Fig. 1. PES of the two-photon He ionization by a pulse train of an envelope fs, pulse duration as, synthesized by the 11th–15th harmonics with relative intensities 0.19:0.4:0.41, respectively, for different total XUV intensities ranging from W/cm to W/cm (top plot); PES peak shifts as a function of the total XUV intensity in the interval ranging from 0 to W/cm (bottom plot).

Fig. 2. XUV EIT schemes in He through coupling of a bound with (a) an AIS or (b) two AISs.

Fig. 3. A VUV LICS scheme in He probed by an XUV field.

Fig. 4. Photoelectron peak shifts observed with ionizing XUV intensity in single-photon ionization of Ar. Xe gas was utilized as the generating medium and a Sn filter was used for spectral selection. The spectra are obtained at two different XUV energies, measured by a calibrated XUV photodiode: 1.8 μJ/pulse (upper black line, shifted in y-axis for clarity), 0.5 μJ/pulse (lower blue line). The blueshifts are attributed to transient space charge induced on surfaces the direct or scattered XUV light is impinging.

Fig. 5. Harmonic spectra recorded by a flat-field XUV spectrometer (FFS) varying the peak gas density of Xe at the harmonic generation region: cm (upper black line, shifted in y-axis for clarity), cm (lower red line). The observed blueshift is of the order of eV.

Fig. 6. Ionization of lithium with a radiation field around 73.1 eV (H47) couples two AIS states. The lower state decays to Li whereas the higher state decays to Li.

Fig. 7. Ionization of lithium with a radiation field around 73.1 eV (H47) coupling two AIS states. We plot the population ratios of Li/Li for various coupling strengths of and for peak intensity W/cm. The multiplication factor for the coupling is shown in the inset. Values for and can be found in Ref. [48].