New experimental information from two-photon spectroscopy and comparison with theory

From a specially designed expt., allowing the measurement of 2-photon absorption spectra and 2-photon polarization parameters over a wide spectral range, new exptl. information is obtained on the location of electronically excited states usually not seen in the UV spectrum. By comparison with theory it is shown that in most of the examples investigated up to now the ordering of the low lying excited states is different from what is usually expected. Correlation strongly influences the ordering esp. in those systems which consist of loosely coupled p-systems

Investigation of Sub-Shell Occupation in Some Ba Compounds by LMTO-ASA Calculations

Ba compounds behave very unusual in XPS-studies as they exhibit only small and often negative binding energy shifts. Four different explanations have been given for these observation. To provide a more quantitative basis for these different models and to shed new light on the unusual behaviour of Ba compounds, we calculated total and 1-projected densities of states for Ba metal and three Ba compounds. All four explanations given up to now are not in qualitative agreement with the obtained results. A different possible explanation is discussed

Hoeher angeregte Elektronenzustaende von zyklisch konjugierten pi-Systemen Schlussbericht

Available from TIB Hannover: DtF QN1(48,17) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman

A new view of valence instabilities in europium compounds: van Hove singularity in EuPdP

Photoemission spectra and the TB-LMTO-ASA band structure of the mixed valent europium compound EuPdP (ZrBeSi type structure) were studied and compared with the results obtained on isostructural SrPdP and EuAuAs and the ThCr2Si2-type compound EuNi2P2. EuNi2P2 is a homogeneous mixed valent compound, whereas the situation in EuPdP is not clear. The Mossbauer spectroscopy suggests static mixed valency in EuPdP corresponding to partially localized 4f electrons, but from the photoemission measurement we know that the situation in EuPdP is different from the static mixed valent Eu3O4 Band structure calculations as well as photoemission spectra indicate a 4f density of states in the vicinity of the Fermi level in EuPdP, which belongs to a van Hove Singularity (vHS). In EuPdP we therefore expect homogeneous mixed valence, similar to EuNi2P2 with itinerant 4f electrons. The absence of homogeneous mixed valence is possibly associated with the layered structure of EuPdP, enabling charge ordering through a Martensitic phase transition. A high density of states associated with a singularity at the Fermi energy seems to be responsible for the structural instability in inhomogeneous mixed valent europium compounds

A new view of valence instabilities in europium compounds: van Hove singularity in EuPdP

Photoemission spectra and the TB-LMTO-ASA band structure of the mixed valent europium compound EuPdP (ZrBeSi type structure) were studied and compared with the results obtained on isostructural SrPdP and EuAuAs and the $\rm ThCr_2Si_2$-type compound $\rm EuNi_2P_2$. $\rm EuNi_2P_2$ is a homogeneous mixed valent compound, whereas the situation in EuPdP is not clear. The Mössbauer spectroscopy suggests static mixed valency in EuPdP corresponding to partially localized $4\,f$ electrons, but from the photoemission measurement we know that the situation in EuPdP is different from the static mixed valent $\rm Eu_3O_4$. Band structure calculations as well as photoemission spectra indicate a $4\,f$ density of states in the vicinity of the Fermi level in EuPdP, which belongs to a van Hove Singularity (vHS). In EuPdP we therefore expect homogeneous mixed valence, similar to $\rm EuNi_2P_2$ with itinerant $4\,f$ electrons. The absence of homogeneous mixed valence is possibly associated with the layered structure of EuPdP, enabling charge ordering through a Martensitic phase transition. A high density of states associated with a singularity at the Fermi energy seems to be responsible for the structural instability in inhomogeneous mixed valent europium compounds