Two-Photon Ionization and Dissociation Dynamics in Atoms and Molecules Studied Using Vacuum Ultraviolet Laser Harmonics and Coincidence Momentum Imaging

The correlated motion of electrons within gas-phase atoms and molecules influences various fundamental physical and chemical processes. In molecular systems, these complex electron dynamics may become coupled to the motion of the nuclei, resulting in many-particle dynamics that drive chemical transformations, such as isomerization or fragmentation. The valence electrons and their dynamics within atoms and molecules can be probed using vacuum ultraviolet (VUV) light to photoionize or photodissociate the system. The charged particles generated in such an interaction encode information on the many-particle system and its properties, with different types of measurements extracting differing levels of this information content. In order to reach a more complete description of the system, including the many-electron dynamics and coupled electron-nuclear motion, higher information content measurements are necessary. This involves going beyond photoelectron or ion yield measurements, instead turning to combined energy- and angle-resolved experiments, while leveraging the power of coincidence methods, allowing the total wave function of the system to be interrogated accurately. In this thesis, the valence electron dynamics in atoms and diatomic molecules are investigated using an intense femtosecond VUV light source to drive two-photon absorption and a coincidence 3-D momentum imaging spectrometer to measure the generated charged particles. This enables energy- and angle-resolved measurements on electrons and ions to be performed in coincidence, providing a multi-modal measurement carrying increased information content, allowing the many-electron and non-adiabatic dynamics to be explored and understood./nThe Ti:sapphire laser system used to generate ultrashort VUV pulses, the VUV beamline, and the experimental endstation, including the charged particle 3-D momentum imaging spectrometer, are discussed thoroughly in the following chapters. In the experiments presented in this thesis, the near-infrared laser pulses from the Ti:sapphire system are frequency doubled to create bright 400~nm femtosecond pulses that are then used to produce VUV radiation via high harmonic generation (HHG). Going to the second harmonic of the fundamental frequency enables high-brightness 9.3~eV femtosecond pulses, the third harmonic, to be generated with pulse energies of roughly 50~nJ, i.e. greater than $10^{10}$ photons at 9.3~eV per pulse. Such pulse energies enable two-photon absorption to be efficiently driven in the VUV, both resonantly and non-resonantly. Techniques for selecting different harmonics from the VUV frequency comb and attenuating the driving field are covered. Energy- and angle-resolved measurements on photoelectrons and photoions are performed using a coincidence 3-D momentum imaging spectrometer, also known as a reaction microscope or COLTRIMS (Cold Target Recoil Ion Momentum Spectrometer), specifically designed for the characteristics of the HHG based VUV light source./nA detailed experimental investigation of the angle-resolved non-resonant one-color two-photon valence ionization dynamics of isolated argon atoms is presented. This study reports the first measurements of the photoelectron angular distribution from non-resonant two-photon ionization of argon, finding that the photoelectron angular distribution is shaped by the interference between different angular momentum components of the photoelectron scattering wave function, which exhibits maximum intensity perpendicular to the ionizing VUV field. By comparing these results with a previous set of theoretical calculations, which have remained unverified for more than a few decades, this work reveals that electron-electron correlation significantly influences the photoionization dynamics. A thorough experimental and theoretical investigation of the energy- and angle-resolved resonant one-color two-photon absorption and dissociation dynamics of single O$_2$ molecules is also provided. This study reports the observation of two narrow and nearly degenerate autoionizing states of different symmetry that are dipole-forbidden, which both can decay through internal conversion to ion-pair states of the same total symmetry. This decay process competes with and interrupts autoionization. These experimental measurements are compared with a set of theoretical calculations, which indicate that these two resonances are excited by parallel-parallel and parallel-perpendicular two-photon transitions, and that the autoionizing states are directly accessed, without intensity borrowing. The calculations also reveal the electronic states participating in the two-photon transitions and Fano line shapes of the involved resonances. Finally, a comprehensive experimental and theoretical study of the energy- and angle-resolved non-resonant one-color two-photon valence ionization dynamics of isolated nitrogen molecules is presented. This investigation represents the first measurements of the photoelectron angular distribution from non-resonant two-photon valence ionization of N$_2$. It is found that the photoelectron angular distributions associated with the X$^2\Sigma_g^+$ and A$^2\Pi_u$ ionic states both vary with changes in photoelectron kinetic energy of only a few hundred meV. The rapid evolution in the photoelectron angular distributions can be attributed to the excitation and decay of dipole-forbidden autoionizing resonances, which belong to series of different symmetries, all of which are members of the famous Hopfield series.

Advisor: Falcone, Roger