T. T. Fister

Multielement spectrometer for efficient measurement of the momentum transfer dependence of inelastic x-ray scattering

Nonresonant x-ray Raman scattering (XRS) is the inelastic scattering of hard x rays from the K shell of low-Z elements or the less tightly bound shells of heavier elements. In the limit of low momentum transfer q, XRS is determined by the same transition matrix element as is measured by x-ray absorption spectroscopies. However, XRS at higher q can often access higher order multipole transitions which help separate the symmetry of various contributions to the local density of states. The main drawback of XRS is its low cross section—a problem that is compounded for a q-dependent study. To address this issue, we have constructed a multielement spectrometer to simultaneously measure XRS at ten different values of q. By means of example, we report new measurements of the XRS from the L- and K-edges of Mg. This instrument is now available to general users at the Advanced Photon Source as the lower energy resolution inelastic x-ray scattering (LERIX) spectrometer.

High multipole transitions in NIXS: Valence and hybridization in 4f systems

Momentum-transfer (q) dependent non-resonant inelastic X-ray scattering measurements were made at the N4, 5 edges for several rare-earth compounds. With increasing q, giant dipole resonances diminish, to be replaced by strong multiplet lines at lower energy transfer. These multiplets result from two different orders of multipole scattering and are distinct for systems with simple 4f0 and 4f1 initial states. A many-body theoretical treatment of the multiplets agrees well with the experimental data on ionic La and Ce phosphate reference compounds. Comparing measurements on CeO2 and CeRh3 to the theory and the phosphates indicates sensitivity to hybridization as observed by a broadening of 4f0-related multiplet features. We expect such strong, non-dipole features to be generic for NIXS from f-electron systems.

Local electronic structure of dicarba-closo-dodecarboranes C2B10H12.

We report nonresonant inelastic X-ray scattering (NRIXS) measurement of core-shell excitations from both B 1s and C 1s initial states in all three isomers of the dicarba-closo-dodecarboranes C2B10H12. First, these data yield an experimental determination of the angular-momentum-projected final local density of states (l-DOS). We find low-energy resonances with distinctive local s- or p-type character, providing a more complete experimental characterization of bond hybridization than is available from dipole-transition limited techniques, such as X-ray absorption spectroscopies. This analysis is supported by independent density functional theory and real-space full multiple scattering calculation of the l-DOS which yield a clear distinction between tangential and radial contributions. Second, we investigate the isomer sensitivity of the NRIXS signal and compare and contrast these results with prior electron energy loss spectroscopy measurements. This work establishes NRIXS as a valuable tool for borane chemistry, not only for the unique spectroscopic capabilities of the technique but also through its compatibility with future studies in solution or in high-pressure environments. In addition, this work also establishes the real-space full multiple scattering approach as a useful alternative to traditional approaches for excited states calculations of aromatic polyhedral boranes and related systems.

Fine structure and chemical shifts in nonresonant inelastic x-ray scattering from Li-intercalated graphite

The authors report measurements of hard x-ray nonresonant inelastic x-ray scattering (IXS) from the Li and C 1s electrons of fully staged LiC6 Li-intercalated graphite prepared by both chemical and electrochemical methods. They find that the Li 1s orbital shifts to higher energies relative to Li metal. Relative to graphite, the C 1s IXS for LiC6 shows a shift for the σ-orbital threshold to lower energies, but no shift for the π* resonance. The findings provide bulk-sensitive evidence for substantial charge transfer from the Li intercalant to the carbon host and establish important groundwork for future in situ electrochemical studies.

An extraction algorithm for core level excitations in non-resonant inelastic x-ray scattering spectra

Non-resonant inelastic X-ray scattering of core electrons is a prominent tool for studying site-selective, i.e. momentum-transfer-dependent, shallow absorption edges of liquids and samples under extreme conditions. A bottleneck of the analysis of such spectra is the appropriate subtraction of the underlying background owing to valence and core electron excitations. This background exhibits a strong momentum-transfer dependence ranging from plasmon and particle-hole pair excitations to Compton scattering of core and valence electrons. In this work an algorithm to extract the absorption edges of interest from the superimposed background for a wide range of momentum transfers is presented and discussed for two examples, silicon and the compound silicondioxide.

Phase separation and Si nanocrystal formation in bulk SiO studied by x-ray scattering

We present an x-ray scattering study of the temperature-induced phase separation and Si nanocrystal formation in bulk amorphous SiOx with x≈1. X-ray Raman scattering at the Si LII,III-edge reveals a significant contribution of suboxides present in native amorphous SiO. The suboxide contribution decreases with increasing annealing temperature between 800–1200 °C pointing toward a phase separation of SiO into Si and SiO2 domains. In combination with x-ray diffraction and small angle x-ray scattering the SiO microstructure is found to be dominated by internal suboxide interfaces in the native state. For higher annealing temperatures above 900 °C growth of Si nanocrystals with rough surfaces embedded in a silicon oxide matrix can be observed.

Deconvolving instrumental and intrinsic broadening in core-shell x-ray spectroscopies

Intrinsic and experimental mechanisms frequently lead to broadening of spectral features in core-shell spectroscopies. For example, intrinsic broadening occurs in x-ray absorption spectroscopy XAS measurements of heavy elements where the core-hole lifetime is very short. On the other hand, nonresonant x-ray Raman scattering XRS and other energy loss measurements are more limited by instrumental resolution. Here, we demonstrate that the Richardson-Lucy RL iterative algorithm provides a robust method for deconvolving instrumental and intrinsic resolutions from typical XAS and XRS data. For the K-edge XAS of Ag, we find nearly complete removal of 9.3 eV full width at half maximum broadening from the combined effects of the short core-hole lifetime and instrumental resolution. We are also able to remove nearly all instrumental broadening in an XRS measurement of diamond, with the resulting improved spectrum comparing favorably with prior soft x-ray XAS measurements. We present a practical methodology for implementing the RL algorithm in these problems, emphasizing the importance of testing for stability of the deconvolution process against noise amplification, perturbations in the initial spectra, and uncertainties in the core-hole lifetime.

Background proportional enhancement of the extended fine structure in nonresonant inelastic x-ray scattering.

We report new measurements and calculations of the nonresonant inelastic x-ray scattering (NRIXS) from Mg and Al for a wide range of momentum transfers, q. Extended oscillations in the dynamic structure factor S(q,omega) due to scattering from the 2p and 2s orbitals (i.e., L edges) are observed out to more than 150 eV past the binding energy. These results are discussed in context of the recently proposed representation of S(q,{omega}) for core shells as an atomic background modulated by interference between different photoelectron scattering paths, in analogy to the standard treatment of extended x-ray absorption fine structure. In agreement with this representation, we find a strong increase in the atomic background with increasing q with a concomitant enhancement in the amplitude of the extended fine structure. This effect should be generic and hence may enable improved measurement of the extended fine structure in a wide range of materials containing low-Z elements.

The LERIX User Facility

We describe the lower energy resolution inelastic x‐ray scattering (LERIX) spectrometer, located at sector 20 PNC‐XOR of the Advanced Photon Source. This instrument, which is now available to general users, is the first user facility optimized for high throughput measurements of momentum transfer dependent nonresonant inelastic x‐ray scattering (NRIXS) from the core shell electrons of relatively light elements or the less‐tightly bound electrons of heavier elements. By means of example, we present new NRIXS measurements of the near‐edge structure for the L‐edges of Al and the K‐edge in Si.

Measurement of the Full XAFS Spectrum of MgO Using Nonresonant Inelastic X‐ray Scattering

We present non‐resonant inelastic x‐ray scattering measurements (NRIXS) of the contribution of each initial state orbital in MgO to the dynamic structure factor. We find good agreement with previous x‐ray absorption fine structure (XAFS) measurements and with previous electron energy loss spectroscopy (EELS) measurements for the corresponding edges. We also find good agreement with ab initio calculations when the interaction between the core hole and the photoelectron is treated properly. These measurements demonstrates the flexibility of a new user facility dedicated to measurement of NRIXS from core‐shell electrons, the lower energy resolution inelastic x‐ray scattering (LERIX) spectrometer located at sector 20 PNC‐XOR of the Advanced Photon Source. The LERIX user facility provides a bulk‐sensitive alternative to, and in some cases extension of, soft x‐ray XAFS measurements.