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Relative cross sections for the valence shell photoionisation (PI) of $\rm^2S$ ground level and $\rm ^2D$ metastable Ca$^{+}$ ions were measured withhigh energy resolution by using the ion-photon merged-beams technique at theAdvanced Light Source. Overview measurements were performed with a full widthat 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 resolutionsreaching the level of $\Delta E=3.3$~meV. The photon energy scale wascalibrated with an uncertainty of $\pm5$~meV. By comparison with previousabsolute measurements %by Kjeldsen et al in the energy range 28~eV -- 30.5~eVand by Lyon et al in the energy range 28~eV -- 43~eV the present experimentalhigh-resolution data were normalised to an absolute cross-section scale and thefraction of metastable Ca$^{+}$ ions that were present in the parent ion beamwas determined to be 18$\pm$4\%. Large-scale R-matrix calculations using theDirac Coulomb approximation and employing 594 levels in the close-couplingexpansion were performed for the Ca$^{+}(3s^23p^64s~^2\textrm{S}_{1/2})$ andCa$^{+}(3s^2 3p^6 3d~^2\textrm{D}_{3/2,5/2})$ levels. The experimental data arecompared with the results of these calculations and previous theoretical andexperimental studies.

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