Einfluss der spektralen Phasen von Femtosekunden-Laserpulsen auf Ionisation und Dissoziation von Ethan und DCl

Inhalt der vorliegenden Doktorarbeit ist das Verständnis und die Kontrolle von fs-laserpulsinduzierter Ionisation und Dissoziation. Eine zentrale Rolle spielt in diesem Zusammenhang die spektrale Phase des Laserpulses, welche den elektrischen Feldverlauf charakterisiert. Durch die Manipulation der s...

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Bibliographic Details
Main Author: Schirmel, Nora
Contributors: Weitzel, Karl-Michael (Prof. Dr.) (Thesis advisor)
Format: Dissertation
Published: Philipps-Universität Marburg 2014
Online Access:PDF Full Text
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Table of Contents: Subject of this dissertation was the understanding and control of ionization and dissociation by femtosecond laser pulses. These processes can be controlled through manipulation of the laser field which can be described by the spectral phase. Two prominent spectral phase contributions are the carrier-envelope phase and the linear chirp. The effects of the CEP on the dissociative ionization of DCl were investigated as well as the effect of higher spectral phase terms on the dissociative ionization of ethane and partially deuterated ethane (CH3CD3). For the investigation of the CEP-dependency, the generated fragment ions, D+ and Cl+, and electrons were detected spatially and mass-resolved. The fragment ions are mainly generated in angles of 60° to the laser polarization. Therefore, by means of CEP the orientation of the molecules to be ionized can be chosen. The angular distribution of the fragment ions is representative for this. The fragment ion emissions from DCl+ are antipodal (difference of pi) in their dependency of the CEP. In the electron signal the CEP-dependency of the emission is considerably more pronounced than in the fragment ion signals. The electrons are mainly detected parallel to the laser polarization on the same side of the molecule where the Cl+ leaves. This is in agreement with an ionization from the HOMOs which are twofold degenerated. The manipulation of the higher spectral phase terms was executed using a 4f-pulse shaper. The ions generated were detected and characterized by means of time-of-flight mass spectrometry. First, the formation of H3+ from ethane was investigated more closely. It was confirmed that H3+ is formed by the reaction C2H62+ ? H3+ + C2H3+. Experiments with deuterated ethane revealed that H-Migration is an important step in the process of H3+ formation. The experiments for understanding the ethane ionization and dissociation reactions are complemented by experiments to control these reactions. The manipulation of the quadratic spectral phase term was found to be very effective in influencing the ion yield of all fragment ions as well as the parent ion. Especially the chirp dependency of the parent ion indicates that the ionization plays an important part in the reaction control. Nevertheless, several effects are of importance: On the one hand the fragmentation is increased with increasing pulse duration. This is primary a pulse duration effect. On the other hand a clear dependency of the ionization efficiency on the sign of the chirp parameter alpha can be observed. The combined effects lead to a maximal ionization yield for negative linear chirps. The chirp effect is most pronounced for small laser intensities and is superimposed by an intensity effect for higher pulse energies. Two different mechanisms of control were identified. Both intra-charge-state and inter-charge-state control are feasible. As example for intra-charge-state control the manipulation of the H3+/H+ ratio was found. By inter-charge-state control, the ratio of CH3+ with two different kinetic energy distributions can be manipulated. For the investigation of the ability of more complicated spectral phases to optimize an ion yield or an ion ratio, a genetic algorithm was deployed. In case of the optimization of single ion yields like the H+ or H3+ ion yield the genetic algorithm revealed that the optimal pulse has a high contribution of a negative linear chirp. This is confirmed by the possibility to recompress the optimized pulse by applying an additional positive linear chirp. Again, the result of the optimal laser pulses found by the genetic algorithm is intensity dependent as found in the systematic variation of alpha. The optimized laser pulses decrease in pulse duration as the intensity of the laser pulse increases and the enhancement of the ion yield compared to the ion yield of the 45 fs-pulse is less pronounced. In these cases the quadratic chirp seemed to be the most important parameter to manipulate ion yields, but the use of more complex chirps is justified as well. For the optimization of ion ratios like Y(H3+) / Y(H+) Ymax(CH3+, lower KE) / Ymax(CH3+, lower KE) the experiments have shown more complex pulses to be more suitable. These pulses induce a pronounced effect in the ion ratio which cannot be a pure intensity effect, since the ion ratio for the optimized spectral phase with reversed sign differs significantly from the optimized ion ratio. Thus, it can be concluded that the spectral phase of femtosecond laser pulses and especially its quadratic term can be used as effective tool in many cases of reaction manipulation leading to different ion yields and ion ratios.