Photoionisation of Cl$^+$ from the $3s^23p^4\;^3P_{2,1,0}$ and the$3s^23p^4\;^1D_2, ^1S_0$ states in the energy range 19 - 28 eV

Abstract: Absolute photoionisation cross sections for the Cl$^+$ ion in its ground and the metastable states; $3s^2 3p^4\; ^3P_{2,1,0}$, and $3s^2 3p^4\; ^1D_2,\; ^1S_0$, were measured recently at the Advanced Light Source ALS) at Lawrence Berkeley National Laboratory using the merged beams photon-ion technique at an photon energy resolution of 15 meV in the energy range 19 -- 28 eV. These measurements are compared with large-scale Dirac Coulomb {\it R}-matrix calculations in the same energy range. Photoionisation of this sulphur-like chlorine ion is characterized by multiple Rydberg series of autoionizing resonances superimposed on a direct photoionisation continuum. A wealth of resonance features observed in the experimental spectra are spectroscopically assigned and their resonance parameters tabulated and compared with the recent measurements. Metastable fractions in the parent ion beam are determined from the present study. Theoretical resonance energies and quantum defects of the prominent Rydberg series $3s^2 3p^3 nd$, identified in the spectra as $3p\rightarrow nd$ transitions are compared with the available measurements made on this element. Weaker Rydberg series $3s^2 3p^3 ns$, identified as $3p \rightarrow ns$ transitions and window resonances $3s3p^4 (^4P)np$ features, due to $3s \rightarrow np$ transitions are also found in the spectra.