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Radiation therapy today, on account of improvements in treatment procedures over the last 60 years, allows precise treatment of static tumors inside the human body. However, irradiation of moving tumors is still a challenging task as moving tumors often leave the treatment beam and the radiation dose delivered to the tumor reduces simultaneously increasing that on healthy tissue. This research work aims to push the frontiers of radiation therapy in order to enable precise treatment of moving tumors with focus on research and development of a unique real-time system enabling active motion compensation through robotic means to compensate tumor motion. During treatment, patients lie on a treatment couch which is normally used for static position corrections of patient set-up errors prior to radiation treatment. The treatment couch used, called HexaPOD, is a parallel manipulator with six degrees of freedom which can precisely position heavy loads inside a small region. Despite the HexaPOD not initially built with dynamics in mind, it is used in this work for sustained motion compensation by moving patients such that tumors stay precisely located at the center of the treatment beam during the complete course of treatment. In order to realize real-time tumor motion compensation by means of the HexaPOD, several challanges need to be addressed. Real-time aspects are covered by the adoption of a hard real-time operation system in combination with measurement and estimation of latencies of all physical quantities in the compensation system such as tumor or breathing position measurements. Accurate timing information is respected consistently in the whole system and all software-induced latencies are adaptively compensated for. This requires knowledge of future tumor positions from predictors. Several predictors for breathing and tumor motion predictions are proposed and evaluated in terms of a variety of different performance metrics. Extensions to prediction algorithms are introduced fusing both breathing and tumor position information to allow for predictions without the need of an explicit correlation model. Predictions determine the future motion path of the HexaPOD in order to compensate for tumor motion. Several control schemes are developed to enable reference tracking for the HexaPOD. Based on linear and non-linear dynamic modelling of the HexaPOD with system identification methods, a first controller is derived in the form of a model predictive controller. A second controller is proposed based on an assumption of the working principle of the HexaPOD's internal controller. Finally, a third controller is derived as combination of the first and second one. For each of these controllers, comparative results with real hardware experiments and humans in the loop as well as choices of free parameters are presented and discussed. Apart from precise tracking, emphasis is placed on patient comfort which is of crucial importance for acceptance of the system. It is demonstrated that smooth trajectories can be realized by the controllers to guarantee that patients feel comfortable while their tumor motion is compensated at sub-millimeter accuracies. Overall errors of the system are analyzed by relating them to tracking and prediction errors. By exploiting the properties of different predictors, it is shown that the startup time until tracking is reached can be reduced to only a few seconds, even in the case of an initially at-rest HexaPOD and with no initial knowledge of tumor motion. This makes the system especially suitable for the relatively short-fractionated treatment sessions for lung tumors. The tumor motion compensation system has been developed solely based on standard clinical hardware, found in most treatment rooms. With a simple and flexible design, existing treatment can be updated in a cost-efficient way to introduce motion compensation capabilities. Simultaneously, the system does not impose any constraints on state-of-the-art treatment types such as intensity modulated radiotherapy or volumetric modulated arc therapy. Supporting different compensation modes, the system can be applied to any moving tumor whether its motion is predictable (lung tumors) or unpredictable (prostate tumors). By integration of adequate tumor position determination methods, the system can be easily extended to other tumors as well.
Adaptive femtosecond quantum control has proven to be a very successful method in many different scientific fields like physics, chemistry or biology. Numerous quantum systems and in particular molecules undergoing chemical reactions have been controlled using shaped femtosecond laser pulses. This method allows to go beyond simple observation and to obtain active control over quantum--mechanical systems. It uses interference phenomena in the time and/or frequency domain to achieve selectivity. The shaped femtosecond laser pulses employed in this scheme have until recently been purely linearly polarized. Therefore, they only address the scalar properties of light--matter interaction and neglect the vectorial character of both the dipole moment $\vec{\mu}$ and the electric field $\vec{E}(t)$. Especially in the quantum control of chemical reactions the investigated systems ---the molecules--- are three dimensional and exhibit complex spatio--tempo\-ral dynamics. With the help of polarization--shaped laser pulses one is now able to follow these dynamics in both, time and spatial direction, and can therefore reach a new level of control over quantum--mechanical systems. In this work, the generation of polarization--shaped laser pulses has been implemented in an optical setup. It requires no interferometric stability as a result of the identical beam path for both polarization components. Dual--channel spectral interferometry was employed as experimental pulse characterization and a mathematical description of the time--dependent polarization state of these pulses was given. The polarization modulation of the shaped pulses by subsequent optical elements was investigated and some solutions to minimize these modulations were presented. Jones matrix calculus with experimentally calibrated matrices was implemented to account for all polarization distortions from the LCD to the position of the experiment and for full characterization of the generated pulse shapes. Adaptive polarization shaping was demonstrated in a purely optical realization of the learning--loop concept. The learning algorithm was able to find the needed linear polarization in order to maximize second harmonic generation in a nonlinear optical crystal. The closed--loop configuration has proven to be capable to clear up more complicated polarization distortion, which was introduced using a multiple order half--wave plate designed for use at a wavelength of 620~nm. The additional deformation of the spectral phase through dispersion in a 10~cm long SF10 glass rod has also been compensated automatically. After these optical demonstration experiments ultrafast polarization shaping was applied to control a quantum system. Polarization sensitivity was shown in pump--probe measurements of the multiphoton ionization of potassium dimer molecules K$_2$. This sensitivity was exploited in a more general way in a learning--loop experiment with polarization--shaped laser pulses. A qualitatively new level of control was demonstrated using the time--dependent polarization state of laser pulses as an active agent. This polarization control was applicable even in randomly aligned molecules, which is a significant simplification of the experimental setup. In addition to these polarization control experiments, the three dimensional dynamics of molecules were also investigated and controlled. The \textit{cis--trans} photoisomerization of NK88 was studied in the liquid phase by transient absorption spectroscopy. The isomerization reaction efficiency was enhanced as well as reduced using linearly polarized laser pulses at 400~nm shaped in spectral phase and amplitude. This experiment demonstrates the ability to control the large scale motion of complex molecular groups with shaped femtosecond laser pulses. The modification of the molecular geometry can be regarded as a first step towards control of chirality in photochemistry. Especially with the successful demonstration of polarization quantum control, which is required in the theoretical models for the selective conversion of one enantiomer into the other, the way is paved towards coherent control of chirality. Besides these fascinating applications of polarization shaping it should now also be possible to extend the wavelength range of these pulses. Apart from second harmonic generation in order to reach the ultraviolet region intra-pulse difference frequency generation could be an option to open the mid-infrared spectral range for polarization shaping. With these new wavelength regions numerous new perspectives arise for quantum control using polarization--shaped laser pulses. Referring once more to the novel of Edwin A. Abbott presented in the introduction one could say that shaped femtosecond pulses really have left Flatland. Or to put it into the words of the sphere, when it teaches the square about the perception of dimensions: \begin{quote} ``Look yonder [...] in Flatland thou hast lived; of Lineland thou hast received a vision; thou hast soared with me to the heights of Spaceland;'' \hfill Edwin A.~Abbott~\cite{abbott1884}, 1884 \end{quote}
In dieser Arbeit wurden zwei Aspekte der Femtochemie mit den Methoden der Femtosekunden--Laserspektroskopie untersucht. Dabei wurden folgende Ziele verfolgt: Einerseits sollte die jüngst entwickelte Technik der adaptiven Pulsformung auf das Problem bindungsselektiver Photodissoziationsreaktionen angewandt werden, zum Anderen bestand die Aufgabe darin, die nichtadiabatische, photoinduzierte Dynamik am Beispiel der Photoisomerisierung von Stilben mit Hilfe der Photoelektronenspektroskopie zeitaufgelöst zu untersuchen. Die Methode der adaptiven Pulsformung wurde mit dem Ziel eingesetzt, eine bindungsselektive Photodissoziation zu verwirklichen. Dazu wurde diese Technik in Verbindung mit einem massenspektroskopischen Nachweis der Photofragmente verwendet. Die Experimente wurden an einigen Spezies der Methylhalogenide CH2XY (X,Y = Halogen) durchgeführt. Diese Verbindungen wurden als Modellsysteme gewählt, da sich gezeigt hat, dass auf Grund stark gekoppelter konkurrierender Dissoziationskanäle durch modenselektive Laseranregung keine Kontrolle erreicht werden kann. Mit dem hier durchgeführten Experiment an CH2ClBr wurde erfolgreich erstmals die Anwendung der adaptiven Femtosekunden-Pulsformung auf das Problem einer bindungsselektiven Photodissoziation demonstriert. Dabei konnte eine Steigerung der Dissoziation der stärkeren Kohlenstoff-Halogen Bindung um einen Faktor zwei erreicht werden. Weiterhin konnte experimentell gezeigt werden, dass das optimierte Produktverhältnis nicht durch eine einfache Variation der Laserpulsdauer oder Laserpulsenergie erzielt werden kann. Es wurde ein möglicher Mechanismus für die Kontrolle diskutiert, der im Gegensatz zu einem unmodulierten Laserpuls die Wellenpaketdynamik auf neutralen dissoziativen Potentialflächen zur Steuerung des Produktverhältnisses involviert. Wie sich aus einer genaueren Analyse des Fragmentspektrums ergab, wird durch den optimalen Laserpuls die Dissoziation in komplexer Weise moduliert. Dies zeigte sich z.B. auch durch eine Änderung des Isotopenverhältnisses in der Ausbeute des dissoziierten Br-Liganden vor und nach der Optimierung. Dieser Frage nach einer isotopenselektiven Photodissoziation wurde in einem weiteren Experiment an CH2Br2 nachgegangen. Dabei konnte jedoch nur eine geringe Variation von etwa fünf Prozent gegenüber dem natürlichen Isotopenverhältnis festgestellt werden. Als größtes experimentelles Problem stellte sich dabei die starke Intensitätsabhängigkeit der Produktausbeuten heraus, was die Suche nach der optimalen Pulsform stark einschränkte. Anhand des molekularen Photodetachments CH2I2-->CH2+I2 wurde gezeigt, dass durch die Analyse der optimalen Pulsformen Informationen über die Dynamik dieses Prozesses gewonnen werden können. Dazu wurde zunächst in einem Pump-Probe-Experiment die Dynamik der I2-Fragmentation nach einer Mehrphotonen-Anregung von CH2I2 mit 266nm Laserpulsen untersucht. Dieses Experiment ergab, dass das Molekül über einen angeregten Zwischenzustand auf einer sehr schnellen Zeitskala über Dissoziationskanälen zerfallen kann. Der dominante Kanal führt zu einer sequentiellen Abgabe einer der I-Liganden und resultiert in den Photoprodukten CH2I und I Im anderen Kanal, dem molekularen Photodetachment, werden die Photoprodukte I2 und CH2 gebildet. In einem Kontrollexperiment wurde dann versucht, das molekulare Photodetachment gegenüber dem dominanten sequentiellen Kanal mit geformten 800nm Laserpulsen zu optimieren. Es wurden Optimierungen mit dem Ziel der Maximierung der Ausbeute an den Photoprodukten I2 und CH2 gegenüber CH2I durchgeführt. Diese Experimente ergaben, dass für beide Fragmente des molekularen Photodetachments eine Steigerung des Produktverhältnisses um etwa einen Faktor drei möglich ist. Dabei zeigte sich, dass eine Maximierung auf ein Produktverhältnis (z.B. I2/CH2I) eine Steigerung des anderen um etwa den gleichen Faktor hervorruft. Dies ist ein deutlicher Hinweis, dass beide Photoprodukte über denselben Dissoziationskanal gebildet werden. Ein weiterer inweis wurde aus der Analyse der optimalen Pulsformen erhalten: In beiden Fällen weisen diese eine markante Doppelpulsstruktur mit einem zeitlichen Abstand von etwa 400fs auf. Dies erinnert stark an die Situation des Pump-Probe--Experiments, wo durch die Analyse des transienten Signals ebenfalls eine optimale Verzögerungszeit zwischen dem Pump- und Probe-Laserpuls von etwa 400fs ermittelt werden konnte, bei der die Produktverhältnisse gerade maximal sind. Im Vergleich zur Massenspektroskopie liefert die Photoelektronenspektroskopie in der kinetischen Energie der Photoelektronen eine zusätzliche Messgröße, die direkt Informationen über die Kerngeometrie des Systems liefern kann. Mit dieser Technik wurde die trans-cis-Photoisomerisierung von Stilben im ersten elektronisch angeregten Zustand S1(1Bu) zeitaufgelöst untersucht. Dabei ging es speziell um die Frage nach der Existenz eines weiteren 1Bu Zustandes, der in neueren theoretischen Untersuchungen diskutiert wurde. In einem Pump-Probe-Experiment wurde dazu das im Molekularstrahl präparierte trans-Stilben durch einen 266nm Laserpuls angeregt und die Dynamik durch einen weiteren 266nm Laserpuls abgefragt. Im Photoelektronenspektrum konnten zwei signifikante Beiträge mit unterschiedlicher Dynamik gefunden werden. Das transiente Signal des ersten Beitrags weist eine Zeitkonstante von etwa 20ps auf und konnte eindeutig der Isomerisierung des S1 Zustandes zugeordnet werden. Im Gegensatz dazu zeigte das Signal des zweiten Beitrags eine Zeitkonstante von 100fs. Dieses Signal könnte aus der Ionisation des S2 Zustandes resultieren, welcher bislang experimentell nicht beobachtet werden konnte.
The subject of this work has been the investigation of dynamical processes that occur during and after the interaction of matter with pulses of femtosecond laser radiation. The experiments presented here were performed in the gas phase and involve one atomic and several model molecular systems. Absorption of femtosecond laser radiation by these systems induces an electronic excitation, and subsequently their ionization, photofragmentation or isomerization. The specific adjustment of the excitation laser field properties offers the possibility to manipulate the induced electronic excitation and to influence the formation of the associated photoproducts. From the perspective of the employed spectroscopic methods, the development of photoelectron spectroscopy and its implementation in laser control experiments has been of particular interest in this thesis. This technique allows for a most direct and intuitive observation of electronic excitation dynamics in atomic as well as in complex polyatomic molecular systems. The propagation of an intermediate electronic transient state, associated to the formation of a particular photoproduct, can be interrogated by means of its correlation to a specific state of the atomic or molecular continuum. Such correlations involve the autoionization of the transient state, or by means of a second probe laser field, a structural correlation, as summarized by the Koopman's theorem (section 2.4.1). The technique of adaptive femtosecond quantum control has been the subject of development in our group for many years. The basic method, by which the temporal profile of near-infrared laser pulses at a central wavelength of 800 nm, can be adjusted, is a programmable femtosecond pulse-shaper that comprises of a zero dispersion compressor and a commercial liquid crystal modulator (LCD). This experimental arrangement was realized prior to this thesis and served as a starting point to extend the pulse-shaping technique to the ultraviolet spectral region. This technological development was realized for the purposes of the experiments presented in Chapter 5. It involves a combination of the LCD-pulse-shaper with frequency up-conversion techniques on the basis of producing specifically modulated laser pulses of central wavelength 266 nm. Furthermore, the optical method X-FROG had to be developed in order to characterize the often complex structure of generated ultraviolet pulses. In the adaptive control experiments presented in this work, the generated femtosecond laser pulses could be automatically adjusted by means of specifically addressing the 128 independent voltage parameters of the programmable liquid-crystal modulator. Additionally a machine learning algorithm was employed for the cause of defining laser pulse-shapes that delivered the desired (optimal) outcome in the investigated laser interaction processes. In Chapter 4, the technique of feedback-controlled femtosecond pulse shaping was combined with time-of-flight mass spectroscopy as well as photoelectron spectroscopy in order to investigate the multiphoton double ionization of atomic calcium. A pronounced absolute enhancement of the double ionization yield was obtained with optimized femtosecond laser pulses. On the basis of the measured photoelectron spectra and of the electron optimization experiments, a non-sequential process was found, which plays an important role in the formation of doubly charged Calcium ions. Then in Chapter 5, the dynamics following the pp* excitation of ethylene-like molecules were investigated. In this context, the model molecule stilbene was studied by means of femtosecond photoelectron spectroscopy. Due to the simplicity of its chemical structure, stilebene is one of the most famous models used in experimental as well as theoretical studies of isomerization dynamics. From the time-resolved experiments described in that chapter, new spectroscopic data involving the second excited electronic state S2 of the molecule were acquired. The second ethylenic product was the molecule tetrakis (dimethylamino) ethylene (TDMAE). Due to the presence of numerous lone pair electrons on the four dimethylamino groups, TDMAE exhibits a much more complex structure than stilbene. Nevertheless, previously reported studies on the dynamics of TDMAE provided vital information for planning and conducting a successful optimisation control experiment of the wavepacket propagation upon the (pp*) S1 excited potential surface of the molecule. Finally, in Chapter 6 the possibility of employing femtosecond laser pulses as an alternative method for activating a metallocene molecular catalyst was addressed. By means of an adaptive laser control scheme, an optimization experiment was realized. There, the target was the selective cleavage of one methyl-ligand of the model catalyst (Cp)^2Zr(CH3)^2, which induces a catalytic coordination position on the molecule. The spectroscopic studies presented in that chapter were performed in collaboration to the company BASF A.G. and constitute a proof-of principle attempt for a commercial application of the adaptive femtosecond quantum control technique.