Strong-field ionization of FEL-prepared doubly excited states in the helium atom

Electron correlation plays a fundamental role in light-matter interaction. To gain new insights into the role of the initial state for the ionization process, we prepared doubly excited states in helium with extreme ultraviolet (XUV) light provided by the free-electron laser in Hamburg (FLASH), and followed their decay in the presence of an additional infrared (IR) field. The XUV photon energy was scanned over the broad region of doubly excited states between 59 eV and 80 eV, reaching as high as the double ionization continuum of helium. The synchronized IR pulses (800 nm wavelength) strong-field ionize the selectively prepared doubly excited states. In a fully differential measurement of the charged ionization products using a reaction microscope (ReMi) we studied the strong-field dynamics as a function of the XUV photon energy. Both single- and double-ionization events have been observed and the impact of different ionization mechanisms will be discussed, also in comparison with simple model calculations.