Ionization and fragmentation of atoms and molecules in strong laser fields

Abstract

This thesis employs a novel momentum imaging technique to investigate the interaction of atoms and molecules with strong laser fields. The goal is to gain better insight into their behavior under such interaction and apply this knowledge to steer and control some of the most important atomic and molecular processes. One of these processes is molecular fragmentation. The first studied experiments deal with the deprotonation process in hydrocarbons after being doubly ionized by intense laser pulses. A feature makes this fragmentation special is that it happens on nanosecond to microsecond timescale, in contrast to femtosecond or picosecond timescale on which most fragmentation processes happen. We find the lifetime of the dications leading to this fragmentation and investigate the effect of laser pulse parameters on this process. Furthermore, quantum chemical simulations are performed to find the origin of this delayed deprotonation. In the next study, we report experiments on direct observation of molecular oxygen production from the fragmentation of doubly ionized carbon dioxide in strong laser fields. Our accompanying simulations and pump-probe measurements indicate that carbon dioxide molecules undergo bending motion during strong-field ionization which supports the formation of molecular oxygen. One of the other processes is frustrated tunneling ionization, in which an ionized electron is recaptured by the parent ion in some high-lying Rydberg states within the three-step model of strong-field interaction. In this regard, we investigate such high-lying Rydberg states produced in strong laser fields by detecting electrons, emitted after the strong-field interaction, together with the parent ions in coincidence measurements. Simulations show that both tunneling ionization by the weak dc field of the reaction microscope spectrometer and photoionization by blackbody radiation contribute to delayed electron emission on nanosecond to microsecond timescale. By employing this technique of detection, in the next step, we demonstrate control over the localization of the high-lying Rydberg wave packets in argon atoms with orthogonally polarized two-color laser fields. The measured ionization signals of high-lying Rydberg states show that the dc-field induced ionization yield oscillates with the relative two-color phase with a period of 2Ï , while the photoionization signal by blackbody radiation shows a period of Ï . Simulations indicate that this observation is a clear signature of the asymmetric localization of electrons recaptured into very elongated (with low angular momentum) high-lying Rydberg states after conclusion of the laser pulse.