Upper panel: photoelectron angular distribution spectra retrieved from an Abel inversion of the measured 2D projection of the electron momentum distribution for the Ne 2p<sup>6</sup> → Ne<sup>+</sup> 2p<sup>5</sup> + e<sup>−</sup> photoionization at 90.5 eV FEL photon energy in the presence of an NIR (800 nm) intensity of 5–6<b>×</b>10<sup>12</sup> W cm<sup>−2</sup>

<p><strong>Figure 2.</strong> Upper panel: photoelectron angular distribution spectra retrieved from an Abel inversion of the measured 2D projection of the electron momentum distribution for the Ne 2p<sup>6</sup> → Ne<sup>+</sup> 2p<sup>5</sup> + e<sup>−</sup> photoionization at 90.5 eV FEL photon energy in the presence of an NIR (800 nm) intensity of 5–6<b>×</b>10<sup>12</sup> W cm<sup>−2</sup>. Shown are the inverted VMIS data. The emission angle is defined with respect to the XUV and the NIR polarization. Lower panel: calculated spectra for the same parameters as used in the experiment.</p> <p><strong>Abstract</strong></p> <p>The angular distribution of photoelectrons ejected during the ionization of Ne atoms by extreme ultraviolet (XUV) free-electron laser radiation in the presence of an intense near infrared (NIR) dressing field was investigated experimentally and theoretically. A highly nonlinear process with absorption and emission of more than ten NIR photons results in the formation of numerous sidebands. The amplitude of the sidebands varies strongly with the emission angle and the angular distribution pattern reveals clear signatures of interferences between the different angular momenta for the outgoing electron in the multi-photon process. As a specific feature, the central photoelectron line is characterized at the highest NIR fields by an angular distribution, which is peaked perpendicularly to both the XUV and NIR polarization directions. Experimental results are reproduced by a theoretical model based on the strong field approximation.</p>