Double and triple photoionization of the Be-like B + ion: Role of double- K -shell-vacancy states

Double and triple photoionization of Be-like B + ions have been studied in the photon-energy range from approximately 250 to 1300 eV. The method of merged photon and ion beams was employed and calculations were performed in support of the experiments making use of different theoretical approaches. The cross section σ 13 for net double ionization in the investigated photon-energy range is dominated by direct removal of one K -shell electron and subsequent single-Auger decay. K -shell ionization is described very well by theory. Accordingly, measured yields of B 3 + photoproducts were normalized to theory to obtain absolute cross sections σ 13 . Aside from the process of direct single ionization plus autoionization, σ 13 additionally contains contributions from photoexcited double- K -shell-vacancy B + resonances, from direct double L -shell ionization, and from direct double ionization involving one L -shell and one K -shell electron. We show that, apart from single K -shell ionization with subsequent Auger decay, all these processes are of minor importance for σ 13 . Nevertheless, the sensitivity of our experiment allowed us to observe the 2 s 2 2 p 3 s P 1 1 double- K -shell-vacancy resonance and to determine its energy, width, strength, and asymmetry parameter. Relativistic Hartree-Fock theory was employed to characterize the role of such resonances in σ 13 . The cross section σ 14 for net triple ionization of B + is expected to arise primarily from direct double K -shell ionization and subsequent single-Auger decay. The experimental cross section is found to be larger than theoretical predictions. Possible additional contributions to σ 14 such as K -shell ionization with accompanying K -shell excitation and subsequent Auger decay are discussed, as is the influence of metastable ions in the primary B + ion beam.