13,318 research outputs found
Femtosecond X-ray emission study of the spin cross-over dynamics in haem proteins
In haemoglobin (consisting of four globular myoglobin-like subunits), the
change from the low-spin (LS) hexacoordinated haem to the high spin (HS)
pentacoordinated domed form upon ligand detachment and the reverse process upon
ligand binding, represent the transition states that ultimately drive the
respiratory function. Visible-ultraviolet light has long been used to mimic the
ligand release from the haem by photodissociation, while its recombination was
monitored using time-resolved infrared to ultraviolet spectroscopic tools.
However, these are neither element- nor spin-sensitive. Here we investigate the
transition state in the case of Myoglobin-NO (MbNO) using femtosecond Fe Kalpha
and Kbeta non-resonant X-ray emission spectroscopy (XES) at an X-ray
free-electron laser upon photolysis of the Fe-NO bond. We find that the
photoinduced change from the LS (S = 1/2) MbNO to the HS (S = 2)
deoxy-myoglobin (deoxyMb) haem occurs in ca. 800 fs, and that it proceeds via
an intermediate (S = 1) spin state. The XES observables also show that upon NO
recombination to deoxyMb, the return to the planar MbNO ground state is an
electronic relaxation from HS to LS taking place in ca. 30 ps. Thus, the entire
ligand dissociation-recombination cycle in MbNO is a spin cross-over followed
by a reverse spin cross-over process
Hard x-ray emission spectroscopy: a powerful tool for the characterization of magnetic semiconductors
This review aims to introduce the x-ray emission spectroscopy (XES) and
resonant inelastic x-ray scattering (RIXS) techniques to the materials
scientist working with magnetic semiconductors (e.g. semiconductors doped with
3d transition metals) for applications in the field of spin-electronics. We
focus our attention on the hard part of the x-ray spectrum (above 3 keV) in
order to demonstrate a powerful element- and orbital-selective characterization
tool in the study of bulk electronic structure. XES and RIXS are
photon-in/photon-out second order optical processes described by the
Kramers-Heisenberg formula. Nowadays, the availability of third generation
synchrotron radiation sources permits applying such techniques also to dilute
materials, opening the way for a detailed atomic characterization of
impurity-driven materials. We present the K{\beta} XES as a tool to study the
occupied valence states (directly, via valence-to-core transitions) and to
probe the local spin angular momentum (indirectly, via intra-atomic exchange
interaction). The spin sensitivity is employed, in turn, to study the
spin-polarized unoccupied states. Finally, the combination of RIXS with
magnetic circular dichroism (RIXS-MCD) extends the possibilities of standard
magnetic characterization tools.Comment: Topical review (17 pages, 13 figures, 188 references). This is an
author-created, un-copyedited version of an article accepted for publication
in Semiconductor Science and Technology. IOP Publishing Ltd is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from i
XAFS analyses of molten metal fluorides
X-ray absorption fine structure studies of molten metal fluorides containing the materials related to nuclear engineering are intensively summarized. By using XAFS spectra data of divalent and trivalent cation metal fluorides in molten state which have been collected by authors’ group for a few years, local structure have been extracted and discussed systematically in conjunction with other spectroscopic studies and numerical calculations. In molten divalent fluorides, tetrahedral coordination of fluorides around a cation is predominant. In the case of pure molten trivalent fluorides, structure with more than 6-coordination has been suggested in some cases, but octahedral coordination structure is much stabilized at heavier rare earth metal fluorides. By mixing with alkali metal fluorides, it is a general trend that inter-ionic distances keep constant, but coordination number of fluorides decreases. In experimental chapter, all the details of sample preparation, furnace installation, X-ray optics setups and data analyses procedures are explained. Finally, future expectations of XAFS technique are also suggested
Resonant Elastic Soft X-Ray Scattering
Resonant (elastic) soft x-ray scattering (RSXS) offers a unique element,
site, and valence specific probe to study spatial modulations of charge, spin,
and orbital degrees of freedom in solids on the nanoscopic length scale. It
cannot only be used to investigate single crystalline materials. This method
also enables to examine electronic ordering phenomena in thin films and to zoom
into electronic properties emerging at buried interfaces in artificial
heterostructures. During the last 20 years, this technique, which combines
x-ray scattering with x-ray absorption spectroscopy, has developed into a
powerful probe to study electronic ordering phenomena in complex materials and
furthermore delivers important information on the electronic structure of
condensed matter. This review provides an introduction to the technique, covers
the progress in experimental equipment, and gives a survey on recent RSXS
studies of ordering in correlated electron systems and at interfaces
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