Multiphoton ionization via Rydberg states and effects of high laser intensity

A photoionization experiment is characterized by the initial bound state of the atom, the polarization, frequency, intensity and also temporal distribution of the ionizing radiation, as well as the partial saves describing the final continuum state. The basic observable is the total ionization rate which can be differentiated with respect to the kinetic energy of the emitted photoelectrons yielding total and partial photoionization cross-sections. Additional information can be extracted if the angular distribution of either electron emission or spin polarization is measured. The angle-resolved measurement reveals interference effects which average to zero whenever the signal is integrated over all angles, quite analogously to the angular correlations observed, e.g., in γ-γ cascades or scattering of particles. At a high enough laser intensity the atom may be ionized by simultaneous absorption of several photons and photoionization is observed below the threshold for one-photon ionization. The actual light intensity required to produce such a signal strongly depends on whether or not an excitation step is in resonance with an atomic transition. The least intensity is required if photons of different frequencies are present in the light beam, each in resonance with atomic frequencies, leading to resonant stepwise ionization. Depending on the quantum numbers of the resonant intermediate states, the angular distribution may exhibit rather high anisotropy. By varying the polarization and frequency of the photons, one can excite different intermediate states and study their influence on the ionization signal. In this context we will also address the question of how well photoionization of Rydberg states can be described using the electric dipole approximation. 31 references 7 figures