Accurate photoionisation cross section for He at non-resonant photon energies

The total single-photon ionisation cross section was calculated for helium atoms in their ground state. Using a full configuration-interaction approach the photoionisation cross section was extracted from the complex-scaled resolvent. In the energy range from ionisation threshold to 59\,eV our results agree with an earlier $B$-spline based calculation in which the continuum is box discretised within a relative error of $0.01\%$ in the non-resonant part of the spectrum. Above the \He^{++} threshold our results agree on the other hand very well to a recent Floquet calculation. Thus our calculation confirms the previously reported deviations from the experimental reference data outside the claimed error estimate. In order to extend the calculated spectrum to very high energies, an analytical hydrogenic-type model tail is introduced that should become asymptotically exact for infinite photon energies. Its universality is investigated considering also H$^-$, Li$^+$, and HeH$^+$. With the aid of the tail corrections to the dipole approximation are estimated.Comment: 20 pages, 7 figures, 2 table

The Soft X-ray Free-Electron Laser FLASH at DESY

FLASH, the Free-electron LASer in Hamburg, is the world’s first free electron laser for extremely bright and ultra-short pulses in the extreme ultraviolet and soft X-ray range. Efficient photon beam transport and diagnostics play an essential role in exploiting the features of this new generation of light sources in a large variety of user experiments. A detailed overview of the FLASH user facility is presented

Towards Time-Resolved Core Level Photoelectron Spectroscopy with Femtosecond X-ray Free Electron Lasers

We have performed core level photoelectron spectroscopy on a W(110) single crystal with femtosecond XUV pulses from the free-electron laser at Hamburg (FLASH). We demonstrate experimentally and through theoretical modelling that for a suitable range of photon fluences per pulse, time-resolved photoemission experiments on solid surfaces are possible. Using FLASH pulses in combination with a synchronized optical laser, we have performed femtosecond time-resolved core-level photoelectron spectroscopy and observed sideband formation on the W 4f lines indicating a cross correlation between femtosecond optical and XUV pulses

Multiphoton ionization of atoms with soft x-ray pulses

International audienceThe new soft x-ray Free-Electron Laser in Hamburg (FLASH) has opened the doorway to totally new experiments of materials research on nanometer 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 photoionisation of rare gas atoms at FLASH by ion mass-to-charge spectroscopy and, thus, also the limits for the application of gas-ionisation detectors for the characterisation of x-ray lasers. By strong beam focusing, we have achieved irradiance levels beyond 10 13 W cm -2 at about 40 eV photon energy and up to 10 16 W cm -2 in the Extreme Ultra-Violet (EUV) at about 90 eV. Here, surprisingly high degrees of photoionisation 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 on the degree of atomic perturbation by the radiation field