G Snell

Characterization of the helical undulator HELIOS I in the 520 to 930 eV range using a multilayer polarimeter

A W/Si multilayer was used to determine the degree of circular polarization of the soft x-ray radiation of the European Synchrotron Radiation Facility helical undulator HELIOS I. The multilayer, manufactured by vapor deposition serves as a wideband tunable polarization analyzer in the photon energy range from 520 to 930 eV. The characterization of the multilayer’s analyzing power, varying from 0.82 to 0.25 for these energies, indicates that it operates close to its calculated specifications. The lack of phase-shifters applicable in this energy range was overcome by a detailed analysis of the unpolarized background identified as radiation from the magnetic lattice. In this way, the degree of circular polarization of HELIOS I was determined to exceed 0.85 for hν>685 eV.

Spin resolved auger electron spectroscopy after photoexcitation with circularly polarized radiation from the BESSY crossed undulator

Abstract The new crossed undulator beamline U2-FSGM at BESSY enables spin resolved spectroscopy of electrons emitted after excitation of atoms, molecules and solids by circularly polarized radiation, even beyond the energy limit of normal-incidence monochromators. It is thus possible to perform spin resolved Auger spectroscopy of an enlarged number of elements. We present first results obtained at the U2-FSGM in the gas phase and from solids. For the gas phase we show spin resolved data for 5p- and 4d-photoionization of Xe and for the N4,5O2,3O2,3 Auger decay in Xe which agree within the uncertainties with theory. They are measured by means of a newly developed time-of-flight apparatus including spin polarization analysis. For solids we present spin resolved M3VV Auger spectra obtained at Cu(100) in a normal incidence, normal emission setup. On the low energy side of the Auger peak they show a change of the preferential spin direction not expected within an atomic model of the Cu M3VV Auger decay. In the center of the peak the preferential spin direction is antiparallel to the spin of the photons exciting the primary M3 hole states.

Spin-resolved electron spectroscopy of the xenon N4,5 O2,3 O2,3 Auger lines

The spin polarization of the Xe N4,5 O2,3 O2,3 Auger lines was measured after ionization by circularly polarized synchrotron radiation of 93.8 eV photon energy. The circularly polarized radiation was produced by converting the linearly polarized light from a conventional undulator by a Mo/Si transmission multilayer acting as a quarter-wave plate. Nearly perfect circular polarization was obtained. In the framework of the two-step model of Auger decay the orientation of the primary hole states 4d-1 2D3/2 and 4d-1 2D5/2 can be derived from the spin polarization of the 1S0 Auger lines. Furthermore, the intrinsic parameters for all lines in this Auger group corresponding to the other 5p-2 final states are determined and compared with theory.

Spin-resolved photoelectron spectroscopy in the autoionization region of atomic bismuth

The photoionization of atomic bismuth in the complex autoionization region above the first ionization threshold Bi+ (6s(2)6p(2))P-3(0) is studied using spin-resolved photoelectron spectroscopy and Hartree-Fock calculations with relativistic extensions. In spin- and angle-resolved measurements with circularly polarized synchrotron radiation the relative photoelectron intensity and the polarization parameters A and xi are determined for the strongest resonances. In calculations taking more than 30 different configurations into account all five dynamical photoionization parameters are determined. The agreement with the experimental data is fairly good. Combining calculated and experimental results for the intensity spectrum and the polarization parameter A as well as earlier absorption data, assignments of the first autoionization resonances above the threshold are obtained leading partially to a revision of previous results.

Spin‐resolved time‐of‐flight electron spectroscopy following excitation with circularly polarized synchrotron radiation (abstract)

A new type of spectrometer for the spin‐polarization sensitive analysis of electrons has been developed. This instrument allows for multichannel detection by employing a retarding‐field Mott polarimeter along with a time‐of‐flight analyzer for the energy selection of the incoming electrons. This method is particularly advantageous for spectra with many lines spread over a wide range of energies, as the whole spectrum can be recorded simultaneously. Such conditions are common for most cases of inner‐shell photoionization, where in addition to the photolines a large variety of corresponding Auger lines occurs. The spectrometer was particularly designed to study such processes. First measurements were taken at the crossed‐field undulator beamline U2 at BESSY. We obtained results for the Xe 5p photoelectrons in the photon energy range between 42 and 60 eV and for Xe 4d photoelectrons and the following NOO Auger decay at 95 eV.