The Ba 4d–4f giant dipole resonance in complex Ba/Si compounds

Abstract

, and the semiconducting BaSi 2 . Here, non-resonant x-ray Raman scattering is used to investigate confinement effects on the shape of the giant resonance in the vicinity of the Ba N IV,V -edge. The distinct momentum transfer dependence of the spectra is analyzed and discussed. The measurements are confronted with calculations of the giant resonance within time-dependent local density approximation in the dipole limit. No modulation of the giant resonance's shape for Ba atoms confined in different local environments was observed, unlike predicted by the calculations. The absence of such shape modulation for complex Ba/Si compounds is discussed providing important implications for further studies of giant resonance phenomena utilizing both theory and experiment.

High-resolution nonresonant x-ray Raman scattering study on rare earth phosphate nanoparticles

“…The term symbol is 1 S 0 . A well-known feature of the → d f 4 4 excitation at the dipole limit is the GDR, [4] which corresponds to a transition → S P 1 0 1 1 . Higher-order multipolar transitions can be observed by EELS and XRS, as thoroughly discussed by Van der Laan [24].…”

“…In the LS coupling scheme, transitions are possible for multipoles k that satisfy | − ′| < < | + ′| L L k L L , where L and L′ are the angular quantum numbers of the initial and final state configuration, respectively. Together with the requirement that in parity conserving (changing) transitions only even (odd) multipoles are possible, [24] this means that in a → d f 4 4 transition only multipoles of k = 1, 3, and 5 (dipole, octupole, and triakontadipole, respectively) contribute. The 1 P 1 state interacts strongly with continuum states, and this mixing gives rise to an asymmetric Fano line shape with a width of 4-5 eV.…”

“…The spectrometer was equipped with seven diced analyzer crystals, and two elastically spherically bent analyzer crystals, all operating in the Johann geometry and employing the Si(nnn) reflection. The Rowland circle diameter, and thus the curvature radii of the analyzers, equaled to 1 m. We used the Si(555) analyzer reflection for LaPO 4 and CePO 4 samples. The total energy resolution when using the diced analyzer crystals was 180 meV as determined from the quasielastic line width, determined mainly by the incident beam bandwidth.…”

“…The measurements were done at two values of θ 2 , namely, 45°and 134°. For the low-momentum transfer setting that was used in the case of La → d f 4 4 , this yields the individual momentum transfer values 2.7, 3.3, 3.9, 4.4, and 4.9 Å −1 , with uncertainties of ±0.2 Å −1 , for each value. For the highmomentum transfer setting for La and Ce, the corresponding values are 8.8, 9.0, 9.3, 9.5, and 9.7 Å −1 , with uncertainties of ±0.1 Å −1 .…”

“…A peculiar aspect found in rare earth atomic spectra is the giant dipole resonance (GDR) [2] that prevails in the XAS spectra of, e.g., → d f 4 4 excitations. The GDR spectra have also been studied in detail in compounds other than ones with rare earths since it has been suggested that the GDR shape modulations are sensitive to the local environment of the absorbing atom, yielding novel access to structural information [3,4]. In rare earths at energies below the GDR, the XAS spectrum typically manifests weak spin-forbidden excitations that gain non-negligible spectral weight due to spin-orbit interaction and the interaction with the GDR.…”