Photoionisation of Ca$^{+}$ ions in the valence-energy region 20$sim$eV~--~56$sim$eV : experiment and theory

Relative cross sections for the valence shell photoionisation (PI) of $rm ^2S$ ground level and $rm ^2D$ metastable Ca$^{+}$ ions were measured with high energy resolution by using the ion-photon merged-beams technique at the Advanced Light Source. Overview measurements were performed with a full width at half maximum bandpass of $Delta E =17$~meV, covering the energy range 20~eV -- 56~eV. Details of the PI spectrum were investigated at energy resolutions reaching the level of $Delta E=3.3$~meV. The photon energy scale was calibrated with an uncertainty of $pm5$~meV. By comparison with previous absolute measurements %by Kjeldsen et al in the energy range 28~eV -- 30.5~eV and by Lyon et al in the energy range 28~eV -- 43~eV the present experimental high-resolution data were normalised to an absolute cross-section scale and the fraction of metastable Ca$^{+}$ ions that were present in the parent ion beam was determined to be 18$pm$4%. Large-scale R-matrix calculations using the Dirac Coulomb approximation and employing 594 levels in the close-coupling expansion were performed for the Ca$^{+}(3s^23p^64s~^2textrm{S}_{1/2})$ and Ca$^{+}(3s^2 3p^6 3d~^2textrm{D}_{3/2,5/2})$ levels. The experimental data are compared with the results of these calculations and previous theoretical and experimental studies.