Photoionization of ${\mathrm{Sc}}^{2+}$ ions by synchrotron radiation: Measurements and absolute cross sections in the photon energy range 23–68 eV

Cross sections for the photoionization (PI) of ${\mathrm{Sc}}^{2+}$ ions with $\left[\mathrm{Ar}\right]3d$ ground-state configuration have been measured by employing the merged ion-photon beams method. The ${\mathrm{Sc}}^{2+}$ ions were produced from metallic vapor in an electron cyclotron resonance ion source, and the photon beam was generated by an undulator in the electron-synchrotron storage ring of the advanced light source of the Lawrence Berkeley National Laboratory. The experimental photon energy range 23–68 eV encompasses the direct $3d$ and $3p$ photoionization thresholds. The experimental photoion spectrum is dominated by autoionizing resonances due to $3p$ excitations predominantly decaying via Coster-Kronig and super-Coster-Kronig transitions. Individual resonances located around $E\approx 40.2\mathrm{eV}$ have been measured with an instrumental energy spread $\Delta E$ as low as 1.2 meV, corresponding to a resolving power of $E/\left(\Delta E\right)$ around 33 500. The fractions of metastable ions in the ${\mathrm{Sc}}^{2+}$ ion beam are obtained by comparing the photoionization cross section with the recently measured [Schippers et al., Phys. Rev. A 65, 042723 (2002)] cross section for the time-reversed process of photorecombination of ${\mathrm{Sc}}^{3+}$ ions. Absolute strengths of several ${3p}^5{3d}^2$ and ${3p}^{5}3d4s$ PI resonances have been determined. They are the same as the corresponding resonance strengths for isoelectronic ${\mathrm{Ca}}^{+}$ ions.