ALBA Synchrotron
The instrument, that has been engineered in-house and manufactured partially-in house and by local companies, allows to energy-analyze the emitted fluorescence and to resolve with good energy resolution the signals from the different de-excitation channels. Thanks to the combined use of a Mythen unidimensional detector and a silicon diced analyzer, it permits the acquisition of the spectrum on a single-shot basis.
It gives access to all the complementary information obtainable by investigating the atomic emission lines and, the rather XAS featureless spectra, obtained with the conventional energy integrated mode, become spectra with fine structure containing key information on electronic levels and magnetic properties.
The figure shows on the left a sketch of the CLEAR spectrometer with, at the lower right, a picture of the presently operating Si(1,1,1) dynamically bent diced analyzer. This instrument is foreseen to work, like the other existing emission spectrometer, in Rowland circle geometry, but, differently from the others, to cover continuously between 2 and 22 keV by means of 4 analyzer crystals (with different crystals reflections) in fully back of forward scattering geometries, exploiting a working wide Bragg angular range (30°-80°). The energy resolution and photon intensity (focal properties) are ensured by the diced analyzer and the analyzer dynamical sagittal bending. While only the Si(1,1,1) analyzer reflection is at the moment available, other analyzers are underway starting from the Si(2,2,0) reflection.
While figure above (right) reports an example of recently measured Co Kβ emission lines, this figure shows an example of high resolution absorption spectra collected by selecting the Co Kβ1,3 emission energy and scanning the incoming energy across the Co K-edge. The combination of a unidimensional detector and a diced analyzer permitted the acquisition of the emission spectrum during the incoming energy scan on a single-shot basis (inset in figure: 2D plot representing the spectral intensity as a function of the emitted (or transferred) and incoming photon energy across the absorption pre-peak). The 2D plots reveal two different final states, not detectable by the classical XAS.
Features
The CLEAR X-ray spectrometer is an energy dispersive spectrometer based on a 1m radius Rowland circle geometry assembled in vacuum, which is highly convenient to avoid misleading signals coming from the sample environment. It has a wide Bragg angle range (40º-80º) and the disposition is a fully back scattering one relative to the sample. A dynamically bent Si(1,1,1) diced analyzer crystal is located inside the spectrometer, which allows to cover an energy range from 6 to 22 keV by means of exploiting the different reflections allowed in Si(1,1,1). The X-ray beam used to study the sample passes through the two halves of the analyzer.
The energy resolution of CLEAR depends on the size of the beam used, the Bragg angle picked for the experiment and the particular reflection being exploited to make the measurement, but it is typically in the range of 0.5-2 eV. Since that range is almost continuous, and since the post processing of the collected data is not to be performed by the user of the spectrometer who can rely on simple ascii files, it proves to be user friendly.
Compatible with all sample setups. Fluorescence is collected in a back-scattering geometry and in vacuum (no additional side window is required, low noise).
Acquisition. The geometry allows energy dispersive images of 6-20 eV (in-Rowland geometry) and 40 eV (out-of-Rowland geometry). Outgoing energy scans in continuous mode allow the investigation of wider energy ranges.
XES, HERFD, RXES, and RIXS. The CLEAR spectrometer is optimized for short time measurements of both emission lines and high-resolution absorption spectra of highly concentrated systems (around half an hour) but for highly diluted systems or weak emission lines, as the ones corresponding to the valence-to-core transitions, as well as RIXS measurements it is still not competitive and needs longer measurement times.