Ultrafast population transfer and ionization mechanisms in intense laser fields

This thesis addresses different aspects of atoms interacting with strong laser fields. The focus is on population and ionization dynamics occurring on ultrafast timescales, when atoms are subjected to intense extreme ultraviolet or infrared laser light. Different ways of modeling these interactions are considered. Comparisons to experimental results are performed in certain cases, and in other cases predictions are made for future experiments.

This thesis comprises five papers.

In paper I, we investigate resonant photoionization of helium atoms through an experiment performed using a free-electron laser, and through numerical simulations and a model based on Rabi oscillations.

In paper II, we consider another photoionization experiment performed using a free-electron laser, where the photon energy is resonant with a transition in the ion. The entanglement between the ion and photoelectron is studied with a model and with numerical simulations.

In paper III, resonant ionization is studied using the effective Hamiltonian formalism, and a prediction is made about stabilization of dressed states.

In paper IV, the RABBIT technique applied to an atom undergoing Rabi oscillations is studied theoretically, through numerical simulations and perturbative calculations.

In paper V, a model for frustrated tunneling ionization, based on the strong-field approximation is studied in detail, and compared with results from numerically solving the time-dependent Schrödinger equation.