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...
Abstract:The temporal coherence properties of soft x-ray free electron laser pulses at FLASH are measured at 23.9 nm by interfering two timedelayed partial beams directly on a CCD camera. The partial beams are obtained by wave front beam splitting in an autocorrelator operating at photon energies from hν = 30 to 200 eV. At zero delay a visibility of (0.63 ± 0.04) is measured. The delay of one partial beam reveals a coherence time of 6 fs at 23.9 nm. The visibility further displays a non-monotonic decay, which can be rationalized by the presence of multiple pulse structure. ©2008 Optical Society of America
The interaction of Al2O3 and CeO2 thin films with sulfur dioxide (2.5 mbar) or with mixtures of SO2 with O2 (5 mbar) at various temperatures (30-400 degrees C) was studied by X-ray photoelectron spectroscopy (XPS). The analysis of temperature-induced transformations of S2p spectra allowed us to identify sulfite and sulfate species and determine the conditions of their formation on the oxide surfaces. Sulfite ions, SO3(2-), which are characterized by the S2p(3/2) binding energy (BE) of approximately 167.5 eV, were shown to be formed during the interaction of the oxide films with pure SO2 at temperatures < or =200 degrees C, whereas sulfate ions, SO4(2-), with BE (S2p(3/2)) approximately 169 eV were produced at temperatures > or =300 degrees C. The formation of both the sulfite and sulfate species proceeds more efficiently in the case of CeO2. The addition of oxygen to SO2 suppresses the formation of the sulfite species on both oxides and facilitates the formation of the sulfate species. Again, this enhancement is more significant for the CeO2 film than for the Al2O3 one. The sulfation of the CeO2 film is accompanied by a reduction of Ce(IV) ions to Ce(III) ones, both in the absence and in the presence of oxygen. It has been concluded that the amount of the sulfates on the CeO2 surface treated with the SO2 + O2 mixture at > or =300 degrees C corresponds to the formation of a 3D phase of the Ce(III) sulfate. The sulfation of Al2O3 is limited by the surface of the oxide film.
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.
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