Direct double ionization of the ${\mathrm{Ar}}^{+}\left(3p^{-1}\right)$ ion by a single photon is investigated both experimentally and theoretically. The photon-ion merged-beams technique was employed at the Advanced Light Source in Berkeley, USA, to measure absolute cross sections in the energy range from 60 to 150 eV. In this range, three contributions to the double ionization of ${\mathrm{Ar}}^{+}$ are to be expected: the removal of two $3p$ electrons, of a $3s$ and a $3p$ electron, and of two $3s$ electrons. Among the possible mechanisms leading to double ionization, the TS1 (two-step one) process dominates in the near-threshold region. In TS1, a photoelectron is ejected and, on its way out, knocks out a secondary electron. This two-step mechanism is treated theoretically by multiplying the calculated cross section for direct single photoionization of a given subshell with the calculated $(e,2e)$ ionization probability for the ejected photoelectron to knock off a secondary electron. The calculated cross section is in very good agreement with the experiment.

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• Single- and few-photon ionization & excitation