Two-color XUV plus NIR femtosecond photoionization of neon in the near-threshold region

Results of angle-resolved electron spectroscopy of near-threshold photoionization of Ne atoms by combined femtosecond extreme ultraviolet and near infrared fields are presented. The dressed-electron spectra show an energetic distribution into so-called sidebands, being separated by the photon energy of the dressing laser. Surprisingly, for the low kinetic energy (few eV) sidebands, the photoelectron energy varies as a function of the emission angle. Such behavior has not yet been observed in sideband creation and has not been predicted in commonly used theoretical descriptions such as strong field approximation and soft photon approach. Describing the photoionization with a time-dependent Schrodinger equation allows a qualitative description of the observed effect, as well as the prediction of fine structure in the sideband distribution.We want to acknowledge the work of the scientific and technical team at FLASH. We acknowledge the Max Planck Society for funding the development and the initial operation of the CAMP endstation within the Max Planck Advanced Study Group at CFEL and for providing this equipment for CAMP@FLASH. The installation of CAMP@FLASH was partially funded by the BMBF grants 05K10KT2, 05K13KT2, 05K16KT3 and 05K10KTB from FSP-302. NMK acknowledges hospitality and financial support from FS-DESY and from the theory group in cooperation with the SQS work package of European XFEL (Hamburg). The participation of the DCU group was made possible by Science Foundation Ireland grant nos. 12/IA/1742 and 16/RI/3696. PJ acknowledges support from the Swedish Research Council and the Swedish Foundation for Strategic Research. MI acknowledges funding by the Volkswagen Foundation within a Peter Paul Ewald-Fellowship. AKK acknowledges the support of the Spanish Ministerio de Economia y Competitividad (grants FIS2016-76617-P and FIS2016-76471-P)