G. Kaindl

Detailed study of pyridine at the C 1s and N 1s ionization thresholds: The influence of the vibrational fine structure

High resolution, vibrationally resolved, near-edge x-ray absorption fine structure (NEXAFS) spectra at the C 1s and N 1s ionization thresholds of pyridine and deuterated d5-pyridine in the gas phase have been recorded. The high resolution of 65 meV (150 meV) at the C s (N 1s) ionization thresholds reveals vibrational structures in the spectra. Detailed ab initio and density functional theory (DFT) calculations were performed to interpret the experimental spectra and to assign the observed peaks. In particular we focused on the previously unexplained intensity ratio for the two components of the C 1s→1π* transition. For this transition the vibrational structure is included through a linear coupling model in the DFT calculations and leads to the experimentally observed ∼2:3 intensity ratio between the two π* components in the C 1s spectrum rather than the ∼3:2 ratio obtained without vibrational effects. After inclusion of relaxation effects in the excited states, in addition to the vibrational effects, both...

M‐edge x‐ray absorption spectroscopy of 4f instabilities in rare‐earth systems (invited)

Soft x‐ray absorption spectroscopy in the M‐edge region is explored as a tool for studying 4f instabilities. The quasiatomic MIV,V spectra, recorded by total‐electron yield under UHV conditions, carry information on the initial‐state 4f occupancy. For compounds of the heavy rare earths, even at high dilution (e.g., Tm0.05Y0.95Se), mean valences can be derived, which agree with those from lattice‐constant systematics (e.g., for TmSe and Sm0.3Y0.7S). A moderate surface sensitivity allows the observation of surface‐induced valence changes on EuPd3, TmS, and SmAl2. The MIV,V spectra studied for 10 Ce compounds fall in two categories: pure 3d94f1 final‐state multiplets for γ‐like compounds, and additional peaks at about 5 eV higher energies for α‐like and intermediate systems. With increasing intensity of the satellites, which are assigned to 3d94f1 final states, the 3d94f2 multiplet structure gets washed out. An interpretation of the spectra with the recent Anderson‐impurity theory of Gunnarsson and Schonhamm...

Performance of the high-resolution SX700/II monochromator

This article reports on the high‐resolution performance of the grazing‐incidence plane grating monochromator SX700/II, installed at BESSY by the Freie Universitat Berlin, in the photon energy range from about 40 to 900 eV. The high resolving power up to 10 000 achieved with this monochromator is based on improving the figure error of the ellipsoidal focusing mirror, on reducing the vertical dimension of the beam source, and on employing a 5‐μm exit slit. We report on high‐resolution gas‐phase studies in the double‐excitation region of He, as well as at core‐excitation thresholds of Ne, Ar, Kr, and Xe in the photon‐energy range from ≂45 eV to ≂900 eV. In addition, high‐resolution core‐excitation spectra at the K thresholds of C, N, and O are presented for gas‐phase CO, N2, and O2. In all cases, high‐n Rydberg states and/or vibrational sidebands of the electronic excitations were resolved. The various contributions to the present instrumental linewidths are discussed as well as the prospects for further imp...

On the Suppression of Superconductivity in Y1-xPrxBa2Cu3O7-δ

Y1-xPrxBa2Cu3O7-δ compounds exhibit a progressive decrease in superconducting transition temperature with increasing x. Using soft-X-ray absorption spectroscopy at the Pr-MIV,V thresholds, Pr is shown to be trivalent for all x in these compounds contrary to expectations, excluding an obvious mechanism for the suppression of Tc based on the tetravalency of Pr. From a discussion of other possible mechanisms it is suggested that Tc decreases through changes in electronic-band structure upon Pr substitution leading ultimately to a semiconducting behaviour for x = 1.0.

Carbon and oxygen K-edge photoionization of the CO molecule

Abstract High-resolution, high-intensity photoionization studies were performed near the carbon and oxygen K-edges of gas-phase CO. At least 37 absorption lines were resolved at the carbon K-edge, and new information was obtained about the π*, Rydberg, and double-excitation resonances, including Rydberg states up to n=7 and vibrational transitions up to v′ = 3. Vibrational structure in oxygen is 1s−1π* and Rydberg states was resolved for the first time. The derived molecular structure parameters are consistent with the Z+1 approximation. High-resolution, high-intensity core-level photoabsorption promises new opportunities in vibrational state-to-state chemistry and surface science.

Commissioning results and performance of the high-resolution Russian-German Beamline at BESSY II

Excellent results were obtained during commissioning of the Russian–German Beamline at bending-magnet D16-1A of the electron-storage ring BESSY II. The beamline is based on a Petersen-type plane-grating monochromator covering the XUV and soft X-ray photon-energy range. Photoionization spectra taken from the 2,–13 resonance of doubly excited He reveal a resolving power of E=DED100,000 at hn=64.12 eV. To our knowledge, this represents the best energy resolution achieved up to now on a bending-magnet beamline in the grazing-incidence photon-energy range. We present results on photon flux and dimensions of the light spot at the sample position, and show representative photoionization spectra of several gases studied in the course of the characterization of the beamline at various photon energies. The high stability of the beamline is demonstrated by the first observation of the weak 4,� 24 resonance of doubly excited He, which had not been observed before. r 2003 Elsevier Science B.V. All rights reserved.

Differences between L3 and L2 x‐ray absorption spectra of transition metal compounds

The differences between L3 and L2 edges of 3d and 4d transition metal complexes and compounds in octahedral symmetry are discussed. The main origin of these differences are the multiplet effects due to the coupling of the 2p core wave function and the 3d and 4d valence wave functions. The 3d and 4d spin–orbit coupling is a second origin of difference. For 3d systems the multiplet effects dominate all other interactions and the L3 and L2 edge are completely mixed and reordered. For 4d systems the core hole spin–orbit coupling is large and the L3 and L2 are separated by about 100 eV with a ratio close to 2:1. The differences between the L3 and L2 edge originate from the weight transfer between the t2g and eg peaks due to the multiplet effect. This weight transfer is about 25% for the L3 edge and about 5% for the L2 edge, which implies that for a comparison to single‐particle calculations the L2 edge is preferable to use. Partly filled 4d systems are low‐spin and the occupation of the t2g states implies a de...

Resonance spectroscopy of γ-radiation on Au (I) and Au (III) compounds

Isomer shift (IS) and electric quadrupole splitting (QS) of the 77 keV γ-rays of Au197 were investigated for a large number of Au (I) and Au (III) compounds at 4.2 °K by nuclear γ-resonance Spectroscopy. A close correlation between the observed isomer shifts and the spectrochemical series of the ligands was observed. For each oxidation state, isomer shift and electric quadrupole splitting show approximately a linear relationship. On the basis of LCAO-MO theory, the experimental results are interpreted by covalency effects in the molecular orbitals, synergic coupling of σ- and π-bonds, and the empirically known donor and acceptor properties of the ligands.

Rashba effect at magnetic metal surfaces

Exchange-split two-dimensional electronic states at the magnetic Gd(0001) surface change their energy dispersion upon magnetization reversal owing to the Rashba effect. The Rashba parameter is found to be substantially enhanced and to change sign when an epitaxial metal-oxide surface layer is formed. The experimental observations are quantitatively described by ab initio calculations giving a detailed account of the near-surface charge-density gradients that are responsible for the Rashba effect.

Bandlike character of 4f electrons in CeRh3

The electronic structure of CeRh 3 was studied by photoemission and bremsstrahlung isochromate spectroscopy (BIS). An analysis of the spectra, taken at different temperatures, on the basis of the Anderson single-impurity model leads to considerable inconsistencies that mark the limits ofapplicability of the model. On the other hand, the BIS spectrum of CeRh 3 can be well described by the results of a local-density-approximation band-structure calculation. These findings confirm a bandlike character of the 4f states in strongly hybridized Ce systems