We investigate resonant strong field ionization of atomic hydrogen with respect to the 1s-2p-transition. By "strong" we understand that Rabi-periods are executed on a femtosecond time scale. Ionization and AC Stark shifts modify the bound state dynamics severely, leading to nonperturbative signatures in the photoelectron spectra. We introduce an analytical model, capable of predicting qualitative features in the photoelectron spectra such as the positions of the Autler-Townes peaks for modest field strengths. Ab initio solutions of the time-dependent Schrödinger equation show a pronounced shift and broadening of the left Autler-Townes peak as the field strength is increased. The right peak remains rather narrow and shifts less. This result is analyzed and explained with the help of exact AC Stark shifts and ionization rates obtained from Floquet theory. Finally, it is demonstrated that in the case of finite pulses as short as 20 fs the Autler-Townes duplet can still be resolved. The fourth generation light sources under construction worldwide will provide bright, coherent radiation with photon energies ranging from a tenth of a meV up to tens of keV, hence covering the regime studied in the paper so that measurements of nonperturbative, relative AC Stark shifts should become feasible with these new light sources.

The fluctuations in the electronic charge density of metallic clusters in response to an approaching electron suppress the single-ionization channel. This conclusion is made on the basis of numerical calculations for the total ionization cross sections using the random-phase approximation with exchange to describe the particle-hole (de)excitations. The general trends can be understood by means of the Thomas-Fermi model. The present theory explains, for the first time, the behavior of the measured total ionization cross section for C(60). The interplay between finite size and non-local screening effects is studied by tracing the changes in the ionization cross sections for Li clusters with an increasing cluster radius.

No abstract

Following the framework of the variable-phase approach, we derive an equation for determining the scattering amplitude of a non-relativistic quantum particle in a non-local potential. Its solution implies the integration of the Volterra integrodifferential equation of the first kind and allows determination of bound-state energies and wavefunctions. A fast numerical scheme for the solution of these equations is suggested and it is demonstrated that the proposed method requires the numerical efforts of the same order as in the local potential case.