tokens: 1 , and (c) 3s 2 3p 4 3 P 0 . Strong Rydberg series are present as are the much weaker series converging to the Cl + () threshold. Similar strong Rydberg series are present as are the much weaker series converging to the Cl + () threshold. Theoretical cross-section calculations were carried out with the darc codes, and convoluted with a Gaussian having a profile of 15 meV. The assigned Rydberg series are indicated as vertical lines grouped by horizontal lines. The corresponding series limits E ∞ of equation ( 2 ) for each series are indicated by vertical lines at the end of the line groups. Several of the nd interloping resonances are seen to disrupt the regular Rydberg resonance pattern of the spectra. Resonance energies and quantum defects for the various series are tabulated in Tables 1 and 2 and compared with the available experimental values. Table 1. Principal quantum numbers n , resonance energies (eV), and quantum defects μ of the Cl + ( Rydberg series seen in the Cl + (3s 2 3p 4 3 P 2,1,0 ) photoionization spectra, converging to the Cl + () threshold. The experimental resonance energies (Hernández et al. 2015 ) are calibrated to ±13 meV. The theoretical results are obtained from the 512-level darc calculations performed within the Dirac–Coulomb R -matrix approximation. The experimental entries in parenthesis are uncertain. Notes. a ALS experimental results (Hernández et al. 2015 ). b Present R -matrix darc calculations. c Rydberg series limits E ∞ for Cl + are from the NIST tabulations (Ralchenko et al. 2014 ). Table 2. Principal quantum numbers n , resonance energies (eV), and quantum defects μ of the Cl + ( Rydberg series seen in the Cl + (3s 2 3p 4 3 P 2,1,0 ) photoionization spectra, converging to the Cl + () threshold. The experimental resonance energies (Hernández et al. 2015 ) are calibrated to ±13 meV. The theoretical results are obtained from the 512-level darc calculations performed within the Dirac–Coulomb R -matrix approximation. The experimental entries in parenthesis are uncertain. Notes. a ALS experimental results (Hernández et al. 2015 ). b Present R -matrix darc calculations. c Rydberg series limits E ∞ for Cl + are from the NIST tabulations (Ralchenko et al. 2014 ). 3.3 Photoionization of the metastable Cl<sup>+</sup>(3s<sup>2</sup>3p<sup>4</sup> <sup>1</sup>D<sub>2</sub> and <sup>1</sup>S<sub>0</sub>) states Photoionization cross-section calculations of the singly ionized atomic chlorine ion from the initial 3s 2 3p 4 1 D 2 and 1 S 0 metastable states were also investigated using the same large-scale R -matrix model within the Dirac–Coulomb approximation ( darc ), as these states were present in the parent ion beam of the recent ALS measurements (Hernández et al. 2015 ). Fig. 3 shows the results for the 3s 2 3p 4 1 D 2 initial state and Fig. 4 the corresponding 3s 2 3p 4 1 S 0 case. Cross-section calculations are shown for photon energies from their respective thresholds to a photon energy of 25 eV and convoluted
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