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

Resonant multiphoton processes in the soft-x-ray regime

The multiphoton ionization of neon atoms was studied in the focus of the soft-x-ray free-electron laser FLASH by photoion spectroscopy. The photon energy was chosen in the range of Ne+ 2p→nℓ resonances between 41 and 42 eV. The experiments were performed at the PG0 branch of the FLASH monochromator beamline while measuring the spectral distribution of every single FLASH pulse. By correlating the ratio of doubly and singly charged neon ions generated and the measured photon energy distribution, the enhancement of this ratio due to resonant Ne+ excitation was found