Towards attosecond XUV-pump XUV-probe measurements in the 100-eV region

Summary form only given. Nonlinear photoionization with energetic FEL pulses has opened up new horizons for the investigation of inner-shell electron dynamics in atomic and molecular systems [1]. So far, however, the limited temporal resolution (typically a few tens of femtoseconds) achievable with FELs has hampered the time resolution of these dynamics.Laser-driven XUV-sources are capable of producing isolated attosecond pulses and offer an alternative to FELs. However, the comparatively low pulse energies delivered by these sources has impeded their application to nonlinear optics. So far, only a few groups have demonstrated multi-photon ionization processes using laserdriven XUV-sources with photon energies below 50 eV [2-5]. Here, we use a novel laser-driven XUV-source to investigate and quantify nonlinear photoemission within the giant 4d resonance of xenon, at photon energies of 93 eV and 115 eV. Our XUV source [6], which is based on high-harmonic generation in the gas phase, is driven by a 10-Hz multi-TW laser system, the `Light-Wave Synthesizer 20' (LWS-20), that delivers sub-two-cycle 75 mJ pulses with a central wavelength around 740 nm [7], and from which we used 40 mJ on target. This high pulse energy allows for increasing the focus diameter to 365 micron full width at half maximum (FWHM), while maintaining a peak intensity on the order of 1015 W/cm2. The 35 m long beamline is designed to accommodate a loose focusing (f =17 m) into the generation chamber as well as a tight refocusing (XUV expansion length = 12.5 m) in the experimental chamber. The XUV pulse energy contained in the Zr transmission window (i.e. between 65 eV to the cutoff at 130 eV) amounts up to ~40 nJ. This is two orders of magnitude larger than what is typically achieved with kHz few-cycle driving pulses [8]. For the study of XUV-matter interactions, the XUV beam is focused into a xenon gas target, where different ionic charge states are generated via XUV photoionization. An ion microscope [9] is used to record the spatial distribution of the different charge states in the focus. The measured data allows for the characterization of the XUV focus and the determination of the two-photon ionization cross sections, which are compared to the predictions made with different theoretical models [10-12]. The present investigation of nonlinear photoemission in the 100-eV spectral region using a laser-driven XUV source represents a major step towards attosecond XUV-pump XUV-probe measurements in the 100-eV region and the study of time resolved non-linear inner-shell dynamics.