Direct double ionization of the He-like $B^{3+}$ ion by a single photon

Within a joint experimental and theoretical research project, double photoionization of the He-like B$^{3+}$ ion by a single photon was studied in the energy range from approximately 350 to 1160 eV. With the parent-ion beam in the experiment containing both $1⁢𝑠^2⁢\;^1⁢𝑆$ ground-state (90.9%) and $1⁢𝑠⁢2⁢𝑠\;^3𝑆$ metastable B$^{3+}$ ions (9.1%), two contributions to the apparent single- and double-ionization cross sections, $\sigma^\mathrm{app}_{34}$ and $\sigma^\mathrm{app}_{35}$, respectively, are involved in the measurements. Ratios 𝑌⁡(5+)/𝑌⁡(4+) of double- over single-ionization yields were experimentally determined. By multiplying the measured ratios by the known cross section $\sigma^\mathrm{app}_{34}$, the apparent cross section $\sigma^\mathrm{app}_{35}$ for double ionization was inferred. Using the information thus obtained, the measured yield of B$^{5+}$ ions produced from hydrogenlike B$^{4+}$ could also be normalized. The results show good agreement with theoretical benchmark cross sections $\sigma_{45}$ for single photoionization of B$^{4+}$. Convergent close-coupling calculations were performed to determine double-photoionization cross sections $\sigma_{35}$ for B$^{3+}$⁡($1s^2$) and B$^{3+}$⁡($1s2s$) ions. The result for the ground state agrees very well with theoretical data obtained previously with an intermediate-energy $R$-matrix approach. Within the systematic uncertainty of the normalized experimental cross sections, calculated and measured data are in agreement with one another.