N Scherer

Complete description of 3p photoionization in calcium

Within the dipole approximation and the LS-coupling limit 3p photoionization in atomic calcium is described completely by two matrix elements and one relative phase. These three parameters are extracted from experimental observables and compared with theoretical calculations (RRPA transformed to the non-relativistic limit) for photon energies around the Cooper minimum in the 3pd channel. Taking into account the intensity-borrowing model, excellent agreement between theoretical and experimental data is found.

Angle-dependent decrease of transition probability in sequential photo double-ionization

The three-body Coulomb interaction in sequential photo double-ionization, frequently called post-collision interaction (PCI), can lead to a collapse of transition probability if the relative angle between the emitted electrons approaches zero. We have studied this effect for a specially selected atomic system, 4d5/2 photoionization in xenon with subsequent N5–O2,3O2,31S0 Auger decay, and compared the experimental data with results from a quantum mechanical PCI calculation and a classical PCI model.

Exchange interference in the angular correlation between magnesium photo- and Coster-Kronig electrons

For the well defined process of 2s photoionization in atomic magnesium with subsequent L1-L2,3M1 1P or 3P Coster-Kronig decay, exchange interference effects are studied for coincident two-electron emission. Both electrons are observed for back-to-back emission in a plane perpendicular to the photon-beam direction, changing the photon energy so that the photoelectron line is moved, from below to above, through the state-selected Coster-Kronig line. The coincidence signal of the singlet Coster-Kronig channel shows the expected constructive interference effect leading to an increase by a factor of 4. However, in the coincidence signal of the triplet Coster-Kronig channel the expected destructive interference appears to be hidden due to (1P1-3P1) spin-orbit induced J-mixing. Implementing this complication into the theory which includes electron exchange and the influence of post-collision interaction, very good agreement is achieved between theoretical and experimental data. This gives quantitative confirmation of the selected process of sequential two-electron emission described in a one-step formulation.

VUV light polarization measured by coincidence electron spectrometry

A method to extract the Stokes parameter describing circularly polarized components of VUV light from suitably selected photoelectron Auger electron coincidences is demonstrated for 2p photoionization in magnesium with subsequent L2,3-M1M1 Auger decay. Including the standard technique of non-coincident angle-resolved photoelectron spectrometry to obtain the Stokes parameters describing linearly polarized components of VUV light, the full polarization properties of VUV light have been measured, as an example, for 80 eV photons delivered from the TGM4 monochromator at BESSY I in Berlin.

Electron-Electron Coincidence Studies in Xenon with Precisely Tailored Equipment

The triple-differential cross section of sequential photo double-ionization shows for small relative angle between the emitted electrons and equal or nearly equal electron energies interesting phenomena due to post-collision interaction and electron exchange. The experimental verification needs particularly tailored equipment. We present three examples.