Strong Field Interactions with Atoms and Molecules

Atoms and molecules exposed to strong fields of magnitude comparable to their internal binding forces undergo ionization. This process sets the ground for multiple strong-field ionization phenomena such as above threshold ionization (ATI). This dissertation addresses two separate ionization problems, the dc Stark ionization of H2O valence orbitals, 1b1, 1b2, and 3a1, within the framework of non-Hermitian quantum mechanics, and ATI for a model-helium atom as part of a review of a previous quantitative approach based on the strong-field approximation (SFA).

Calculations of the dc ionization parameters, dc Stark shift and exponential decay rates, for the H2O valence orbitals are carried out by solving the Schrödinger equation in the complex domain. Two independent models are implemented in the study of static ionization of the molecular orbitals (MOs). In the first one, a spherical effective potential obtained from a self-consistent calculation of H2O orbital energies is combined with an exterior complex scaling approach to express the problem as a system of partial differential equations that is solved numerically using a finite-element method. In the second approach, a model potential for the H2O molecule is expanded in a basis of spherical harmonics and combined with a complex absorbing potential that results in a complex eigenvalue problem for the Stark resonances.

The second part of this investigation is concerned with the study of ATI for atoms subjected to a strong laser field. The convergence of the ionization spectrum for a model-helium atom is addressed in a systematic study that is carried out following Keldysh's formalism of SFA. A generalized compact expression for the ionization amplitude that incorporates electron rescattering into the analysis is explored as well. Additionally, a model based on the concept of quantum paths is implemented in the numerical evaluation of the SFA transition amplitude. In this analysis, a coherent sum over all allowed quantum trajectories that render the action stationary is carried out. This calculation allows to generate an ATI spectrum that converges to the numerical Keldysh amplitude as the number of trajectories increases.