In the spectral range of the extreme ultraviolet at a wavelength of 13.3 nm, we have studied the photoionization of xenon at ultrahigh intensities. For our ion mass-to-charge spectroscopy experiments, irradiance levels from 10(12) to 10(16) W cm(-2) were achieved at the new free-electron laser in Hamburg FLASH by strong beam focusing with the aid of a spherical multilayer mirror. Ion charges up to Xe21+ were observed and investigated as a function of irradiance. Our surprising results are discussed in terms of a perturbative and nonperturbative description.

At the soft-x-ray free-electron laser FLASH in Hamburg, we have studied multiphoton ionization on neon and helium by ion mass-to-charge spectroscopy. The experiments were performed in a focused beam at 42.8 and 38.4 eV photon energy and irradiance levels up to 10 14 W/cm 2 . Direct, sequential, and resonant two-, three-, and four-photon excitations were investigated by quantitative measurements as a function of the absolute photon intensity. The atomic and ionic photoionization cross sections derived indicate a clear dominance of sequential compared to direct multiphoton processes. DOI: 10.1103/PhysRevA.75.051402 PACS number͑s͒: 32.80.Rm, 32.80.Fb, 42.50.Hz Recent progress has been achieved in generating soft-xray pulses of high power by means of free-electron lasers ͑FELs͒ ͓1-4͔ and higher-harmonics generation technique ͓5͔. In the near future, several large x-ray FEL facilities will be realized to study fast processes in materials and chemical reactions by ultrashort laser shots ͓1,2,6͔. However, highly intense soft x rays ͑xuv͒ and vacuum-ultraviolet radiation cause nonlinear response of matter such as atomic multiphoton ionization ͓7-13͔ which has to be taken into consideration in any FEL experiment. In this context, we report on the strength of two-, three-, and four-photon multiple ionization obtained by quantitative measurements of ion time-offlight ͑TOF͒ spectroscopy at the new xuv Free-electronLASer in Hamburg ͑FLASH͒ ͓4͔. In order to distinguish and compare different sequential and direct multiphoton excitation schemes, our experiments were performed on neon ͑Ne͒ and helium ͑He͒ atoms at two different photon energies, namely 42.8 and 38.4 eV, i.e., just above and below the threshold for sequential two-photon double ionization of Ne via Ne + at 41.0 eV. Significant differences in the respective nonlinear dependences on photon intensity could be observed. Figure 1 summarizes the multiphoton processes we discuss here. They refer to sequential ͓Fig. 1͑d͔͒ and direct ͓Fig. 1͑g͔͒ two-photon processes, a combination of both, i.e., a four-photon excitation ending up in a triply charged ion ͓Fig. 1͑e͔͒, and three-photon double ionization via virtual and resonance states ͓Figs. 1͑b͒ and 1͑c͔͒. The investigations were performed at the microfocus beamline BL2 at FLASH with an experimental setup described in detail previously ͓12,15,16͔. It consists of a calibrated online gas-monitor photodetector and a conventional ion TOF spectrometer. In order to avoid effects due to space charge and secondary ionization, the pressure of the target gas homogeneously filling the vacuum chamber was controlled below 2 ϫ 10 −4 Pa. The FEL radiation was distributed among subsequent photon pulses separated by 200 ms with up to 3 ϫ 10 12 photons per pulse and a pulse duration of ⌬t = ͑25± 8͒ fs ͓4,17͔. A focal spot size of A = ͑5.0± 0.7͒ ϫ 10 −6 cm 2 was realized by means of an ellipsoidal mirror ͓16͔. Ions generated in the focus were extracted toward the TOF spectrometer by a static electric field parallel to the polarization vect...

We have developed different types of photodetectors that are based on the photoionization of a gas at a low target density. The almost transparent devices were optimized and tested for online photon diagnostics at current and future x-ray free-electron laser facilities on a shot-to-shot basis with a temporal resolution of better than 100 ns. Characterization and calibration measurements were performed in the laboratory of the Physikalisch-Technische Bundesanstalt at the electron storage ring BESSY II in Berlin. As a result, measurement uncertainties of better than 10% for the photon-pulse energy and below 20 m for the photon-beam position were achieved at the Free-electron LASer in Hamburg ͑FLASH͒. An upgrade for the detection of hard x-rays was tested at the Sub-Picosecond Photon Source in Stanford.

Exceptional behavior of light-matter interaction in the extreme ultraviolet is demonstrated. The photoionization of different rare gases was compared at the free-electron laser in Hamburg, FLASH, by applying ion spectroscopy at the wavelength of 13.7 nm and irradiance levels of thousands of terawatts per square centimeter. In the case of xenon, the degree of nonlinear photoionization was found to be significantly higher than for neon, argon, and krypton. This target specific behavior cannot be explained by the standard theories developed for optical strong-field phenomena. We suspect that the collective giant 4d resonance of xenon is the driving force behind the effect that arises in this spectral range.

In order to measure the photon flux of highly intense and extremely pulsed vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) radiation in absolute terms, we have developed a gas-monitor detector which is based on the atomic photoionization of a rare gas at low particle density. The device is indestructible and almost transparent. By first pulse-resolved measurements of VUV free-electron laser radiation at the TESLA test facility in Hamburg, a peak power of more than 100 MW was detected. Moreover, the extended dynamic range of the detector allowed its accurate calibration using spectrally dispersed synchrotron radiation at much lower photon intensities.