Transitioning from a dipole to a relativistic response in atoms and molecules
Date
2021
Authors
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Publisher
University of Delaware
Abstract
Light-matter interaction has been a major avenue of study for decades. With the advent of the laser, precise control, manipulation and study of atoms and molecules has been possible, and as the power output of lasers has increased, so has our understanding of the interaction of light with matter. As the peak power of lasers increase from the terawatt to petawatt and soon the exawatt, our comprehension of the interaction with atoms and molecules with these intense light elds must grow. Our understanding of ionization processes such as tunneling ionization, dissociative ionization and enhanced ionization have guided us through intensities of up to 10^19 W/cm2, but as the intensity of light reaches farther into the relativistic regime, these ideas must be extended. The work in this thesis presents both theoretical and experimental results that try to extend the process of ionization into this province. ☐ We present a classical, relativistic Monte Carlo calculation investigating the role of the magnetic eld on bound state dynamics and ionization for atoms in ultrastrong external radiation fields. Atoms with atomic numbers 1 < Z < 20 in external fields from 0.01 a.u. to 103 a.u. are studied. We show that in calculations of ionization rates, the dipole approximation of the field yields accurate values compared to a full treatment of classical electromagnetic laser field for intensities up to 10^23 W/cm2. The calculations indicate the quasi-static approximation is valid for external field frequencies less than one-twentieth of the Kepler orbit time for the ionizing state. For ionization at fields above 100 a.u., the magnetic field affects the atom by altering the portion of the bound state that ionizes and deflecting the mean emission angle of the photoelectron angular distributions, though no change in the width of the final state angular emission about this mean occurs. The photoelectron distribution and change in the ionizing bound state are seen as a first indication of nonperturbative, magnetic field effects for atoms in ultrastrong radiation fields. ☐ The ionization of chlorinated methane molecules (CH3Cl) in strong and ultrastrong (10^14-10^18 W/cm2) laser fields is experimentally investigated. The charge states of carbon (Cm+, 1 <= m <= 4) exhibit characteristics of molecular ionization, such as Coulomb explosion, and kinetic energies of release have been determined. The energy spectra of the ions show no dependence on the intensity, indicating that the energy comes entirely from a dissociation process. The charge states of chlorine show similar results, with a some correlation between the charge states of carbon and chlorine. A simple 1D classical model of an aligned CCl ion is used to model the interaction and shows that enhanced ionization is a driving influence for lower charge states of carbon and chlorine, but as the intensity grows the response becomes more atomic in nature. ☐ Recollision of an electron with a parent ion for a laser driven atomic system is investigated in the relativistic regime via a strong field quantum description and Monte-Carlo semi-classical approach. We find the relativistic recollision energy cutoff is independent of the ponderomotive potential Up, in contrast to the well-known 3.2Up-scaling. The relativistic recollision energy cutoff is determined by the ionization potential of the atomic system and achievable with non-negligible recollision flux before entering a "rescattering free" interaction. The ultimate energy cutoff is limited by the available intensities of short wavelength lasers and cannot exceed a few thousand Hartree, setting a boundary for recollision based attosecond physics.
Description
Keywords
Dipole, Relativistic response, Atoms, Molecules, Light matter, Lasers