Photoionization of Sc²⁺ ions by synchrotron radiation: Measurements and absolute cross sections in the photon energy range 23–68 eV

Cross sections for the photoionization (PI) of Sc²⁺ ions with [Ar]3d ground-state configuration have been measured by employing the merged ion-photon beams method. The Sc²⁺ 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≈40.2eV have been measured with an instrumental energy spread ΔE as low as 1.2 meV, corresponding to a resolving power of E/(ΔE) around 33 500. The fractions of metastable ions in the Sc²⁺ 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 Sc³⁺ ions. Absolute strengths of several 3p⁵3d² and 3p⁵3d4s PI resonances have been determined. They are the same as the corresponding resonance strengths for isoelectronic Ca⁺ ions.