Ch. Heckenkamp

High‐flux normal incidence monochromator for circularly polarized synchrotron radiation

A 6.5‐m normal incidence monochromator installed at the storage ring BESSY, which is optimized for a high throughput of circularly polarized off‐plane radiation at moderate resolution is described. The monochromator employs two exit slits and is specially designed and used for low‐signal experiments such as spin‐ and angle‐resolved photoelectron spectroscopy on solids, adsorbates, free atoms, and molecules. The Monk–Gillieson mounting (plane grating in a convergent light beam) allows for large apertures with relatively little astigmatism. With two gratings, a flux of more than 1011 photons s−1 bandwidth−1 (0.2–0.5 nm) with a circular polarization of more than 90% in the wavelength range from 35 to 675 nm is achieved.

A high flux normal incidence monochromator for circularly polarized synchrotron radiation

Design and performance of a 6.3 m normal incidence UHV-monochromator (see fig. 1) of the Gillieson type for synchrotron radiation are described. The monochromator will be used for simultaneously spin polarization and emission angle-resolved photoelectron spectroscopy; it combines a high photon flux and a high degree of circular polarization of the monochromatized radiation with a moderate resolution.

Experimental characterization of the Xe 5p photoionization by angle- and spin-resolved photoelectron spectroscopy

Spin- and angle-resolved photoelectron spectroscopy with elliptically polarized radiation has been used to fully characterize the dynamics of photoelectron emission from free Xe atoms in the 5p-autoionization and continuum region. An advantageous reaction geometry and its experimental realization at the storage ring BESSY are discussed. The three independent experimental parameters which characterize the angular dependence of the photoelectron spin-polarization vector are reported for the wavelength range from 100nm to 40 nm. The results are compared with theoretical predictions based on RRPA-, RPAE- and semiempirical MQDT-calculations. The combination of existing data for the differential photoionization cross section with the spinpolarization parameters is used to completely decouple the photoionization channels: The transition matrix elements and their relative phases are determined separately for every single dissociation channel. The results are discussed in the context of the MQDT. Correlation effects and the influence of spin-orbit interaction on the continuum states most clearly show up when the Dill-Fano angular-momentum-transfer formalism is applied.

Application of a threshold electron spectrometer in spin resolved photoelectron analysis

The paper shows that threshold photoelectron spectroscopy can be successfully performed, even if the spin polarization is the object of interest. Using circularly polarized radiation from the Bonn 2.5 GeV synchrotron photoelectrons from argon atoms produced directly at threshold have been analyzed with respect to their energy by means of a threshold electron spectrometer and with respect to their spin in order to test this combined method. The comparison of the experimental result with predictions of a RRPA-calculation shows quantitatively good agreement.

A laser-generated plasma as a source of VUV continuum radiation for photoelectronic spectroscopy

The feasibility of using laser-generated plasmas as VUV continuum sources for photoelectron spectroscopy has been demonstrated by measuring the spectral intensity distribution of the VUV continuum in the wavelength region from 79 to 43 nm by energy analysis of the photoelectrons ejected from argon atoms. The maximum photon flux obtained after reflection at a gold-coated spherical mirror was of the order of 1011 photons nm-1 per pulse at 50 nm for a laser energy of 830 mJ. The results show a shift of the emission maximum to lower wavelengths with increasing laser energy.

Energy dependence of the electron spin polarisation parameters for Hg 5d photoionisation with circularly polarised light

Circularly polarised VUV radiation from the storage ring BESSY was used to measure all three spin polarisation parameters of photoelectrons from the Hg 5d shell in the photon energy range from threshold to approximately 35 eV. The experimental results are compared with theoretical calculations in the relativistic and non-relativistic random-phase-approximation scheme (RRPA and RPAE respectively) and Dirac-Slater (DS) calculations. A discussion of the influence of relativistic and potential barrier effects on the experimental data is given.

Spin-Polarization Measurements in the Autoionization Region of Mercury

The autoionization region of mercury was investigated with spin- and angle-resolved photoelectron spectroscopy between the Hg+ (5d9 6s2)2D5/2 and Hg+ (5d9 6s2)2D3/2 thresholds. Spin-polarization parameters A and ξ and intensity were measured separately for the photoelectrons corresponding to the two final ionic states accessible (2S1/2, 2D5/2) as function of the radiation wavelength. The data are dominated by structure due to autoionization resonances and differ quantitatively for the two peaks in the photoelectron spectra. The results are compared with earlier measurements below the 2D5/2-threshold.

A rotatable electron spectrometer system for spin- and angle-resolved photoemission experiments with circularly polarized synchrotron radiation

An electron-optical system is described which has been especially designed for spin- and angle-resolved photoelectron spectroscopy with circularly polarized synchrotron radiation. One hv- and one uhv-type of this system (for studies at atoms/molecules and solids/adsorbates, respectively) are in operation at the storage ring BESSY. The system employs, thus, a light source and an electron spin detector (high energy Mott detector) both fixed in space. This is realized by a rotatable electron spectrometer and a 90° deflector by which the electron beam is directed along the axis of rotation of the spectrometer, independent of the emission angle chosen.

Singlet-triplet mixing in Hg 6s photoionisation via autoionising transitions

New experimental photoabsorption cross section and angle-resolved photoelectron spin polarisation data for mercury are presented. For the first three autoionisation resonances above the Hg 6s threshold an analysis is performed in terms of transition amplitudes to the singlet and triplet continua. These quantities reveal that photoionisation dynamics is largely influenced by a rather complicated spectral behaviour of the triplet matrix element, including three changes of sign. Results are compared with quantum defect differences and ratios of oscillator strengths from the discrete spectrum.