High-energy-resolution X-ray monochromator calibration using the detailed-balance principle.

A new method is presented to calibrate an X-ray energy scale with sub-meV relative accuracy by using the detailed-balance principle of the phonon creation and annihilation. This method is conveniently used to define or verify the energy scale of high-energy-resolution monochromators that are used in inelastic X-ray scattering and nuclear resonant inelastic X-ray scattering instruments at synchrotron radiation facilities. This method does not rely on sample properties and its precision only depends on the statistical data quality. Well calibrated instruments are essential for reliable comparison of data sets obtained at different synchrotron radiation beamlines, of data with theoretical predictions, and of data from other techniques such as neutron or light scattering. The principle of the detailed-balance method is described in this paper and demonstrated experimentally.

Feasibility of in-line instruments for high-resolution inelastic X-ray scattering.

Inelastic X-ray scattering instruments in operation at third-generation synchrotron radiation facilities are based on backreflections from perfect silicon crystals. This concept reaches back to the very beginnings of high-energy-resolution X-ray spectroscopy and has several advantages but also some inherent drawbacks. In this paper an alternate path is investigated using a different concept, the `M(4) instrument'. It consists of a combination of two in-line high-resolution monochromators, focusing mirrors and collimating mirrors. Design choices and performance estimates in comparison with existing conventional inelastic X-ray scattering instruments are presented.

Synchrotron Mössbauer spectroscopy using high-speed shutters.

A new method of performing Mössbauer spectroscopy with synchrotron radiation is demonstrated that involves using a high-speed periodic shutter near the focal spot of a microfocused X-ray beam. This fast microshuttering technique operates without a high-resolution monochromator and has the potential to produce much higher signal rates. It also offers orders of magnitude more suppression of unwanted electronic charge scattering. Measurement results are shown that prove the principle of the method and improvements are discussed to deliver a very pure beam of Mössbauer photons (E/ΔE ≃ 10(12)) with previously unavailable spectral brightness. Such a source will allow both Mössbauer spectroscopy in the energy domain with the many advantageous characteristics of synchrotron radiation and new opportunities for measurements using X-rays with ultra-high energy resolution.

Vibrational properties of alpha- and sigma-phase Fe-Cr alloy.

Experimental and theoretical studies, of the Fe-partial phonon density of states (PDOS) for Fe52.5Cr47.5 alloy having alpha and sigma phases were carried out. The former using the nuclear resonant inelastic x-ray scattering method, and the latter with the direct one. Characteristic features of PDOS, which distinguish one phase from the other, were revealed and successfully reproduced by the theory. Data pertinent to the dynamics such as the Lamb-Mössbauer factor, f, the kinetic energy per atom, E(k), and the mean force constant, D, were directly derived, while vibrational specific heat at constant volume, C(V), and vibrational entropy, S were calculated using the Fe partial PDOS. Based on the values of f and C(V), we determined Debye temperatures, Theta(D). An excellent agreement for some quantities derived from experiment and first-principles theory, like C(V) and quite good ones for others like D and S were obtained.

Phonon density of states of iron up to 153 gigapascals.

We report phonon densities of states (DOS) of iron measured by nuclear resonant inelastic x-ray scattering to 153 gigapascals and calculated from ab initio theory. Qualitatively, they are in agreement, but the theory predicts density at higher energies. From the DOS, we derive elastic and thermodynamic parameters of iron, including shear modulus, compressional and shear velocities, heat capacity, entropy, kinetic energy, zero-point energy, and Debye temperature. In comparison to the compressional and shear velocities from the preliminary reference Earth model (PREM) seismic model, our results suggest that Earth's inner core has a mean atomic number equal to or higher than pure iron, which is consistent with an iron-nickel alloy.

Vibrational dynamics of myoglobin determined by the phonon-assisted Mössbauer effect.

The phonon-assisted Mössbauer effect is used to determine the partial phonon density of states of the iron within the active center of deoxymyoglobin, carboxymyoglobin, and dry and wet metmyoglobin between 40 and 300 K. Between 0 and 1 meV the iron density of states increases quadratically with the energy, as in a Debye solid. Mean sound velocities are extracted from this slope. Between 1 and 3 meV a nearly quadratic "Debye-like" increase follows due to the similar strength of intermolecular and intramolecular forces. Above 3 meV, optical vibrations are characteristic for the iron-ligand conformation. The overall mean square displacements of the heme iron atom obtained from the density of states agree well with the values of Mössbauer absorption experiments below 180 K. In the physiological temperature regime the data confirm the existence of harmonic vibrations in addition to the protein specific dynamics measured by Mössbauer absorption. In the Debye energy regime the mean square displacement of the iron is in agreement with that of the hydrogens measured by incoherent neutron scattering demonstrating the global character of these modes. At higher energies the vibration of the heavy iron atom at 33 meV in metmyoglobin is as large as that of the lightweight hydrogens at that energy. A freeze dried, rehydrated (h=0.38 g H2O/g protein) metmyoglobin sample shows an excess of states above the Debye law between 1 and 3 meV, similar to neutron scattering experiments. The room temperature density of states below 3 meV exhibit an increase of the density compared to the low temperature data, which can be interpreted as mode softening.

Laser ablation-miniature mass spectrometer for elemental and isotopic analysis of rocks.

A laser ablation-miniature mass spectrometer (LA-MMS) for the chemical and isotopic measurement of rocks and minerals is described. In the LA-MMS method, neutral atoms ablated by a pulsed laser are led into an electron impact ionization source, where they are ionized by a 70 eV electron beam. This results in a secondary ion pulse typically 10-100 µs wide, compared to the original 5-10 ns laser pulse duration. Ions of different masses are then spatially dispersed along the focal plane of the magnetic sector of the miniature mass spectrometer (MMS) and measured in parallel by a modified CCD array detector capable of detecting ions directly. Compared to conventional scanning techniques, simultaneous measurement of the ion pulse along the focal plane effectively offers a 100% duty cycle over a wide mass range. LA-MMS offers a more quantitative assessment of elemental composition than techniques that detect ions directly generated by the ablation process because the latter can be strongly influenced by matrix effects that vary with the structure and geometry of the surface, the wavelength of the laser beam, and the not well characterized ionization efficiencies of the elements in the process. The above problems attendant to the direct ion analysis has been minimized in the LA-MMS by analyzing the ablated neutral species after their post-ionization by electron impaction. These neutral species are much more abundant than the directly ablated ions in the ablated vapor plume and are, therefore, expected to be characteristic of the chemical composition of the solid. Also, the electron impact ionization of elements is well studied and their ionization cross sections are known and easy to find in databases. Currently, the LA-MMS limit of detection is 0.4 wt.%. Here we describe LA-MMS elemental composition measurements of various minerals including microcline, lepidolite, anorthoclase, and USGS BCR-2G samples. The measurements of high precision isotopic ratios including (41)K/(39)K (0.077 ± 0.004) and (29)Si/(28)Si (0.052 ± 0.006) in these minerals by LA-MMS are also described. The LA-MMS has been developed as a prototype instrument system for space applications for geochemical and geochronological measurements on the surface of extraterrestrial bodies.

Observation of phonons with resonant inelastic x-ray scattering.

Phonons, the quantum mechanical representation of lattice vibrations, and their coupling to the electronic degrees of freedom are important for understanding thermal and electric properties of materials. For the first time, phonons have been measured using resonant inelastic x-ray scattering (RIXS) across the Cu K-edge in cupric oxide (CuO). Analyzing these spectra using an ultra-short core-hole lifetime approximation and exact diagonalization techniques, we can explain the essential inelastic features. The relative spectral intensities are related to the electron-phonon coupling strengths.

Determination of the magnetic spin direction from the nuclear forward-scattering line intensities.

An expression is derived for the line intensities in a nuclear forward-scattering energy spectrum that is obtained via a Fourier transformation of the time dependence of the wavefield. The calculation takes into account the coherent properties of the nuclear forward-scattering process and the experimental limitations on the observable time window. It is shown that, for magnetic samples, the spin direction can be determined from the ratios between the different lines in the energy spectrum. The theory is complemented with experimental results on alpha-iron.

A cryogenically stabilized meV-monochromator for hard X-rays.

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.

Anharmonic motions of Kr in the clathrate hydrate.

The anomalous glass-like thermal conductivity of crystalline clathrates has been suggested to be the result of the scattering of thermal phonons of the framework by 'rattling' motions of the guests in the clathrate cages. Using the site-specific (83)Kr nuclear resonant inelastic scattering spectroscopy in combination with conventional incoherent inelastic neutron scattering and molecular-dynamics simulations, we provide unambiguous evidence and characterization of the effects on these guest-host interactions in a structure-II Kr clathrate hydrate. The resonant scattering of phonons led to unprecedented large anharmonic motions of the guest atoms. The anharmonic interaction underlies the anomalous thermal transport in this system. Clathrates are prototypical models for a class of crystalline framework materials with glass-like thermal conductivity. The explanation of the unusual molecular dynamics has a wide implication for the understanding of the thermal properties of disordered solids and structural glasses.

Wavelength-dispersive double flat-crystal analyzer for inelastic X-ray scattering.

A double flat-crystal analyzer for inelastic X-ray scattering is described. The general correlation between the energy and direction of the X-rays transmitted by the analyzer allows one to collect data for a range of energy transfers simultaneously. Such an analyzer with 120 meV resolution was built to operate at the copper K edge. Experimental results show that this X-ray optic can be an alternative to a conventional spherical-focusing backscattering analyzer in resonant inelastic X-ray scattering experiments or when flexible energy resolution or high momentum resolution is required.

Crystal monochromator with a resolution beyond 10(8).

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.

Observation of the 22.5-keV resonance in (149)Sm by the nuclear lighthouse effect.

We have observed coherent nuclear resonant scattering of synchrotron radiation at the 22.5-keV resonance of (149)Sm. High-speed rotational sample motion led to an angular deflection of the resonantly scattered radiation off the nonresonant primary beam. This allowed us to determine the resonance energy of the first excited nuclear level of (149)Sm to be 22496(4) eV. Because of the angular deflection of the resonant photons, time spectra of coherent nuclear resonant scattering can be recorded as a function of a spatial coordinate. Time resolutions of a few 10 ps can be expected, which are beyond the limits of existing x-ray detection schemes.