Photoionization dipole (<em>E</em>1) and quadrupole (<em>E</em>2) cross sections

<p><strong>Figure 2.</strong> Photoionization dipole (<em>E</em>1) and quadrupole (<em>E</em>2) cross sections. Left: 3p ionization of Ar and unpolarized Ar<sup>+</sup>(3p<sup>5</sup> <sup>2</sup>P); right: 2p ionization of Ne and unpolarized Ne<sup>+</sup>(2p<sup>5</sup> <sup>2</sup>P). Curves for different LS multiplet states of the <em>n</em>p<sup>4</sup> configuration as in figure <a href="" target="_blank">1</a>. Dotted curves refer to neutral Ar and Ne.</p> <p><strong>Abstract</strong></p> <p>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.</p>