## Non-dipole parameter γ + 3δ of the photoelectron angular distribution in the single-photon 3p ionization of Ar and unpolarized Ar^{+} as a function of the photoelectron energy

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**Figure 1.** Non-dipole parameter γ + 3δ of the photoelectron angular distribution in the single-photon 3p ionization of Ar and unpolarized Ar^{+} as a function of the photoelectron energy. Inset shows the region of energies around the Cooper minimum. Ionization of Ar^{+} to different multiplet states of the residual doubly charged ion: Ar^{2 +} (3p^{4} ^{3}P) (solid); Ar^{2 +} (3p^{4} ^{1}S) (dashed); Ar^{2 +} (3p^{4} ^{1}D) (chain). Ionization of Ar: this work (dotted); RPA [27, 28] (crosses).

**Abstract**

As an extension to recent developments in the theory of non-dipole effects in nonlinear photoprocesses in the XUV/x-ray wavelength regime, the sequential two-photon double ionization of the 3p shell in atomic argon is studied theoretically and compared to similar processes in the 2p shell of neon. The calculations predict distinct non-dipole effects in the region of the Cooper minimum in argon at photon energies around 50 eV, where they are of similar importance as at photon energies of more than 500 eV. The non-dipole effects should therefore be clearly observable in experiments performed at the present x-ray free electron lasers.