Single and double photoionization of the Li-like B²⁺ ion: Strong ionization-excitation contributions

Single and double photoionization of the Li-like B²⁺(1s²2s²S) ion were studied both experimentally and theoretically in the energy range from approximately 250–1200 eV. The cross section σ₂₃ for net single ionization in that range is dominated by direct removal of one K-shell electron, a process that is described very well by theory. Accordingly, measured yields of B³⁺ photoproducts were normalized to theory to obtain absolute cross sections σ₂₃. Aside from direct single ionization, there are additional contributions to σ₂₃ from photoabsorption resonances. The parameters of the Fano profile of the most prominent of these resonances, the triply excited 2s²2p²P state, were calculated by employing the convergent close coupling (CCC) approach and the theoretical results were experimentally verified. Using the normalization procedure obtained from the investigation of single ionization, also the measured yields of B⁴⁺ product ions could be put on an absolute cross-section scale. The resulting experimental cross section σ₂₄ is in good accord with the CCC calculations revealing unexpectedly strong contributions that arise from ionization with excitation of B²⁺(1s²2 s²S) forming intermediate autoionizing B³⁺(nln'l’) levels, which subsequently decay to B⁴⁺ by Auger-electron emission. In addition, contributions to σ₂₄ from resonant excitation of B²⁺(nln’ n”l”) with n′, n’, n” ≥ 2 could be identified. These triply excited resonances with an empty K shell can decay by simultaneous or sequential emission of two electrons and thus contribute to net double photoionization of the parent B²⁺ ion.