AB INITIO RATES OF MULTIPHOTON IONIZATION OF THE ISOELECTRONIC SPECIES H, HE+, AND LI2+ IN THE VUV AND XUV FREQUENCY REGIONS

Ab initio theoretical investigation of two- and three-photon ionization of the isoelectronic species H, He${}^{+}$, and Li${}^{2+}$ in the vacuum ultraviolet and x-ray ultraviolet frequency regions, is carried out using the fully nonperturbative Floquet-coupled-channel method. We present the results for the total ionization rates in the wavelength region between approximately 260 to 10 nm, and in the intensity domain between about ${10}^{10}$ and ${10}^{17}$ W/cm${}^{2}$, depending on the member of the isoelectronic species investigated. Comparison with the perturbation theoretical results at moderate intensities is made, a corresponding scaling law is tested, and the finite values of the rates of ionization at the resonance frequencies are obtained. The dependence of the rates on the intensity of the field shows the presence of the so-called ``stability windows,'' for all three species investigated here. The role of polarization on the ionization rates shows a general dominance of the linear polarization over the circular polarization in the intensity and frequency domains investigated, for all the three species concerned.