Angular dependence of the (high-energy side) second sideband (electron energy ~71.9 eV) shown for different effective optical intensities (i.e

<p><strong>Figure 4.</strong> Angular dependence of the (high-energy side) second sideband (electron energy ~71.9 eV) shown for different effective optical intensities (i.e. different delay times). The experimental results are shown in the upper panel. The lower panel displays the corresponding simulated angular distribution. For better comparison, the curves are normalized on the total amount of signal for the sideband.</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>