Author: Dietrich Kiesewetter

Publisher:

ISBN: OCLC:1147878007

Category: Electrons

Page: 153

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The techniques provided by attosecond physics are versatile and powerful methods for probing the dynamics of electrons on an ultra-fast time scale. In this work, two experiments that apply and extend these techniques to measure different quantities or new phase spaces are presented. In the first experiment, the binding potential of the gas used in attosecond metrology is studied from its influence on the experiment. This influence is typically treated as an artifact to be calculated and removed, but this cannot be done accurately close to threshold where the binding potential is most influential. A simple pseudo-potential is developed to extract a coarse description of the binding potential. It is defined by the average value and slope of the potential experienced by an escaping electron within the first 1.4 femtoseconds after its ionization, followed by a constant region which later asymptotically approaches zero. A simple correspondence between the parameters of this pseudo-potential and the experimental observables is found using the attosecond metrology technique reconstruction of attosecond beating by interference of two-photon transitions (RABBITT), augmented with additional readily-available measurements. This technique is applied to study the binding potentials of helium and neon. The experimentally extracted pseudo-potential parameters suggest the core of the helium potential is both deeper and steeper than that of neon, and that neon contains an effective repulsive potential outside the core region. The model is corroborated by the good agreement of the extracted long-range character of the binding potentials with an advanced numerical simulation of a similar experiment. In the second experiment, an experimental scheme to dissect strong field processes is introduced. The study of strong field processes is limited by the coupling of the ionization and driven continuum motion steps by the strong field and the weighting of the contributing phase space by the strong field ionization rate. In the proposed scheme, the strong field ionization step is replaced by single-photon ionization using a sub-cycle extreme-ultraviolet (XUV) pulse, bypassing the restrictive ionization rate weighting and selectively ionizing a narrow phase space. By changing the delay between the XUV pulse and the strong field, the phase space under study is controlled. The experimental application of this scheme to study non-sequential double ionization (NSDI) of helium is explored and found to require wavelengths longer than the 800 nm light used in most strong field experiments and intensities much higher than probed in previous XUV-seeded strong field studies. Preliminary experiments to demonstrate the phase control of the XUV-seeding process, performed at both 800 nm and 1300 nm with a range of intensities both below and within the targeted regime for NSDI of helium, show the expected ionization phase dependence of detected photoelectrons and ions. The XUV-seeded photoelectron yield is found to have a significant on-axis enhancement over either the XUV or strong field alone. Many of the other observed features are well attributed to photoelectron streaking, as corroborated by numerical simulations of the experiment. Suggested improvements and experimental designs for the next iteration of this experiment are outlined.