Stimulating electron competition with extreme-ultraviolet-initiated high-order harmonic generation

We address extreme-ultraviolet (XUV) initiated high-order harmonic generation (XIHHG) in argon atoms observing a resonant enhancement of the high-energy harmonics when the XUV pulse energy is tuned to the well-known window resonance. We attribute this effect to a three-stage mechanism: excitation of the 3s3p6 4p window resonance, followed by field-assisted autoionization and strong-field driven recollision. We also verify that the observed effect is robust against intensity-averaging, increasing the possibility that it may be observed experimentally. The resonant XIHHG mechanism represents an important addition to the attosecond toolset, as experimental schemes to identify signatures of correlation have heretofore proved elusive.