Abstract
In this work, we present the photon energy dependence of the two- and three-photon cross sections of the two-electron Li+, Ne8+ and Ar16+ ions, following photoionization from their ground state. The expressions for the cross sections are based on the lowest-order (non-vanishing) perturbation theory for the electric field, while the calculations are made with the use of an ab initio configuration interaction method. The ionization cross section is dominated by pronounced single photon resonances in addition to peaks associated with doubly excited resonances. In the case of two-photon ionization, and in the non-resonant part of the cross section, we find that the 1D ionization channel overwhelms the 1S one. We also observe that, as one moves from the lowest atomic number ion, namely Li+, to the highest atomic number ion, namely Ar16+, the cross sections generally decrease.
Highlights
During the last decade, the advent of new light sources, capable of delivering intense and/or ultrashort, coherent radiation in the soft- and hard-X-ray regime, either directly by free-electron lasers (FEL) or indirectly by high-harmonic generation techniques, has renewed interest in experimental and theoretical photoionization studies of multiply charged ions
It is worth noting that three new FELs are under development and set to start user operations soon (E-XFEL, Swiss FEL, PAL) with extra pulse parameter specifications compared to the current ones
The structure of the text is as follows: in Section 2, we present the theoretical method in sufficient detail for a self-contained formulation of the present study; in Section 3, we present and discuss our results about the calculated energies and the lowest-order (non-vanishing) perturbation theory (LOPT) two- and three-photon ionization partial cross sections
Summary
The advent of new light sources, capable of delivering intense and/or ultrashort, coherent radiation in the soft- and hard-X-ray regime, either directly by free-electron lasers (FEL) or indirectly by high-harmonic generation techniques, has renewed interest in experimental and theoretical photoionization studies of multiply charged ions. It appears that, nowadays, in these wavelength regimes, excitation/ionization processes involving more than one photon are routinely feasible Their quantitative description requires theoretical approaches capable of coping with the non-linear features of the interaction between X-ray radiation with atomic and molecular systems. Excitation and ionization of the valence shells of the neutral species of these elements have been extensively studied and experimental reports and theoretical calculations about their photoionization cross sections are available in literature. To provide one example illustrating the need for such data, we mention the case of neon’s multiple ionization in the pioneering experiment of Young et al [3] at LCLS with a FEL pulse at a photon energy circa 1110 eV (pulse peak intensity and FWHM duration were estimated to be 1017 W/cm and 100 fs, respectively). The cross sections and the energies are presented in more traditional units, namely, eV for the energies, and cm s and cm s2 for the two- and three-photon cross sections, respectively
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