Abstract
Two-photon double ionization of He is studied at the Free Electron Laser in Hamburg (FLASH) by inspecting He2+ momentum () distributions at 52 eV photon energy. We demonstrate that recoil ion momentum distributions can be used to infer information about highly correlated electron dynamics and find the first experimental evidence for ‘virtual sequential ionization’. The experimental data are compared with the results of two calculations, both solving the time-dependent Schrödinger equation. We find good overall agreement between experiment and theory, with significant differences for cuts along the polarization direction that cannot be explained by the experimental resolution alone.
Highlights
Differential cross sections for non-sequential double ionization of He by 52 eV photons from the Free Electron Laser in Hamburg, FLASH
Many of the earlier studies concentrated on evaluation of the total cross section for two-photon double ionization (TPDI) and the wide range of values obtained by different investigators has led to considerable speculation about the proper treatment of electron correlation in the various theoretical treatments
The results shown below are obtained from the 11 fs pulse, which has a bandwidth (FWHM) of 0.4 eV, similar to the averaged spectral width of the FLASH pulses
Summary
The experiments were performed at the ‘unfocussed beamline’ (BL3) at the FEL in Hamburg, FLASH [1]. Due to the large number of electrons emerging from single ionization with ionization yields exceeding those for TPDI by more than three orders of magnitude [42], and due to the small count rate of true He2+ events (enforced to reduce the influence of space charge effects on the momentum measurements, as discussed later), no meaningful coincident electron spectra could be recorded This will only become feasible at significantly enhanced FLASH repetition rates. The H+2 ions that emerge from residual gas ionization are created in the focal volume (intersection of FEL beam and gas jet) and along the whole FEL beam trajectory with typical kinetic energies of 25 meV (300 K room temperature motion) As a result, their momentum distribution in all spatial directions is much broader compared to those of the He2+ events, such that a linear fit to the H+2 distribution in each direction outside the location where the true He2+ ions occur provides a very precise estimate of the background
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