Sub-femtosecond tracing of molecular dynamics with strong laser fields

Abstract

Molecules can undergo ionization during the interaction with intense laser pulses. The ionization may lead to a change of the electronic state of the molecule which is accompanied by a rearrangement of molecular structure. Considering double ionization, the change of structure modifies the conditions for the second ionization step. Thus the electronic and nuclear dynamics are coupled, and a complete understanding of this coupled dynamics requires to measure both of them simultaneously. However, due to the large difference in their mass, the electrons and nuclei move on different time scales. The nuclei move at a time scale ranging from picoseconds to about ten femtoseconds and the motion of the electrons may range into the attoseconds. While a number of methods for the measurement of attosecond electronic and femtosecond nuclear dynamics have been developed, it is in general difficult to capture simultaneously both the fast electronic and the slower nuclear motion. One way to achieve this is a use of the Coulomb explosion imaging in combination with an angular streaking method. While the angular streaking exploits the mapping of a fast rotating electric field vector of a laser pulse onto the electron momentum, the Coulomb explosion imaging uses the kinetic energy released upon the ionization to determine the intermolecular distance. In this Thesis, using the outlined experimental framework, we investigate the coupled dynamics between the nuclei and the electrons. In the first chapter, the timing between the two steps of double ionization provides a sub-femtosecond timing reference for the nuclear motion. Using the few-cycle duration of a laser pulse, the interval between the emission of two electrons from a molecule is reduced down to half femtosecond. This allows imaging of the early stages of a molecular vibrational wave-packet with an unprecedented spatio-temporal resolution surpassing the resolution of a standard pump-probe technique. The second chapter extends this approach with the field shaping via the carrier-envelope phase. Near-circularly polarized laser pulses are used to manipulate the electron emission dynamics during a stretch motion of a molecule. The third and fourth chapters treat the ionization dynamics at longer internuclear distances. The third chapter focuses on the covalent bonds where electrons can move between the nuclei, and it is shown that the laser-driven electronic motion in the molecule might influence the ionization process. The fourth chapter investigates molecular dimers where the charge is bound to a molecule, the mobility across the dimer is suppressed and the influence of the near static charge on the ionization is studied.