69 research outputs found
Laboratory von H\'amos X-ray Spectroscopy for Routine Sample Characterization
High energy resolution, hard X-ray spectroscopies are powerful element
selective probes of the electronic and local structure of matter, with diverse
applications in chemistry, physics, biology and materials science. The routine
application of these techniques is hindered by the complicated and slow access
to synchrotron radiation facilities. Here we propose a new, economic, easily
operated laboratory high resolution von H\'amos type X-ray spectrometer, which
offers rapid transmission experiments for X-ray absorption, and is also capable
of recording X-ray emission spectra. The use of a cylindrical analyzer crystal
and a position sensitive detector enabled us to build a maintenance free,
flexible setup with low operational costs, while delivering synchrotron grade
signal to noise measurements in reasonable acquisition times. We demonstrate
the proof of principle and give examples for both measurement types. Finally,
tracking of a several day long chemical transformation, a case better suited
for laboratory than synchrotron investigation, is also presented
Angular dependence of core hole screening in LiCoO2: A DFT+U calculation of the oxygen and cobalt K-edge x-ray absorption spectra
Angular dependent core-hole screening effects have been found in the cobalt
K-edge x-ray absorption spectrum of LiCoO2, using high-resolution data and
parameter-free GGA+U calculations. The Co 1s core-hole on the absorber causes
strong local attraction. The core-hole screening on the nearest neighbours
cobalt induces a 2 eV shift in the density of states with respect to the
on-site 1s-3d transitions, as detected in the Co K pre-edge spectrum. Our DFT+U
calculations reveal that the off-site screening is different in the
out-of-plane direction, where a 3 eV shift is visible in both calculations and
experiment. The detailed analysis of the inclusion of the core-hole potential
and the Hubbard parameter U shows that the core-hole is essential for the
off-site screening, while U improves the description of the angular dependent
screening effects. In the case of oxygen K-edge, both the core-hole potential
and the Hubbard parameter improve the relative positions of the spectral
features
Determining the Quintet Lifetimes in Side-ring Substituted [Fe(terpy)2]2+ Complexes
We previously studied the effect of 4' substitution in iron(II)-bis-terpyridine complexes, showing that the photoexcited high-spin quintet-state is stabilized by electron-donating substituents. In this paper we explore the effects of electron-donating ( X = NH2 , Cl ) and withdrawing ( X = NO2 ) substituents in the 5,5" positions on the stability and lifetime of the quintet-state. We used a simple densitiy-functional theory (DFT) based method that had been proven fairly accurate in the case of 4' substitution to estimate the energy barrier of the quintet-singlet transition and thereby predict the quintet state lifetime. We synthetized the complexes and used ultrafast transient optical absorption spectroscopy to experimentally determine the quintet lifetimes, in order to test the applicability of these quantum-chemistry based predictive methods for these side-ring substitution cases. UV-Visible spectra of the complexes have shown that the metal-to-ligand charge transfer (MLCT) and ligand-localized transitions of these complexes change according to the previous observations. We have shown that in the 5,5" positions, electron withdrawing groups stabilize the quintet state, while donating groups destabilize it. This is in stark contrast to the effects previously observed for the 4' case, and indicates that unlike the latter case, the simple concept of inductive and mesomeric effects may not be adequate to describe the changes due to 5,5" substitution, warranting further study of the area
Screening in YBCO at large wave vectors
We present experimental inelastic x-ray scattering (IXS) and ab initio
time-dependent density-functional-theory (TDDFT) studies of YBa2Cu3O7-{\delta}.
The response of the low-lying Ba 5p and Y 4p core electrons is shown to
interact strongly with the Cu 3d and O 2p excitations, with important
consequences on screening. The agreement between IXS and TDDFT results is
excellent, apart from a new type of excitations, mainly related to loosely
bound Ba electrons and significantly affected by correlations. This points to
correlation mechanisms not fully described by TDDFT that might have a role in
giving rise to antiscreening.Comment: 6 pages, 3 figure
On the microscopic origin of the magneto-electronic phase separation in Sr doped LaCoO3
The nanoscopic magneto-electronic phase separation in doped La1-xSrxCoO3
perovskites was studied with local probes. The phase separation is directly
observed by M\"ossbauer spectroscopy in the studied doping range of 0.05 <= x
<= 0.25 both at room-temperature as well as in the low temperature magnetic
phase. Extended with current synchrotron based X-ray spectroscopies, these data
help to characterize the volume as well as the local electric and magnetic
properties of the distinct phases. A simple model based on a random
distribution of the doping Sr ions describes well both the evolution of the
separated phases as well as the variation of the Co spin state. The experiments
suggest that Sr doping initiates small droplets and a high degree of doping
driven cobalt spin-state transition, while the Sr-free second phase vanishes
rapidly with increasing Sr content
Metal-ligand interplay in strongly-correlated oxides: a parametrized phase diagram for pressure induced spin transitions
We investigate the magnetic properties of archetypal transition-metal oxides
MnO, FeO, CoO and NiO under very high pressure by x-ray emission spectroscopy
at the K\beta line. We observe a strong modification of the magnetism in the
megabar range in all the samples except NiO. The results are analyzed within a
multiplet approach including charge-transfer effects. The pressure dependence
of the emission line is well accounted for by changes of the ligand field
acting on the d electrons and allows us to extract parameters like local
d-hybridization strength, O-2p bandwidth and ionic crystal field across the
magnetic transition. This approach allows a first-hand insight into the
mechanism of the pressure induced spin transition.Comment: 5 pages, 3 figure
Probing Spin-Vibronic Dynamics Using Femtosecond X-ray Spectroscopy
Ultrafast pump-probe spectroscopy within the X-ray regime is now possible owing to the devel- opment of X-ray Free Electrons Lasers (X-FELs) and are opening new opportunities for direct probing the correlated evolution of the nuclei, the electronic and spin degrees of freedom on the femtosecond timescale. In this contribution we use excited state wavepacket dynamics of the photoexcited decay of a new Fe(II) complex, [Fe(bmip)2]2+ (bmip=2,6-bis(3-methyl-imidazole- 1-ylidine)pyridine), to simulate the experimental observables associated with femtosecond Fe K- edge X-ray absorption near-edge structure (XANES) and X-ray emission (XES) spectra. We show how the evolution of the nuclear wavepacket is translated into the experimental observable and the sensitivity of these approaches for following excited state dynamics
Temperature and pressure-induced spin-state transitions in LaCoO3
We report the continuous variation of the spin moment of cobalt in LaCoO3
across its temperature and pressure-induced spin transitions evidenced with
K\beta emission spectra. The first thermal transition is best described by a
transition to an orbitally nondegenerate intermediate spin (S=1) state. In
parallel, continuous redistribution of the 3d electrons is also indicated by
partial fluorescence yield X-ray absorption spectra. At high pressure, our
study confirms that the material becomes low spin between 40 and 70 kbar at
room temperature
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