| Atoms exposed to high-intensity femtosecond laser pulses have double-ionization rates many times larger than any theory of independently ionizing electrons predicted. This non-sequential or (“excess”) double ionization has appeared in a number of experiments using a wide range of different atoms and molecules. Unlike most strong-field processes, non-sequential double ionization diminishes in magnitude as the intensity of the incident laser field approaches the intensity of the Coulomb field. In this thesis, numerical simulations examine the time evolution of two-electron atoms in order to understand the physical processes leading to non-sequential double ionization.; The results of simulations which directly solve Schrödinger's equation reveal non-sequential double ionization is the result of a two-step process. In the first step, one electron is pulled from the core, but not necessarily ionized, by the applied laser field. In the second step, this electron revisits the nucleus. Given the appropriate phase of the laser field during this revisit, two electrons may share energy and escape the Coulomb potential. Once double ionization has occurred, the ionized electron probability density leaves the vicinity of the nucleus, forming double-ionization jets with inter-connected velocities.; This description is notable in its nearly classical description of non-sequential double ionization. To gain further insight, the two-electron Hamiltonian was also modeled classically. Purely classical calculations using large ensembles of two-electron atoms were examined and the same general behavior is seen in both classical and quantum simulations. An analysis of the precise positions and momenta of classical trajectories reveals further information about the energy range of the revisiting electron as well as the timing and energy of outgoing double-ionization jets. Specifically, the revisiting electron is found to return to the nuclear core in opposition to the laser force. |
