Potential barrier effects in multiphoton ionization processes and intense laser acceleration of electrons in highly charged ions

This dissertation deals with two different subfields of atomic physics, thus it is divided into two different parts. The first part (Chapters 2 and 3) investigates the potential barrier effects in multiphoton ionization of rare gas atoms, which are resonance-like enhancements of generalized multiphoton cross sections for XUV ionization of Ar, Kr, and Xe. The resonant-like behavior is demonstrated and analyzed within a single-active-electron, central-potential model. It is found that the resonance originates from the potential barriers experienced by intermediate- and final-state photoelectron wave packets corresponding to absorption of one, two, or three photons. The resonance-like profiles in the generalized three-photon ionization cross sections are shown to be similar to those found in the generalized two-photon ionization cross sections. Owing to the similarities found in both two- and three-photon generalized cross sections, we expect such potential barrier effects to be general features of multiphoton ionization processes in most atoms with occupied p and d subshells. The complexity of Cooper minima in multiphoton ionization processes is also discussed. The second part (Chapter 4) presents intense laser acceleration of electrons in hydrogenlike highly charged ions, which can be ionized and subsequently accelerated up to GeV energies by a tightly-focused laser pulse with intensity approaching 1022 W/cm2 . Both quantum mechanical and classical relativistic approaches are briefly reviewed. Up to fifth order corrections to the paraxial approximation of the laser field are taken into account in classical relativistic simulations. It is shown that both the angle and energy of the laser accelerated electron depend on its initial position in the laser focus. The results presented in Chapters 2 and 3 have been published, while those in Chapter 4 are currently under preparation for submission