Experimentally recorded VMIS data for the Ne photoionization at 90.5 eV FEL photon energy is shown for the case with only FEL (a) and FEL in the presence of an NIR (800 nm) intensity of ~3<b>×</b>10<sup>12</sup> W cm<sup>−2</sup> (b)

<p><strong>Figure 1.</strong> Experimentally recorded VMIS data for the Ne photoionization at 90.5 eV FEL photon energy is shown for the case with only FEL (a) and FEL in the presence of an NIR (800 nm) intensity of ~3<b>×</b>10<sup>12</sup> W cm<sup>−2</sup> (b). Shown is the angular dependence of the emitted photoelectrons (the polarization direction of FEL and NIR laser is vertical). In (a), the 2p as well as the 2s (inner ring) are clearly visible (the radius represents the electron momentum). In (b), the NIR laser broadens the central line substantially due to sideband formation. Thus, 2p and 2s contributions are not clearly visible anymore. The broadening decreases for larger angles with respect to the polarization until finally at 90°, there is almost no effect of the NIR laser on the photoelectrons. The red double arrow indicates the energy range shown in figure <a href="http://iopscience.iop.org/0953-4075/46/16/164026/article#jpb463604f2" target="_blank">2</a>. The low kinetic energy feature in the centre results from NIR-generated ATI electrons in the rest gas. The data are binned according to the sideband amplitude (b) and averaged over several hundred single-shot images. The enlarged view of the data (inset) shows that successive sidebands are separated by only ~3 pixels (corresponding to a separation of 1.5 eV), thus demonstrating the excellent resolution capabilities of the VMIS.</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>