Double- K -hole resonances in single photoionization of He-like B3+ ions

Within a joint experimental and theoretical research project, single photoionization of He-like B3+ ions was investigated in the energy range from approximately 250 to 1200 eV. With the parent-ion beam in the experiment containing both 1s2S1 ground-state and 1s2sS3 metastable B3+ ions, double-core-hole resonances could be studied. Two series of hollow resonant states were observed, one populated by K-shell double excitation 1s2S1→2 ′P1 (a=s,p; ′=p,s; n=2,3,,6) at photon energies up to about 510 eV, the other by K-shell single excitation 1s2sS3→2′P3 (a=s,p; ′=p,s; n=2,3, ,6) at energies up to about 310 eV. High resolving powers up to approximately 29000 were achieved. The relativistic many-body perturbation theory was employed to determine level-to-level cross sections for K-shell excitation with subsequent autoionization. The resonance energies were calculated with inclusion of electron correlation and radiative contributions. The energy uncertainties of the most prominent resonances are estimated to be below ±1meV. Convergent close-coupling (CCC) calculations provided single-photoionization cross sections σ34 for B3+ including the resonant and nonresonant channels. Apart from the resonances, σ34 is dominated by direct ionization in the investigated energy range. The contribution σ34dir of the latter process to σ34 was separately determined by using the random-phase approximation with exchange and relativistic Hartree-Fock calculations which agree very well with previous calculations. Direct ionization of one electron accompanied by excitation of the remaining electron was treated by the CCC theory and found to be a minor contribution to σ34.