficiency of localization as well as the length the most suited pulses strongly depend on temperature.

In dieser Arbeit wurde die Ionisation einiger zweiatomiger Moleküle (H2, N2 und O2) in intensiven Laserfeldern untersucht. Hierbei wurden verschiedene Modelle zur Beschreibung der Tunnelionisation sowohl untereinander als auch mit der Lösung der zeitabhängigen Schrödingergleichung (TDSE) verglichen. Die kernabstandsabhängige Ionisationswahrscheinlichkeit wurde für verschiedene Intensitäten betrachtet und die Gültigkeit modifizierter atomarer bzw. Molekularer Modelle zur Beschreibung der Tunnelionisation analysiert. Es wurde herausgefunden, dass Modelle, die auf der quasistatischen Näherung beruhen (wo die Ionisation unabhängig von der Frequenz des Laserfeldes ist), nur in einem kleinem Frequenz- und Intensitätsbereich hinreichend genaue Ergebnisse liefern, dem Tunnelregime. Modelle mit einem frequenzabhängigen Faktor stimmen hingegen sowohl im Tunnel- als auch im Mehrphotonenregime mit den genaueren TDSE Ergebnissen überein. Weiterhin wird auch die Abweichung zur Franck-Condon Näherung verdeutlicht. Es wurde ein kleiner Einfluss auf die Revival-Zeit des im Wasserstoffmolekül-Ion gestarteten Wellenpakets gefunden. Die Berücksichtigung von Bond-Softening führt weiterhin zu einer Verringerung der Revival-Zeit mit steigender Spitzenintensität des Lasers. Außerdem wird die Anisotropie der Ionisation von H2 als Funktion der Laserintensität in linear und zirkular polarisiertem Licht mit dem molekularen Tunnelmodell MO-ADK untersucht. Gute Übereinstimmung mit den experimentellen Beobachtungen wurde gefunden, insbesondere wenn der Effekt des Fokusvolumens des Laserfeldes berücksichtigt wird. Die Anwendbarkeit des Zwei-Zentren-Modells auf größere Moleküle, N2 und O2, wird ebenfalls getestet. Es wird beobachtet, dass dies für N2 (symmetrisches HOMO) funktioniert, für O2 (asymmetrisches HOMO) jedoch nicht. / The ionization of some diatomic molecules, H2, N2, and O2, exposed to intense laser fields has been studied by comparing various molecular tunneling–ionization models with each other and with the numerical solution of the time-dependent Schrödinger equation (TDSE). The internuclear-distance dependent ionization yields over a wide range of laser peak intensities are investigated and the validity of the modified atomic and molecular tunneling models is examined. It is found that those models that depend on the quasi-static approximation, where ionization is independent on the oscillation frequency of the applied laser field, are useful for laser-induced ionization processes in only a very small region of the frequency and intensity domain of laser fields, i.e. in the tunneling regime. The models that include a frequency dependent factor are in a good agreement with the accurate TDSE calculations in both the multiphoton and the tunneling ionization regimes. Furthermore, the deviation from Franck-Condon-like distribution is also clarified. A small effect on the revival time of the vibrational wavepacket of hydrogen molecular ion, due to this deviation, has been found. Consideration of the bond-softening effect leads to a decrease in the revival time with increasing laser-peak intensity. The anisotropy of H2 as a function of laser intensity in linear and circular polarized fields using molecular tunneling model (MO-ADK) are also studied and a good agreement with the experimental observations, especially if the focal volume of the laser field is considered, has been obtained. The applicability of the two-center model for larger molecules, N2 and O2, is tested. It is found that it works with N2 (symmetric HOMO) but fails in O2 (ansymmetric HOMO).