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 Li2+ in the vacuum ultraviol et and x-ray ultraviolet frequency regions, is carried out using the f ully nonperturbative Floquet-coupled-channel method. We present the re sults 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 isoelectroni c species investigated. Comparison with the perturbation theoretical r esults at moderate intensities is made, a corresponding scaling law is tested, and the finite values of the rates of ionization at the reson ance frequencies are obtained. The dependence of the rates on the inte nsity of the field shows the presence of the so-called ''stability win dows,'' for all three species investigated here. The role of polarizat ion on the ionization rates shows a general dominance of the linear po larization over the circular polarization in the intensity and frequen cy domains investigated, for all the three species concerned.