Ultrafast Photoionization Dynamics Studied with Coincidence Momentum Imaging Spectrometers

Sammanfattning: The time scale of the dynamics in atoms and molecules varies from attoseconds (10-18) to picoseconds (10-12) depending on the size of the particles. To study such dynamics, ultrafast light pulses are needed to trigger and capture the reaction. One of the most direct methods consists in ionizing the system and observing the following response. In this work, we use two different light sources with coincidence momentum imaging spectrometers. In the first experiment, we study electron wave packet interferences, while the second experiment focuses on electron and nuclear dynamics after multiple photoionization.In the first study, the experiment is performed with XUV attosecond pulses and a weak few-cycle infrared (IR) field. The attosecond pulses are produced from high harmonic generation (HHG) driven by an intense high-repetition-rate few-cycle IR field based upon optical parametric chirped pulse amplification (OPCPA). A weak fraction of the IR field is sent together with the attosecond pulses into a helium gas target. The photoelectrons are measured with a newly built momentum imaging spectrometer, which detects electrons over a full solid angle. The photoelectron momentum distribution is focused to be asymmetric relative to the plane perpendicular to the polarization axis and carrier-envelope phase (CEP) dependent. This asymmetry in photoemission can be explained by an interference phenomenon controlled by the relative amplitude and phase of the attosecond pulses.In the second study, the dissociation dynamics of the polyatomic molecule methyl iodide (CH3I) was investigated with the intense XUV free electron laser in Hamburg (FLASH). We developed a model to describe the ionization and dissociation dynamics assuming a 3-fold symmetry. This model allowed us to study the charge transfer mechanism from the iodine atom to the methyl group (CH3). Another mechanism contributing to the ionization of this methyl group was identified.This thesis contributes to the development of ultrafast and high-repetition-rate femtosecond lasers and attosecond light sources as well as three-dimensional momentum spectrometers. Hopefully, these tools will lead to further knowledge on ultrafast atomic and molecular physics.