A T Payne

High-accuracy X-ray absorption spectra from mM solutions of nickel (II) complexes with multiple solutions using transmission XAS.

A new approach is introduced for determining X-ray absorption spectroscopy (XAS) spectra on absolute and relative scales using multiple solutions with different concentrations by the characterization and correction of experimental systematics. This hybrid technique is a development of standard X-ray absorption fine structure (XAFS) along the lines of the high-accuracy X-ray extended range technique (XERT) but with applicability to solutions, dilute systems and cold cell environments. This methodology has been applied to determining absolute XAS of bis(N-n-propyl-salicylaldiminato) nickel(II) and bis(N-i-propyl-salicylaldiminato) nickel(II) complexes with square planar and tetrahedral structures in 15 mM and 1.5 mM dilute solutions. It is demonstrated that transmission XAS from dilute systems can provide excellent X-ray absorption near-edge structure (XANES) and XAFS spectra, and that transmission measurements can provide accurate measurement of subtle differences including coordination geometries. For the first time, (transmission) XAS of the isomers have been determined from low-concentration solutions on an absolute scale with a 1-5% accuracy, and with relative precision of 0.1% to 0.2% in the active XANES and XAFS regions after inclusion of systematic corrections.

Structure determination from XAFS using high-accuracy measurements of x-ray mass attenuation coefficients of silver, 11 keV–28 keV, and development of an all-energies approach to local dynamical analysis of bond length, revealing variation of effective thermal contributions across the XAFS spectrum

We use the x-ray extended range technique (XERT) to experimentally determine the mass attenuation coefficient of silver in the x-ray energy range 11 kev-28 kev including the silver K absorption edge. The results are accurate to better than 0.1%, permitting critical tests of atomic and solid state theory. This is one of the most accurate demonstrations of cross-platform accuracy in synchrotron studies thus far. We derive the mass absorption coefficients and the imaginary component of the form factor over this range. We apply conventional XAFS analytic techniques, extended to include error propagation and uncertainty, yielding bond lengths accurate to approximately 0.24% and thermal Debye-Waller parameters accurate to 30%. We then introduce the FDMX technique for accurate analysis of such data across the full XAFS spectrum, built on full-potential theory, yielding a bond length accuracy of order 0.1% and the demonstration that a single Debye parameter is inadequate and inconsistent across the XAFS range. Two effective Debye-Waller parameters are determined: a high-energy value based on the highly-correlated motion of bonded atoms (σ(DW) = 0.1413(21) Å), and an uncorrelated bulk value (σ(DW) = 0.1766(9) Å) in good agreement with that derived from (room-temperature) crystallography.

Helium-like titanium x-ray spectrum as a probe of QED computation

We discuss the first absolute energy measurements of the intercombination and forbidden transitions ( xyz ,, ) in trapped Ti 20+ ions to 15 parts per million accuracy. We present new measurements on helium-like titanium, in which the orbital radius is reduced and QED terms are magnified by the increased nuclear charge. The measured transition energies are higher than predicted.

X-ray measurements in helium-like atoms increased discrepancy between experiment and theoretical QED

A recent 15 parts per million (ppm) experiment on muonic hydrogen () found a major discrepancy with quantum electrodynamics (QED) and independent nuclear size determinations. Here we find a significant discrepancy in a different type of exotic atom: a medium-Z nucleus with two electrons. Investigation of the data collected is able to discriminate between available QED formulations and reveals a pattern of discrepancy of almost six standard errors of experimental results from the most recent theoretical predictions, with a functional dependence proportional to Zn where . In both the muonic and highly charged systems, the sign of the discrepancy is the same, with the measured transition energy higher than predicted. Some consequences are possible or probable, and some are more speculative. This may give insight into effective nuclear radii, the Rydberg, the fine-structure constant, or unexpectedly large QED terms.

Voigt profile characterization of copper Kα

We report a characterization of the Cu Kα profile and a transferable determination of the 2p satellite line using a new Voigt methodology which generates improved fits, smaller residuals and details of Compton profile features. The Kα1, 2 emission of Cu was obtained from a rotating anode through a monolithic Si channel-cut monochromator. Least-squares fitting of a minimum set of Voigt profiles reached a noise limit. Sufficient statistical information and resolution permits the determination of major and minor peak components in a fully-free least-squares analysis rather than the previous constrained single peak-by-peak method. Relative energies of the component Voigts within each profile, linewidths and Kα1/Kα2 peak intensity ratios, are compared to the previous best empirical sum of Lorentzian-slit peaks, clearly demonstrating that a sum of Voigt profiles provides a superior fit to the observed profile. 104 profiles at accelerating voltages from 20 kV through 50 kV provided a stable unique profile across the broad range of 2.5 − 6.25 times the characteristic energy. This robustness proves the stability of Cu Kα for use in high accuracy calibration, and supports the validity of the impulse approximation across this range of energy. The lineshape, contributions to noise broadening, the quantum yield and the Fano factor, relevant to spectral profiling, are discussed.

Simulation and minimization of nonlinear effects in backgammon-type multi-wire gas proportional counters

The backgammon-type multi-wire gas proportional counter (MWPC) enables highly efficient detection of x-ray photons in two dimensions with good resolution and a wide range of energies and count rates. We develop a simulation which accurately describes the internal geometry of the detector and assists in the identification of several sources of nonlinear detector output. The sophistication of the model allows real solutions to be generated to minimize these effects. The result is an increase of 20% in the linear range, significant improvements to detector linearity, and improved output uniformity across both detector dimensions. These insights and methodology can be applied to other detector types and to future optimizations and developments of detector performance.