Novel field-ionization behavior of very high Rydberg states of Ar (
n=50−−250) is observed using sequential pulsed electric fields. Fast autoionization acts as a filter against adiabatic field ionization near the
2P1/2 threshold, whereas both diabatic and adiabatic field ionization occur near the
2P3/2 threshold. Sequences of up to 9 equivalent pulses yield recurrent field ionization signals near the
2P3/2 threshold, provided that the field returns close to zero between pulses, demonstrating a statistical redistribution of the Rydberg population amongst all states of a given Stark manifold near zero field.
Theoretical and experimental studies show the strong influence of the three-photon nearly resonant 2p state on four- and five-photon ionization of atomic hydrogen near the threshold for four-photon ionization. Changes in five-photon ionization occur when the four-photon ionization channel opens. The angular distributions of photoelectrons from five-photon ionization of H are studied at five wavelengths which cover the range from four-photon resonance with high-lying Rydberg states (n\ensuremath{\ge}10) to direct four-photon ionization into the continuum. The role of resonances in this ionization process is discussed. A fair agreement is found in comparing experimental and theoretical results.