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Time-resolved and ultrafast hard X-ray imaging, scattering and spectroscopy are powerful tools for elucidating the temporal and spatial evolution of complexity in materials. However, their temporal resolution has been limited by the storage-ring timing patterns and X-ray pulse width at synchrotron sources. Here we demonstrate that dynamic X-ray optics based on micro-electro-mechanical-system resonators can manipulate hard X-ray pulses on time scales down to 300 ps, comparable to the X-ray pulse width from typical synchrotron sources. This is achieved by timing the resonators with the storage ring to diffract X-ray pulses through the narrow Bragg peak of the single-crystalline material. Angular velocities exceeding 107 degrees s−1 are reached while maintaining the maximum linear velocity well below the sonic speed and material breakdown limit. As the time scale of the devices shortens, the devices promise to spatially disperse the temporal width of X-rays, thus generating a temporal resolution below the pulse-width limit., It is desirable to improve spatiotemporal control of light generated by synchrotron user facilities or table-top X-ray sources. Here the authors demonstrate manipulation of hard X-rays using microelectro mechanical systems (MEMS) oscillators on timescales of 300 ps, approaching the synchrotron pulse width.

Microelectromechanical systems (MEMS) are miniature devices integrated into a vast range of industrial and consumer applications. Optical MEMS are developed for dynamic spatiotemporal control in lightwave manipulation and communication as modulators, switches, multiplexers, spectrometer, etc. However, they have not been shown to function similarly in sub-nm wavelength regimes, namely, with hard x-rays, as high-brilliance pulsed x-rays have proven powerful for addressing challenges in time-domain science, from energy conversion to neurobiological control. While desirable temporal properties of x-ray pulses can be enhanced by optics, conventional x-ray optics are inherently massive in size, hence, never dynamic. We demonstrate highly ultrafast x-ray optics-on-a-chip based on MEMS capable of modulating hard x-ray pulses exceeding 350 MHz, 103× higher than any other mechanical modulator, with a pulse purity >106 without compromising the spectral brilliance. Moreover, the timing characteristics of the devices can be tuned on-the-fly to deliver optimal pulse properties to create a host of dynamic x-ray instruments and applications, impossible with traditional optics of 109× bulkier and more massive. The advent of the ultrafast optics-on-a-chip heralds a new paradigm of x-ray photonics, time-domain science, and accelerator diagnostics, especially at not only the future-generation light sources that offer coherent and high-frequency pulses but also lab-based facilities that normally do not offer timing structures.

Time-resolved synchrotron x-ray measurements often rely on using a mechanical chopper to isolate a set of x-ray pulses. We have started the development of micro electromechanical systems (MEMS)-based x-ray optics, as an alternate method to manipulate x-ray beams. In the application of x-ray pulse isolation, we recently achieved a pulse-picking time window of half a nanosecond, which is more than 100 times faster than mechanical choppers can achieve. The MEMS device consists of a comb-drive silicon micromirror, designed for efficiently diffracting an x-ray beam during oscillation. The MEMS devices were operated in Bragg geometry and their oscillation was synchronized to x-ray pulses, with a frequency matching subharmonics of the cycling frequency of x-ray pulses. The microscale structure of the silicon mirror in terms of the curvature and the quality of crystallinity ensures a narrow angular spread of the Bragg reflection. With the discussion of factors determining the diffractive time window, this report showed our approaches to narrow down the time window to half a nanosecond. The short diffractive time window will allow us to select single x-ray pulse out of a train of pulses from synchrotron radiation facilities.

Synchrotron beamlines typically use macroscopic, quasi-static optics to manipulate x-ray beams. We present the use of dynamic microelectromechanical systems-based optics (MEMS) to temporally modulate synchrotron x-ray beams. We demonstrate this concept using single-crystal torsional MEMS micromirrors oscillating at frequencies of 75 kHz. Such a MEMS micromirror, with lateral dimensions of a few hundred micrometers, can interact with x rays by operating in grazing-incidence reflection geometry; x rays are deflected only when an x-ray pulse is incident on the rotating micromirror under appropriate conditions, i.e., at an angle less than the critical angle for reflectivity. The time window for such deflections depends on the frequency and amplitude of the MEMS rotation. We demonstrate that reflection geometry can produce a time window of a few microseconds. We further demonstrate that MEMS optics can isolate x rays from a selected synchrotron bunch or group of bunches. With ray-trace simulations we explain the currently achievable time windows and suggest a path toward improvements.

We take this opportunity to introduce our research and development effort on MEMS-based dynamic x-ray optics that exploit the properties to manipulate and control the x-ray pulses at synchrotron sources for time-resolved experiments.

Hard x-ray research taking advantage of both the coherence and timing properties of next generation low-emittance storage rings will thrive in exploring the spatial and temporal correlations at the nanometer and nanosecond scales. Current gating technology with fast detectors and mechanical choppers will not be able to handle the high repetition-rates of these sources and requires the development of a fast gating device that can fill the needs of the timing community. Here, we introduce a MEMS device that has the potential to meet the future requirements of x-ray timing science. We demonstrate a device that is frequency matched to the operating sub-frequency of a high rep-rate bunch mode at the APS (Advanced Photon Source) to select single pulses from the pulse train with separations of ∼153 ns.

A new method of performing Mössbauer spectroscopy using a fast shutter in combination with microfocused synchrotron radiation is demonstrated., 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 ≃ 1012) 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.

We have performed temporally resolved X-ray excited optical luminescence measurements on Si nanowires. By performing Si K edge X-ray excitation measurements we have determined that the short-lifetime components have primarily Si character, while the long-lifetime component stems from the SiO2 shell and SiO2−Si interface. The lifetime of the short-lifetime component decreases with increasing luminescence energy, i.e., as the size of the nanosilicon species decreases.

From the beginning of its discovery the Mossbauer effect has continued to be one of the most powerful tools with broad applications in diverse areas of science and technology. With the advent of synchrotron radiation sources such as the Advanced Photon Source (APS), the European Synchrotron Radiation Facility (ESRF) and the Super Photon Ring-8 (SPring-8), the tool has enlarged its scope and delivered new capabilities. The popular techniques most generally used in the field of materials physics, chemical physics, geoscience, and biology are hyperfine spectroscopy via elastic nuclear forward scattering (NFS), vibrational spectroscopy via nuclear inelastic scattering (NRIXS), and, to a lesser extent, diffusional dynamics from quasielastic nuclear forward scattering (QNFS). As we look ahead, new storage rings with enhanced brilliance such as PETRA-III under construction at DESY, Hamburg, and PEP-III in its early design stage at SLAC, Stanford, will provide new and unique science opportunities. In the next two decades, x-ray free-electron lasers (XFELs), based both on self-amplified spontaneous emission (SASE-XFELs) and a seed (SXFELs), with unique time structure, coherence and a five to six orders higher average brilliance will truly revolutionize nuclear resonance applications in a major way. This overview is intended to briefly address the unique radiation characteristics of new sources on the horizon and to provide a glimpse of scientific prospects and dreams in the nuclear resonance field from the new radiation sources. We anticipate an expanded nuclear resonance research activity with applications such as spin and phonon mapping of a single nanostructure and their assemblies, interfaces, and surfaces; spin dynamics; nonequilibrium dynamics; photochemical reactions; excited-state spectroscopy; and nonlinear phenomena.

We have measured the time-resolved, X-ray excited optical luminescence spectra from two types of Mg{sub x}Zn{sub (1-x)}O core-shell, heterostructured nanowires: type I, with a small x, wurtzite core, encased in a larger x, wurtzite sheath; and type II, with a wurtzite core (x {approx} 0), encased in a rock-salt sheath (x > 0.62). By monitoring the X-ray energy dependence of the various luminescence peaks, we have determined the local environment of the sites where these peaks originate.

We report the synthesis and characterization of 1D Si-CdSe heteronanostructures with different morphologies such as coaxial, biaxial, sandwiched, pattern wrapping, coiling, structures etc., via a one-step metal catalyzed thermal evaporation method. Both Si and CdSe exhibit single crystalline characteristics in the heterostructures, as revealed by scanning transmission electron microscopy. The Si nanowires formed directly from the Si substrate via the solid-liquid-solid process acts as the absorption site for CdSe deposition as well as the template for the formation of 1D Si-CdSe heterostructures. Time-resolved X-ray excited optical luminescence from the 1D Si-CdSe heteronanostructures reveals two main emission features at 530 and 637 nm with slow and fast decay lifetime, respectively. The 530 and 637 nm emission is associated with the Si and CdSe component of the heterostructures, respectively.

Photonic microsystems played an essential role in the development of integrated photonic devices, thanks to their unique spatiotemporal control and spectral shaping capabilities. Similar capabilities to markedly control and manipulate X-ray radiation are highly desirable but practically impossible due to the massive size of the silicon single-crystal optics currently used. Here we show that micromechanical systems can be used as X-ray optics to create and preserve the spatial, temporal and spectral correlation of the X-rays. We demonstrate that, as X-ray reflective optics they can maintain the wavefront properties with nearly 100% reflectivity, and as a dynamic diffractive optics they can generate nanosecond time windows with over 100-kHz repetition rates. Since X-ray photonic microsystems can be easily incorporated into lab-based and next-generation synchrotron X-ray sources, they bring unprecedented design flexibility for future dynamic and miniature X-ray optics for focusing, wavefront manipulation, multicolour dispersion, and pulse slicing., Microelectromechanical systems (MEMS) are essential in a wide range of photonics applications but have not been demonstrated for X-ray optics. Here, Mukhopadhyay et al. use single-crystal silicon to demonstrate a MEMS system that can preserve and manipulate the spatial, temporal and spectral correlations of the X-rays.

Stanley L. Ruby (1924–2004) made major contributions to Mossbauer spectroscopy and was the first to suggest the feasibility of observing the Mossbauer effect using synchrotron radiation. In this article we recall his scientific legacy that have inspired his scientific colleagues.

Many advances have been made in chemical structure research over the past three decades using synchrotron radiation. Synchrotron radiation has a number of unique properties. They include high brightness, high collimation, broad energy spectrum, variable polarization, coherent power, and subnanosecond pulse width. The third-generation storage rings with wiggler and undulator sources and lower electron beam dimensions are delivering over 1012 times higher brightness than laboratory-based sources. The future of synchrotron sources looks very promising with the development of energy recovery linac sources and free-electron laser sources. These will permit dynamic studies of chemical structure with subpicosecond time resolution. Commensurate with the development of X-ray sources, major progress has occurred in optical schemes to meet the challenging needs of chemical structure research. High-resolution monochromatization and submicron focusing of X rays present new avenues for the future.

The electric quadrupole and magnetic hyperfine interactions rneasured from the 161 Dy Mossbauer resonance in crystalline Dy(OH)3 and from the 166 Er resonance in crystalline Ho(OH)3 and Er(OH)3 are interpreted using the crystal field and molecular exchange field model . The crystal field parameters established from previous optical spectroscopy results account weIl for these hyperfine parameters. The crystal-field and magnetic properties of these ferromagnetic insulators are described weIl within the model.

We demonstrate the use of electrostatically driven micro-electromechanical systems (MEMS) devices to control and deliver synchrotron x-ray pulses at high repetition rates. Torsional MEMS micromirrors, rotating at duty cycles of 2 kHz and higher, were used to modulate grazing-incidence x rays, producing x-ray bunches shorter than 10 μs. We find that dynamic deformation of the oscillating micromirror is a limiting factor in the duration of the x-ray pulses produced, and we describe plans for reaching higher operating frequencies using mirrors designed for minimal deformation.

We describe a MEMS micromirror for modulation of X-ray pulses needed for critical time-resolved process studies of cyclical and far-from-equilibrium phenomena. This MEMS micromirror leverages grazing-angle reflection of X-ray pulses to generate its temporal modulation response. The generated responses are 1.5μs wide for 8keV X-rays and have been successfully used to isolate single X-ray pulses separated 1.59μs from their suppressed neighboring pulses.

This is the Golden Jubilee year for the Mossbauer Effect Nobel Prize [1]. Since its discovery, the measurements of hyperfine interactions, recoilless fraction (the so-called Lamb–Mossbauer factor), and the second-order Doppler shift obtained from Mossbauer spectra have shed light on the magnetic, structural, and dynamical properties of matter. The applications of this tool have contributed to every area of science and technology including nuclear physics, condensed-matter physics, general physics, chemical physics, materials science, earth science, planetary science, environmental science, life science, and medical science. This volume addresses many of these applications in detail.

It is shown that the hyperfine field in a crystal can be characterized in a short measurement using resonant nuclear diffraction at a high‐brightness synchrotron x‐ray beamline. In addition to the strength of the field, this technique is sensitive to the field direction and to the crystal‐structure factor of the nuclear sites. It is possible to separately measure the field at different crystallographic subgroups of nuclei. These principles are illustrated with an experiment using a crystal of 57Fe‐enriched yttrium iron garnet, and future applications are discussed.

We report the results of theoretical calculations of copper K-edge x-ray-absorption near-edge spectra (XANES) in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ and related oxides ${\mathrm{Cu}}_{2}$O, CuO, and ${\mathrm{KCuO}}_{2}$. The final bound states were obtained from the self-consistent-field discrete-variational X\ensuremath{\alpha} method (SCF DV X\ensuremath{\alpha}), and continuum states were found by the multiple-scattering method using the muffin-tin truncation of the SCF DV X\ensuremath{\alpha} potentials. Composition of the final-state wave functions was analyzed. The 1s ionization potentials of the three reference compounds obtained from the SCF DV X\ensuremath{\alpha} transition-state calculations were used to set up the relative energy scale for the calculated cross section. The principal features of measured Cu K-edge XANES for ${\mathrm{Cu}}_{2}$O, CuO, and ${\mathrm{KCuO}}_{2}$ were reproduced satisfactorily by our calculations along with their relative energy positions. Our calculated polarized XANES of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ were compared with measured spectra, and two shakeup features were identified with radiation polarized along the crystal c axis. By quantitatively comparing the measured spectra with a model based on our calculated cross sections, their intensities were found to be \ensuremath{\sim}24% and 10% of the main transition with shakeup energies of 5.7 and 9.5 eV, respectively. We suggest that these multielectron excitation features involve Cu 3d\ensuremath{\rightarrow}4p transitions.

We report x‐ray excited optical luminescence (XEOL) from one‐dimensional nanostructures of ZnO excited with photon energies across the Zn K‐edge. The optical luminescence shows an UV and a green emission band characteristic of near band edge and defect emission, respectively. The optical channels were used in turn to monitor the Zn K‐edge XAFS to high k values. The densities of states of oxygen character in the valence band were also studied with x‐ray emission spectroscopy (XES). The Zn K‐edge decay dynamics was examined with time‐resolved x‐ray excited optical luminescence.

Over the past few decades, scientists have focused their attention on the development of concepts and designs, leading to demonstrations, of unique x-ray sources to perform femtosecond and attosecond science. The rewards of such an effort in the x-ray wavelength range will revolutionize the subfields of atomic physics, molecular physics, biology, condensed matter physics and material science. A brief review of this subject and its impact on emerging areas of science will be presented. The storage-ring-based synchrotron radiation sources are today's workhorses in providing both time-averaged and time-resolved structural and chemical information with subnanosecond to subsecond resolution using x-ray imaging, spectroscopy and scattering techniques. On the other hand, many phenomena are ultrafast with characteristic periods of a few femtoseconds to tens of picoseconds. These include electronic motions around a nucleus in an atom, atomic and molecular vibrational motions in matter, spin dynamics, chemical and biological reactions, and phase transitions in response to photoexcitation. Probing such phenomena using photon-excited pump-probe experiments will require both optical and x-ray sources with comparable resolution. In the future, sources based on atypical concepts in storage-rings, table-top plasma sources, laser-based high harmonic generation (HHG) sources, linac-based sources, such as energy-recovery linacs (ERLs) and x-ray free-electron lasers (FELs), will likely meet these demands.

A unique capability to vary the length of magnetic period in an undulator can be achieved with a geometry based on staggered arrays of permeable poles placed in a magnetic solenoid. The new classes of solenoid‐derived undulators, which do not use permanent magnets, have the potential to provide wide tunability of the first and third energy harmonics by varying the length of the magnetic period and solenoid field, and deliver only small powers at undesired harmonics. The magnetic and spectral properties of such “variable‐period undulator” operating on the proposed PETRA III storage ring are presented to demonstrate the superior performance of such a device in comparison to conventional permanent‐magnet hybrid undulators. They have the potential to be useful as synchrotron radiation sources at both currently operating and future low‐emittance storage rings, energy‐recovery linacs (ERLs), and free‐electron lasers (FELs).

The use of circularly polarized radiation is advantageous for the study of magnetic materials using x‐ray scattering techniques. The APS is an ideal source of x‐ray radiation for such studies. We present a description of the elliptical multipole wiggler (EMW) [S. Yamamoto, H. Kawata, H. Kitamura, and M. Ando, Phys. Rev. Lett. 62, 2672 (1989)] to be constructed at the APS. This device has been chosen for reasons of tunability and special polarization properties. This insertion device is capable of producing circularly polarized x rays on axis. The EMW period will be λu=16 cm, the number of full strength poles in the hybrid structure is 31, and the device length is 2.8 m. The hybrid magnetic structure produces a peak vertical magnetic field with Ky=14 and the electromagnet provides horizontal magnetic field with Kx=1–2. The frequency of the horizontal field change is up to 10 Hz. The beamline will consist of three stations operating in tandem with only one station receiving x rays at any one time. The three...

A concept for variable-period undulators for the production of synchrotron radiation from both medium- and high-energy storage rings is described. This concept is based on a staggered array of permeable poles placed in a magnetic solenoid that produces a longitudinal field. The concept permits variations in the short magnetic period of the undulator of as much as 100%. The unique capabilities of such undulators will allow them to be tuned by the variation of the period length and of the solenoid field. The device can be operated at either constant flux or constant power, independent of X-ray energy. It is expected that the new concept will have a major impact on the production and applications of X-rays because of the inherent simplicity and flexibility of the design and the absence of radiation damage. Analyses of the magnetic and mechanical design concepts are presented.

It is shown that the operation of the Advanced Photon Source (APS) storage ring at 6 GeV will (1) deliver higher brilliance at x-ray energies used by a majority of users due to natural reduction in electron beam emittance at lower storage ring energy, and (2) lower the total power produced by insertion devices thus permitting stored currents up to 300 mA with minimal changes in accelerator or beamline hardware. While higher brilliance x-ray beams can be realized from the APS undulators by only increasing the stored current for the present modes of operation, this however leads to serious high heat load concerns. This report includes detailed analyses of radiation brilliance, undulator tunability, power, power density, and total and coherent flux as a function of x-ray energy from various harmonics of undulator-A, for operation at 6.0, 6.5 and 7.0 GeV with 100, 140, 200 and 300 mA currents. A discussion of a smaller period (2.7 cm) undulator's spectral performance is also presented. It is shown that the APS can be immediately operated at 6 GeV with 200-300 mA current to benefit user science in the x-ray energy range below 35 keV. This may not require any hardware change either in the storage ring or the beamlines, making this switch without any interruptions to user research programs. Finally, the suggested 6 GeV operation with 300 mA should be considered a temporary step towards higher brilliance operation. A need for re-optimization of the APS technical operations, defined by storage-ring energy, current, undulator period, and power, are also addressed in this report.

Stanley L. Ruby, who made major contributions to Mossbauer spectroscopy and who inspired the community with the idea of observing the Mossbauer effect using synchrotron radiation, passed away on October 18, 2004, in Los Gatos, California. His boundless intellectual curiosity and passion for life was an inspiration to all around him, especially his scientific colleagues.

One-dimensionally confined nonferromagnetic layers of Cu grown between ferromagnetic layers of Co have an average induced magnetic moment that decreases with Cu film thickness. We studied the effects of changing the nature of confinement to three dimensions by embedding Cu nanoparticles in a Co matrix and measuring the induced moments using x-ray magnetic circular dichroism. The nanoparticle spin moments were more than twice that of films of comparable thickness due to the three dimensional confinement and greater interfacial area.

The authors have monitored the optical luminescence from one-dimensional Si–CdSe nanoheterostructures as a function of x-ray energy at the Se L3 edge (∼1430eV). The wires consist of a diamond Si core encased in a wurtzite CdSe sheath. The time-resolved luminescence spectrum consists of a short-lifetime band centered at 637nm and a long-lived band at 530nm. By monitoring the intensities of these bands following excitation of a Se 2p3∕2 electron, the authors are able to show that the 637nm band is associated with the CdSe sheath while the 530nm band emanates from the Si core.

The authors have found that the directionality of the orbital populated following core-level x-ray absorption of a hexagonal nanostructure has a strong influence on the resulting optical luminescence yield spectra. For ZnO, there is an enhancement of the band gap exciton luminescence following O 1s to 2pz relative to 2px,y excitation. The defect luminescence O 1s excitation spectrum also shows sensitivity to the nature of the defect (surface or bulk).

We have monitored the optical luminescence from ZnS nanowires as a function of x-ray energy at the Zn L edge (1022 eV). The x-ray absorption spectrum obtained using the 338 nm, band edge emission as a signal resembles that of the wurtzite form of ZnS, while that obtained using the 430 and 520 nm defect emissions, resembles that of the sphalerite phase. Wurtzite is the dominant phase of the wire, while sphalerite is only found at the end of the wire adjacent to the gold particle used for nucleation and in small, highly localized regions of the wire. Therefore, the present results support the idea that the defect luminescence centers are caused by Au ions (520 nm) and vacancies (430 nm), which are located in regions of sphalerite and show how x-ray excited optical luminescence may be used to probe the local environment of such centers.

We have monitored the changes that occur in the x-ray-excited optical luminescence, absorption, and photoemission spectra as a function of vacuum annealing time and temperature for ZnS nanowires. All measurements were done in situ. Initial heating causes desorption of surface oxides and a concurrent reduction in the intensity of all the luminescence peaks, which we attribute to the creation of surface states that quench the luminescence. Extended annealing causes diffusion of Au from the particle used to nucleate the wire growth, which results in an increase in intensity of its associated luminescent band at 520nm. Changes were also observed in the ZnL- and SK-edge x-ray absorption spectra, which are consistent with this interpretation.

Citations are added to the paper by Shenoy et al. [J. Synchrotron Rad. (2003). 10, 205–213].

The relationship between the zeolite framework and the coordination number and electron-transfer properties of Eu2+ and Eu3+ have been investigated by time-resolved luminescence (TRL) EXAFS, EPR and Mossbauer spectroscopy. TRL-determined lifetimes are sensitive to non-framework oxygen atoms and can be used to help distinguish between coordination involving OH groups and the framework oxygen atoms. The suitability and some of the present limitations of this application of TRL, which is new to the study of zeolites, are described. The average coordination of hydrated Eu3+ in the large-pore zeolite Y is identical to that in aqueous solution, while the coordination number in zeolites A and ZSM-5 is smaller, with framework oxygen atoms displacing water from the Eu3+ coordination sites. Europium metal dissolved in liquid ammonia can be used to generate Eu(NH3)62+ and solvated electrons. With zeolite X, this solution results in the formation of Na43+ and two kinds of defects. These are assigned to an electron trapped in an oxygen vacancy arising from a Lewis acid site and a V-type hole center in which the unpaired electron resides on a non-bridging oxygen atom bonded to silicon.

The M\"ossbauser effect of the 80.6-keV resonance in $^{166}\mathrm{Er}$ has been used to study the magnetic and electronic properties of Er${\mathrm{H}}_{2}$. The onset of magnetic hyperfine splitting shows a magnetic transition occurring at 2.4 \ifmmode\pm\else\textpm\fi{} 0.1 K. The extrapolated saturation hyperfine field is 2480 \ifmmode\pm\else\textpm\fi{} 20 k${\mathrm{O}}_{\mathrm{e}}$, corresponding to a magnetic moment of $(2.75\ifmmode\pm\else\textpm\fi{}0.08){\ensuremath{\mu}}_{B}$ on the Er ion in the ordered state. Measurements of the hyperfine splitting in an external magnetic field show that the crystal-field ground state is a ${\ensuremath{\Gamma}}_{6}$ Kramers doublet. These data, as well as an analysis of previously published magnetic-susceptibility data, indicate that the first excited crystal-field level is \ensuremath{\gtrsim} 150 K above the ground state. Measurements of the hyperfine spectra of Er as a solute in Y${\mathrm{H}}_{2}$ give a ${\ensuremath{\Gamma}}_{7}$ ground state, which can arise from a small change in the ratio of fourth-order to sixth-order crystal-field parameters on going from this case to that of Er${\mathrm{H}}_{2}$. All of the above results can be understood on the basis of a "hydridic" (i.e., negatively charged) model for hydrogen in this compound.

The measurement of Cu K-edge x-ray absorption near-edge structure of monovalent, divalent, and trivalent copper oxides has been used to identify the presence of different valence of copper in the La/sub 2-//sub x/(Sr,Ba)/sub x/CuO/sub 4/ (x = 0.0--0.3) system. The results indicate the coexistence of Cu II and Cu III states in these compounds. Theoretical calculations based on a cluster of CuO/sub 4/O/sub 2/La/sub 8/LaSr predict the presence of nearly degenerate electronic ground states representing nominally Cu II and Cu III configurations.

Analyses of the EXAFS of exchanged divalent manganese cations in small pore (A) and large pore (Y) zeolites reveal that the local environments of the cations in the zeolite are similar to the environments of Mn2+ in solution. Further, the EXAFS show no long‐range order about the exchanged cations in the zeolite. The implications of the similarity of complexes in the zeolites and in aqueous solution and the lack of long‐range order about the intrazeolite cations are discussed.

We report measurements of the specific heat, Moessbauer effect, and electrical resistivity of NpSn/sub 3/, which identify its weak antiferromagnetism (T/sub N/=9.5 K) as arising from itinerant 5f electrons. This is the first known verification of itinerant f-electron antiferromagnetism in any system. (AIP)

Moessbauer studies of the ternary Chevrel phase and rare earth rhodium boride superconductors have been made. Anomalous phonon properties at the Sn site in SnMo/sub 6/S/sub 8/, SnMo/sub 6/Se/sub 8/, and La/sub 0/ /sub 98/Sn/sub 0/ /sub 02/Mo/sub 6/Se/sub 8/ have been investigated. Studies of polarization of conduction electrons at the site of the magnetic ion have been made by means of the /sup 151/Eu Moessbauer effect in Eu/sub x/Sn/sub 1-x/Mo/sub 6/S/sub 8/ and the effects of such polarization on superconducting properties discussed. The Moessbauer effect in /sup 166/Er has been used to investigate the electronic ground state in the ternary compound ErRh/sub 4/B/sub 4/ both in the superconducting and magnetically ordered states.

The Mossbauer spectra of 166 Er in ErH 3 and YH 3 have been measured. Using and axial Hamiltonian, the electronic gyromagnetic rations for the ground state of Er 3+ in YH 3 have been deduced: g | = 11.6 ± 0.1 and g ⊥ = 4.4 ± 0.1. The analysis yields crystal field parameters which provide the correct λ 7 ground state with these g -values.

X-ray absorption near edge structure (XANES) spectra of gaseous Kr, solid Kr and solid KrF2 have been obtained and compared to differentiate between continuum resonances near the edge and bound state transitions. By comparing experimental and ab initio (MSW-Xα) calculated spectra the influence of environment as well as molecular bonding on XANES is demonstrated.

Microscopic studies of the electronic properties of Chevrel phase superconductors are reported. Phonon anomalies at the Sn site in SnMo6Se8, SnMo6Se8 and La0.98Sno.02Mo6Se8 have been investigated. Studies of polarization of conduction electrons by the Eu ion in Eux Snl-xMo6S8 have been performed, and the effects of Such polarization on superconducting properties are discussed.

We present here the results of our detailed M\"ossbauer measurements on the mixed-valence system ${\mathrm{EuNi}}_{2}$${\mathrm{P}}_{2}$. The linewidth of the resonance exhibits a broad peak at \ensuremath{\sim}40 K. We argue that this feature is anomalous in that it cannot be understood in terms of the known mechanisms that can give rise to line broadening. The system exhibits strong mixed valence---the average valence being \ensuremath{\sim}2.5---even at T=1.4 K. This is the first Eu-based material having truly mixed valence behavior as T\ensuremath{\rightarrow}0; all others tend to approach the trivalent configuration as T\ensuremath{\rightarrow}0.

The M-edge x-ray-absorption near-edge spectra was calculated for bulk ${\mathrm{UO}}_{2}$ and gas-phase ${\mathrm{UCl}}_{4}$ from a multiple-scattering approach within the muffin-tin--potential approximation. The muffin-tin potential was constructed from the self-consistent relativistic discrete-variational X\ensuremath{\alpha} potential. The spectra were interpreted by analyzing the photoelectron-trapping time in different channels inside each atomic sphere. The energy levels of bound states down to the valence levels of the ligands were calculated within the same framework as the continuum in order to set the absolute energy scale. The calculated spectra were compared and found to agree quantitatively with our experimental results.

The magnetic moment on the Er atom in the reentrant superconductor Er${\mathrm{Rh}}_{4}$${\mathrm{B}}_{4}$ at 100 mK is found from M\"ossbauer-effect measurements to be $(8.3\ifmmode\pm\else\textpm\fi{}0.2){\ensuremath{\mu}}_{B}$. This value is 30% larger than that found by neutron diffraction, indicating that only a fraction of the moment shows long-range magnetic order. The combination of M\"ossbauer and neutron diffraction results suggest that it is the presence of superconducting fluctuations in the magnetically ordered state which affects the nature of the ordered state.