Search

161Dy time-domain synchrotron Mossbauer spectroscopy (SMS), just recently presented as a novel tool to investigate Dy(III)-based single-molecule magnets (SMMs), has now been applied on two polynuclear Dy6-based wheel compounds possessing special features of single-molecule toroics (SMTs). The magnitude of the magnetic hyperfine field revealed for the Dy6 ring system [Dy6(teaH)6(NO3)6]·8MeOH is with B0 = 574.3(5) T characteristic for Dy(III) with a 6H15/2 ground state. The ligand substituted compound [Dy6(Me-teaH)6(NO3)6]·6MeCN possesses within the experimental errors the same magnetic hyperfine characteristics (B0 = 574.9(5) T). Consequently, the herein studied organic ligand variation has no perceptible influence on the magnetic hyperfine field sensed by the 161Dy(III) nuclei in these particular homometallic Dy(III)6-wheels.

We present a novel method for detecting the solid–liquid phase boundary of compressed iron at high temperatures using synchrotron Mossbauer spectroscopy (SMS). Our approach is unique because the dynamics of the iron atoms are monitored. This process is described by the Lamb–Mossbauer factor, which is related to the mean-square displacement of the iron atoms. Focused synchrotron radiation with 1 meV bandwidth passes through a laser-heated ^(57)Fe sample inside a diamond-anvil cell, and the characteristic SMS time signature vanishes when melting occurs. At our highest compression measurement and considering thermal pressure, we find the melting point of iron to be T_M=3025±115 K at P=82±5 GPa. When compared with previously reported melting points for iron using static compression methods with different criteria for melting, our melting trend defines a steeper positive slope as a function of pressure. The obtained melting temperatures represent a significant step toward a reliable melting curve of iron at Earth's core conditions. For other terrestrial planets possessing cores with liquid portions rich in metallic iron, such as Mercury and Mars, the higher melting temperatures for compressed iron may imply warmer internal temperatures.

New aspects of synchrotron Mössbauer microscopy have been reported, including micrometer spatial resolution, forward as well as backscattering geometry, and the ability to measure samples with natural isotopic abundance, such as meteorites., New aspects of synchrotron Mössbauer microscopy are presented. A 5 µm spatial resolution is achieved, and sub-micrometer resolution is envisioned. Two distinct and unique methods, synchrotron Mössbauer imaging and nuclear resonant incoherent X-ray imaging, are used to resolve spatial distribution of species that are chemically and magnetically distinct from one another. Proof-of-principle experiments were performed on enriched 57Fe phantoms, and on samples with natural isotopic abundance, such as meteorites.

We have developed a multi-wavelength Fast Temperature Readout (FasTeR) spectrometer to capture a sample’s transient temperature fluctuations, and reduce uncertainties in melting temperature determination. Without sacrificing accuracy, FasTeR features a fast readout rate (about 100 Hz), high sensitivity, large dynamic range, and a well-constrained focus. Complimenting a charge-coupled device spectrometer, FasTeR consists of an array of photomultiplier tubes and optical dichroic filters. The temperatures determined by FasTeR outside of the vicinity of melting are, generally, in good agreement with results from the charge-coupled device spectrometer. Near melting, FasTeR is capable of capturing transient temperature fluctuations, at least on the order of 300 K/s. A software tool, SIMFaster, is described and has been developed to simulate FasTeR and assess design configurations. FasTeR is especially suitable for temperature determinations that utilize ultra-fast techniques under extreme conditions. Working in parallel with the laser-heated diamond-anvil cell, synchrotron Mössbauer spectroscopy, and X-ray diffraction, we have applied the FasTeR spectrometer to measure the melting temperature of ^(57)Fe_(0.9) Ni_(0.1) at high pressure.

In the first known kinetic application of the technique, synchrotron 57Fe-Mossbauer spectroscopy was used to follow the rate of heterogeneous electron transfer between aqueous reagents and a solid phase containing Fe. The solid, a synthetic 57Fe-enriched Fe(III)-bearing pyroaurite-like phase having terephthalate (TA) in the interlayer [Mg3Fe(OH)8(TA)0.5 · 2H2O], was reduced by Na2S2O4 and then reoxidized by K2Cr2O7 by means of a novel flow-through cell. Synchrotron Mossbauer spectra were collected in the time domain at 30-s intervals. Integration of the intensity obtained during a selected time interval in the spectra allowed sensitive determination of Fe(II) content as a function of reaction time. Analysis of reaction end member specimens by both the synchrotron technique and conventional Mossbauer spectroscopy yielded comparable values for Mossbauer parameters such as center shift and Fe(II)/Fe(III) area ratios. Slight differences in quadrupole splitting values were observed, however. A reactive diffusion model was developed that fit the experimental Fe(II) kinetic data well and allowed the extraction of second-order rate constants for each reaction. Thus, in addition to rapidly collecting high quality Mossbauer data, the synchrotron technique seems well suited for aqueous rate experiments as a result of the penetrating power of 14.4 keV X-rays and high sensitivity to Fe valence state.

A new time integrated method for the study of resonant nuclear scattering of synchrotron radiation in the forward direction or in Bragg directions is introduced. This method gives in principle similar information as the well known time differential method. A brief comparison of both methods is presented. The idea is to excite coherently the nuclei incorporated in two absorbers, one moving with respect to the other. The fields radiated by the nuclei from both absorbers interfere and each time the nuclear energy in one absorber matches, by Doppler modulation, the nuclear energy of the other, an extremum in the time integrated intensity is observed. The results of the first experiments at the Advanced Photon Source at the Argonne National Laboratory will be presented.

Synchrotron Mossbauer Spectroscopy, SMS, is an emerging technique that allows fast and accurate determination of hyperfine field parameters similar to conventional Mossbauer spectroscopy with radioactive sources. This new technique, however, is qualitatively different from Mossbauer spectroscopy in terms of equipment, methodology, and analysis to warrant a new name. In this paper, the authors report on isomer shift and quadrupole splitting measurements of Mohr`s salt, Fe(NH{sub 4}){sub 2}(SO{sub 4}){sub 2}{center_dot}6H{sub 2}O for demonstration purposes. Theoretical calculations were performed and compared to experiments both in energy and time domain to demonstrate the influence of thickness distribution and preferential alignment of powder samples. Such measurements may prove to be useful when the data collection times are reduced to few seconds in the third generation, undulator based synchrotron radiation sources.

We have constructed the first working model of a novel diffraction geometry for the high-resolution monochromatization of hard (5–30 keV) synchrotron radiation. The monochromator has an energy bandpass of 6.7 meV FWHM and an angular acceptance of 22 μrad (4.5 arcseconds) for 14.4 keV X rays - an energy resolution ΔEE of 4.6 × 10−7. With a second set of crystals, an energy bandpass of 23 meV FWHM and an angular acceptance of 7 μrad was achieved for 23.8 keV photons. The performance of the monochromator has been verified, in a nearly ideal measurement, using nuclear-fluorescent X rays with an energy bandwidth on the order of 10 neV.

We have built a high-energy-resolution monochromator for nuclear resonant scattering from the 9.4 keV nuclear transition of 83Kr. The monochromator consists of two highly asymmetric silicon single crystals arranged in a dispersive geometry and produces an energy bandwidth of 2.3 meV. This monochromator has been successfully used for a study of nuclear forward scattering and nuclear resonant inelastic scattering from a 83Kr sample that was solidified in a diamond anvil cell at 2.15 GPa and room temperature.

A white beam collimator has been built to increase the efficiency of a diamond high-heat-load premonochromator. The collimator is composed of a water-cooled beryllium compound refractive lens. We demonstrate a 30% increase in spectral flux of the diamond premonochromator (at 22.6 keV) by using the collimator.

Grazing‐incidence antireflection (GIAR) films are used to suppress electronic reflectivity, while maintaining good nuclear resonant reflectivity to help monochromatize the synchrotron radiation down to the μeV level of energy resolution. Such monochromators will have superior throughput by a factor of 100–500 compared to natural linewidth of individual nuclei. 119SnO2 GIAR films of various thicknesses were designed and fabricated on Pd‐coated quartz substrates. The performance of these films was evaluated based on the measurement of off‐resonance x‐ray reflectivity using MoKα radiation and conversion electron Mossbauer spectroscopy. A Pd film was deposited on a quartz substrate at room temperature and was subsequently annealed at 500 °C in an Ar atmosphere at a substrate temperature of 200 °C, SnO2 was sputtered onto the Pd by reactive sputtering in an Ar and O2 atmosphere. Employing quantitative analysis of surface and interface roughness, a series of experiments was also conducted to determine optimal s...

With an undulator-based synchrotron source and a tunable x-ray monochromator, we produce an x-ray beam with a flux of 4×108 photons/s in an energy bandwidth of 920±110 μeV (ΔE/E≈6.2×10−8) at a photon energy of 14.4 keV. The tunability of the monochromator allows for a measurement of lattice excitations in α-Fe using the technique of inelastic nuclear resonant scattering from 57Fe. The phonon density-of-states are extracted from the measurement and compared to the calculated phonon density-of-states derived from neutron-scattering measurements.

The design and performance results for a cryogenically stabilized high-resolution monochromator for 23.880 keV (lambda = 52 pm) X-rays are presented. The four-crystal-reflection monochromator is suitable for nuclear resonant scattering measurements from 119Sn compounds using synchrotron radiation. The design includes a low-vibration cryostat that maintains two of the four crystal reflections at a temperature where the coefficient for thermal expansion of the crystalline material (silicon) vanishes. Test results demonstrate a 1.3 meV bandwidth with negligible broadening due to vibrations and a spectral efficiency of 37% when used with an undulator source.

A silicon (840) channel‐cut crystal with a large asymmetry is used to linearly polarize synchrotron radiation generated by an undulator source at the 14.413 keV nuclear resonance of 57Fe. The resulting σ‐polarized radiation is then scattered from an 57Fe foil placed in a magnetic field that effects σ→π polarization conversion within the resonant bandwidth. A second crystal of the same type is placed in a crossed position to suppress the nonresonant radiation while transmitting the π‐polarized resonant radiation. A polarization suppression factor of 6×10−7 has been obtained. The suppression of the nonresonant radiation allowed monitoring the decay of the nuclear levels after 1 ns of their excitation.

Monochromatization with crystal diffraction has been achieved to a resolution (lambda/delta lambda) beyond 10(8). The monchromator is specifically designed for 23.880 keV synchrotron radiation (lambda = 51.9 pm) for applications involving nuclear resonant scattering from 119Sn. The design uses asymmetrically cut silicon (12 12 12) crystal reflections from two single-crystalline monoliths oriented in a dispersive geometry. A transmitted energy bandwidth of 140 +/- 20 mu eV was measured, corresponding to a resolution of 1.7 x 10(8). Methods of improving efficiency, wavelength stability and resolution are discussed.

We present a novel miniature multi-axis driving structure that will allow positioning of two crystals with better than 50-nrad angular resolution and nanometer linear driving sensitivity. The precision and stability of this structure allow the user to align or adjust an assembly of crystals to achieve the same performance as does a single channel-cut crystal, so we call it an “artificial channel-cut crystal.” In this paper, the particular designs and specifications, as well as the test results, for a two-axis driving structure for a high-energy-resolution artificial channel-cut crystal monochromator are presented.

A 4-bounce dispersive crystal monochromator reduces the bandpass of synchrotron radiation to a 10-50 meV range without sacrificing angular acceptance. The monochromator includes the combination of an asymmetrical channel-cut single crystal of lower order reflection and a symmetrical channel-cut single crystal of higher order reflection in a nested geometric configuration. In the disclosed embodiment, a highly asymmetrically cut (α=20) outer silicon crystal (4 2 2) with low order reflection is combined with a symmetrically cut inner silicon crystal (10 6 4) with high order reflection to condition a hard x-ray component (5-30 keV) of synchrotron radiation down to the μeV-neV level. Each of the crystals is coupled to the combination of a positioning inchworm and angle encoder via a respective rotation stage for accurate relative positioning of the crystals and precise energy tuning of the monochromator.

The insertion-device-based, third-generation, synchrotron radiation sources now under construction in Europe, the USA, and Japan bring new opportunities and challenges in the design and manufacture of x-ray optics. These high brightness sources provide new opportunities to overcome some of the outstanding problems associated with nuclear resonant monochromatization of synchrotron radiation. New methods such as polarizing monochromators, and zone plates provide alternative methods for production of (mu) eV-neV resolution in the hard x-ray regime. The design principles, and characterization, and performance of crystal monochromators and of nuclear coherent scattering optics, including Grazing Incidence Anti Reflection (GIAR) films, multilayers, zone plates, as well as single crystals are discussed.