Momentum mapping of multi-electron ionization dynamics

Chemical events are generally perceived as a bond breaking and bond creation which proceed on time-scales ranging from tens of femtoseconds to microseconds. These processes, however, are preceded and ultimately determined by much faster processes - intra-molecular electron dynamics - that occur on an attosecond time-scale. The broader goal of this project is to apply the methods of Attophysics to the study of electronic processes in atoms and molecules to bridge the gap between the conventional understanding of "slow" chemistry and the underlying much faster electronic dynamics. The emergence of Attophysics is linked to the development of laser-driven XUV sources of isolated sub- femtosecond pulses that permit direct attosecond resolution. However, such XUV pulses predominantly probe the strongly bound inner-shell electrons of atoms or molecules and are not suitable to probe the dynamics of chemically-relevant outer shell electrons. The latter can be interrogated with strong laser fields in combination with electron parent-ion recollision, typically by recording higher-order harmonic spectra, which are sensitive to the bound electronic structure. Despite its simplicity, such an optical experiment reveals little or no information on ionization and/or fragmentation dynamics. Therefore the method of choice in our project relies on the detection of the emerging charged particles rather than photons. Our approach is based on the powerful apparatus for cold target recoil ion momentum spectroscopy (COLTRIMS) - recently constructed and tested in the Vienna lab - which is capable of measuring three-dimensional momentum vectors of the electrons and ions originating from the interaction of a single atom/molecule with a strong laser field. Information on attosecond dynamics can be retrieved from the measured momentum spectra of the electrons or ions, provided their emission is linked to a suitably fast reference process such as the oscillations of the electric field in a strong laser pulse, which can modulate the measured spectra on a sub-femtosecond time-scale. The aim of the proposed project is to study correlated multi-electron ionization dynamics in atoms and (dissociating) molecules. The complexity of tracing attosecond electron dynamics dramatically scales with the number of the electrons that need to be resolved. To gain the extra degrees of freedom required to manipulate a complex multi-electron system, we will drive the COLTRIMS experiment with fully controlled two-color polarization- and phase-encoded strong laser fields synthesized with a carrier-envelope phase (CEP) stabilized Ti:sapphire laser setup in our lab. By combining these two advanced techniques - particle momentum mapping and controllable strong laser field shaping - we strive to observe the internal electron dynamics that lead to the ionization event. The proposed project will consist of two main research routes: (1) Multi-electron recollision-free strong-field ionization dynamics, and (2) attosecond time-space-energy mapping of recollision induced impact ionization dynamics. The first route focuses on time-resolving the process of detachment of several electrons in strong circularly polarized laser pulses. By temporally restricting the process of multi-electron ionization to a single laser oscillation period we can map the direction of electron emission to their instant of emission and at the same time investigate the transition from uncorrelated (sequential) emission to a possibly correlated manner. The second route is dedicated to the temporally resolved measurement of strongly correlated double ionization dynamics initiated by the impact of an electron that was driven back to the parent ion by the strong laser field. By temporally and spatially shaping the trajectories of the recolliding wave packets in two spatial dimensions we can in addition to the well known time-energy mapping inherent to electron recollision establish a mapping between time and space and thus extract significantly more information about the process.

Chemical events are generally perceived as bond breaking and bond creation, which proceed on time-scales ranging from about ten femtoseconds to nanoseconds. These processes, however, are preceded and ultimately determined by much faster processes intra-molecular electron dynamics that occur on an attosecond time-scale (1 attosecond =10^-18 seconds). This project was investigating electronic processes in atoms and molecules in order to bridge the gap between the conventional understanding of slow chemistry and the underlying much faster electronic dynamics. Intense ultrashort laser pulses with a precisely known evolution of the electric field, partly synthesized from pulses of different colours, were used as a temporal reference on the attosecond time-scale. The three-dimensional momentum vectors of ions and electrons emerging during the interaction of these laser pulses with atoms and molecules were measured using electron-ion coincidence spectroscopy. Attosecond resolution was achieved by linking these vectors to the known laser electric field evolution. Highlights of this project include (i) the development and first demonstration of a method that allows the measurement of the complex phase of a bound quantum state during a very fast restructuring with sub-10-attosecond resolution, (ii) the first demonstration of attosecond laser-control over fragmentation reactions in polyatomic molecules, (iii) the finding that a controlled distortion of the bound electronic cloud can be used to dictate the dynamical pathway in a fragmentation reaction of a polyatomic molecule, and the first successful experimental application of this finding, (iv) the discovery of a new ionization mechanism in polyatomic molecules, (v) the demonstration that electron-electron correlation, one of the most fundamental mechanisms in physics, can be controlled on the attosecond time-scale using precisely shaped two-dimensional laser fields.