A. A. Sorokin

Spatio-temporal coherence of free electron laser pulses in the soft x-ray regime

The temporal coherence properties of soft x-ray free electron laser pulses at FLASH are measured at 23.9 nm by interfering two time-delayed 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 h nu = 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.

Multi-photon ionization of molecular nitrogen by femtosecond soft x-ray FEL pulses

At the new free-electron laser (FEL) for vacuum ultraviolet (VUV) and soft x-ray radiation FLASH of the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, multi-photon double ionization of molecular nitrogen has been observed and studied by ion time-of-flight spectroscopy. The experiments have been performed at the microfocus beamline BL2 with photon pulses of 25 fs duration and irradiance levels up to 2 × 1013 W cm−2 at a photon energy of 38 eV, i.e. well above the first ionization/dissociation thresholds of the target. A new and important aspect of our experiments has been the reliable determination of absolute photon numbers per pulse with the help of a calibrated setup allowing the investigation of nonlinear effects by quantitative measurements. Results obtained are discussed in terms of a sequential two-photon ionization scheme.

Performance of the monochromator beamline at FLASH

The performance of the high-resolution monochromator at FLASH, the vacuum ultra-violet free-electron laser at DESY in Hamburg, was investigated. Using different techniques we measured the relative and absolute transmission of the monochromator beamline for FLASH radiation in the fundamental and higher (up to third) harmonics. The focal spot size of the soft x-ray beam at the output of the beamline was determined as well. In addition, a new way of acquiring the resolving power of the monochromator is discussed. This method has also been applied to calibrate the energy scale of the monochromator system with an accuracy of 5 meV at 48 eV photon energy.

Multiphoton ionization of atoms with soft x-ray pulses

The new soft x-ray free-electron laser in Hamburg (FLASH) has opened the doorway to totally new experiments of materials research on nanometre and femtosecond scales. However, the mechanisms of photon–matter interaction are not well understood under the conditions of ultra-high photon intensities in conjunction with short wavelengths. In this context, we have quantitatively investigated nonlinear photoionization of rare gas atoms at FLASH by ion mass-to-charge spectroscopy and, thus, also the limits for the application of gas-ionization detectors for the characterization of x-ray lasers. By strong beam focusing, we have achieved irradiance levels beyond 1013 W cm−2 at about 40 eV photon energy and up to 1016 W cm−2 in the extreme ultraviolet at about 90 eV. Here, surprisingly high degrees of photoionization were observed on Xe atoms. By comparison with other rare gas targets, it emerged that the excitation of inner-shell resonances might play a significant role in the degree of atomic perturbation by the radiation field.

Photon-matter interaction at short wavelengths and ultra-high intensity – Gas-phase experiments at FLASH

In different focused beams at the new soft X-ray Free-electron LASer in Hamburg FLASH, ion time-of-flight spectroscopy on gas targets was performed. Irradiation levels above 1013 up to 1016 W cm-2 were achieved in the vacuum and extreme ultra-violet. The first group of experiments was performed on nitrogen molecules and neon and helium atoms at wavelengths around 30 nm, i.e. at photon energies around 40 eV. Absolute cross sections for one- and two-photon ionization could be derived. The second group of experiments was performed on different rare gases at the wavelength of 13.3 nm, i.e. at the photon energy of 93 eV. As an example, the generation of Xe21+ was observed which requires a total energy of at least 5 keV absorbed per atom, starting from neutral Xe, within the FLASH pulse duration of about 10 fs. Here, the situation might be beyond the multiphoton scheme and perturbation theory.

Total electron-impact ionization cross sections of helium

By accurate measurement of total cross section ratios for electron-impact ionization and photoionization, total electron-impact ionization cross sections of He were determined. Relative standard uncertainties as low as 1.6 to 2% in the electron energy range from 100 to 4000 eV were achieved using published cross section data for photoionization. Results are discussed in terms of theoretical predictions and by comparison with former experimental results. The data are in excellent agreement with the results by Rejoub et al (2002 Phys. Rev. A 65 042713) and represent an accurate and reliable database for testing theoretical models.

Pulse energy measurements of extreme ultraviolet undulator radiation

A detector system based on the photoionization of rare gases at low particle densities has been developed for absolute photon flux measurements of highly intense and extremely pulsed radiation in the VUV and EUV spectral range. Due to its wide dynamic range, the device can be calibrated with spectrally dispersed synchrotron radiation at low photon intensities but applied with high power sources. The detector is free of degradation and almost transparent, and therefore suitable for intensity monitoring. We describe here the application of the detector for flux measurements at a beamline for undispersed deflected undulator radiation in the PTB Radiometry Laboratory at the electron storage ring BESSY II. In the single-bunch operation mode of BESSY II, measurements of the pulse energy down to 3 nJ for single EUV pulses have been performed. This demonstrates the capability of the gas detector for a broad range of applications with pulsed EUV sources.

Atomic Physics Using Ultra-Intense X-Ray Pulses

The combination of short wavelengths and ultrahigh intensities as provided by the new soft and hard X-ray free electron laser sources opens the doorway to totally new experiments on photon-matter interaction. It concerns, in particular, new classes of nonlinear inner-atomic processes. In the present contribution, recent results on sequential and nonsequential multi-photon ionization of gas phase targets are presented and discussed; including processes where also inner shells are affected. Moreover, examples are given how linear and nonlinear photoionization may be used for online photon diagnostics at these new radiation sources.

A new soft x‐ray autocorrelator—direct evaluation of the temporal properties of FEL pulses at 24 nm

To provide two jitter‐free soft x‐ray pulses for femtosecond x‐ray pump and probe experiments a split and delay unit (autocorrelator) has been constructed for the VUV—FEL in Hamburg (FLASH). Here we report experiments applying this autocorrelator to examine the average temporal properties of FEL pulses delivered from FLASH at 24 nm (51.8 eV). In a linear autocorrelation experiment the spatio‐temporal coherence properties are measured for both the first and the third harmonic of the FEL pulses. Furthermore, we report on the first evaluation of the pulse length from the time‐resolved observation of doubly charged helium ions produced by direct two‐photon double ionization at 24 nm. In summary the determination of the longitudinal pulse parameter of FLASH at 24 nm to 6 fs and 29±5u2009fs for the coherence time and the pulse length (FWHM) respectively proofs the autocorrelator as a valuable tool for time resolved two pulse X‐ray experiments.

Autocorrelation Experiments with Soft X-ray FEL Pulses

We report first direct measurements of the average temporal coherence and pulse length of soft X-ray fs pulses from the free-electron laser at DESY (FLASH) by means of linear and nonlinear autocorrelation