LAUSR

It has been shown that for nonlinear atomic ionization, transfer of the degree of circular polarization from incident to fluorescence light is maximum at the Cooper minimum (see Phys. Rev. A \textbf{100}, 011401(R) (2019)). Until the present, however, it is still a challenge to produce and detect circularly-polarized light at xuv and x-ray photon energies. We here show, that transfer of linear polarization is strongly enhanced at the Cooper minimum in two-photon ionization and can be readily detected using current free-electron laser facilities by measurements of the degree of linear polarization of the characteristic $L\alpha_1$ and $L\alpha_2$ lines. Two-photon ionization of $2p_{3/2}$ electron of tungsten atom is proposed to demonstrate the presented phenomena and realization of such experiment is discussed in detail.