Strong-Field Extreme-Ultraviolet Dressing of Atomic Double Excitation.

We report on the experimental observation of a strong-field dressing of an autoionizing two-electron state in helium with intense extreme-ultraviolet laser pulses from a free-electron laser. The asymmetric Fano line shape of this transition is spectrally resolved, and we observe modifications of the resonance asymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of Microjoules. A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, driven by classical extreme-ultraviolet (XUV) electric fields, evidences strong-field-induced energy and phase shifts of the doubly excited state, which are extracted from the Fano line-shape asymmetry.

Two-photon excitation and relaxation of the 3d-4d resonance in atomic Kr.

Two-photon excitation of a single-photon forbidden Auger resonance has been observed and investigated using the intense extreme ultraviolet radiation from the free electron laser in Hamburg. At the wavelength 26.9 nm (46 eV) two photons promoted a 3d core electron to the outer 4d shell.

Theory of multiphoton multielectron ionization of xenon under strong 93-eV radiation.

We present a theoretical interpretation of recent experimental results on multiphoton multiple ionization of xenon by soft-x-ray radiation of photon energy approximately 93 eV and intensity up to 10(16) W/cm2 [A. A Sorokin, Phys. Rev.