Control of molecular processes with intense laser pulses

Abstract

This thesis describes the results of experimental research on the possibilities of controlling and probing molecular processes by ultrafast, intense laser pulses. Three directions were investigated: The first is control of femtosecond nuclear and electronic beating dynamics in polyatomic molecules, using CO2 as an example, by determining the pathways that are taken during double ionization with two delayed laser pulses. The strong field sequential double ionization in CO2 proceeds via populating different intermediate states in CO2+, depending on the selective removal of valence or inner valence electrons by appropriately choosing the peak intensities of the two delayed laser pulses. This allowed control over these pathways and the dynamics taking place on these states. As a second research direction, using the acetylene molecule as an example, it was investigated whether multielectron ionization and fragmentation processes, which are of fundamental importance in particular for hydrocarbon molecules, can be controlled using impulsive laser-alignment by exploiting the angular dependence of the multi-electron ionization probability. Ionization in acetylene is enhanced when C-H inter-nuclear distance is stretched (Enhanced ionization) and the binding energy of the inner valence - orbitals is lowered. The involvement of inner valence - electrons in ionization, makes this ionization mechanism highly directional along the laser polarization direction and can be controlled actively by impulsive laser-alignment. The third class of experiments was dedicated to the investigation, whether electronic trapping processes that result in the population of highly-excited Rydberg states and which are thought to be of high importance for determination of chemical reactivity can be controlled by the cycle-shape of the laser electric field that is determined by the carrier-envelope offset phase of few-cycle laser pulses. For these experiments the well-studied prototype system of the argon dimer was used. In the experiments we observed a strong CEP dependence of the sum momenta of fragment ions for both ionization and frustrated ionization (electron trapping) from argon dimers. The analysis of electron trapping mechanism as a function of CEP reveals that the trapped electron imparts negligible momentum to the argon dimer and such electrons are born close to the field maximum. All experimental data in this thesis were recorded using the method of coincidence ion momentum imaging with a reaction microscope.