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High-order harmonic generation enhanced by XUV light

The combination of high-order harmonic generation (HHG) with resonant XUV excitation of a core electron into the transient valence vacancy that is created in the course of the HHG process is investigated theoretically. In this setup, the first electron performs a HHG three-step process whereas, the second electron Rabi flops between the core and the valence vacancy. The modified HHG spectrum due to recombination with the valence and the core is determined and analyzed for krypton on the 3d -- > 4p resonance in the ion. We assume an 800 nm laser with an intensity of about 10^14 W/cm^2 and XUV radiation from the Free Electron Laser in Hamburg (FLASH) with an intensity in the range 10^13--10^16 W/cm^2. Our prediction opens perspectives for nonlinear XUV physics, attosecond x rays, and HHG-based spectroscopy involving core orbitals.