Valence-shell photoionization of chlorinelike Ar⁺ ions

Absolute cross-section measurements for valence-shell photoionization of Ar⁺ ions are reported for photon energies ranging from 27.4 to 60.0 eV. The data, taken by merging beams of ions and synchrotron radiation at a photon energy resolution of 10 meV, indicate that the primary ion beam was a statistically weighted mixture of the 2P3/2o ground state and the 2P1/2o metastable state of Ar⁺. Photoionization of this Cl-like ion is characterized by multiple Rydberg series of autoionizing resonances superimposed on a direct photoionization continuum. Observed resonance line shapes indicate interference between indirect and direct photoionization channels. Resonance features are spectroscopically assigned and their energies and quantum defects are tabulated. The measurements are satisfactorily reproduced by theoretical calculations based on an intermediate coupling semirelativistic Breit-Pauli approximation.