Timothy I. Morrison

Re-investigation of titanium silicalite by X-ray absorption spectroscopy: Are the novel titanium sites real?

EXAFS studies have been carried out on titanium silicalite-1 (TS-1). The titanium atom is coordinated to four oxygen atoms at the distance of 1.80(1) Å. There is no evidence for the presence of 5- and 6-coordinated titanium species or the novel 4-coordinated titanium species involving [TiOx] units sharing edges with [SiO4] units. The EXAFS spectrum of TS-1 appears to be consistent with the model that titanium resides at the tetrahedral sites of the framework.

Phosphorus promotion of Mo/Al2O3 hydrotreating catalysts

Abstract Phosphorus-promoted and unpromoted Mo/Al 2 O 3 catalysts were studied, in the calcined and sulphided state, by extended X-ray absorption fine structure (EXAFS) spectroscopy and, in the calcined state, by X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) in hydrogen. EXAFS indicates phosphorus has little effect on the structure of oxidized or sulphided molybdenum on the surface of alumina. With or without phosphorus, EXAFS indicates the supported MoS 2 crystallites contain only 7–8 molybdenum atoms and are roughly 10Ain size. XPS results suggest that at least for catalysts in the calcined state, the addition of phosphorus leads to molybdenum species that are more easily reduced by hydrogen, implying a difference in the nature of the bonding between the surface molybdenum oxide and phosphorus-promoted alumina. TPR, however, was not sufficiently sensitive to indicate a substantial difference between the phosphorus and non-phosphorus catalysts. Finally, temperature-programmed desorption of ammonia indicated that phosphorus does not appreciably alter the acid strength of the alumina support.

Modeling of pair distribution functions for XAFS in disordered systems

Abstract A commonly used expression for modeling the XAFS of highly disordered systems is shown to generate substantial systematic errors in the coordination number in many cases of practical interest. This expression is corrected and generalized. Further, a simpler and more flexible model distribution is proposed, and the corresponding XAFS expression is derived. Comparison with experimental and simulated data show that the new expressions are useful in cases of high disorder for which the cumulant expansion loses its utility, and they explicitly account for the k -dependence of the mean free path.

Numerical modeling of tapered undulators

Abstract Tapered-undulator radiation will be used in at least one of the third generation synchrotron sources. While the spectral and angular characteristics of the radiation have been calculated for some specific configurations, the effects of tapering on the spectrum are still not well characterized. In this paper we have used an FFT-based method to calculate the spectral and angular distributions of the radiation from a tapered undulator with vertically-polarized magnetic field. The effect of tapering on the horizontal symmetry of the angular distribution of the radiation is investigated. A simple expression for the presence or absence of horizontal reflection symmetry as a function of the undulator parameters is derived. We have shown that for APS-type undulators and energies as high as 25 keV the radiation may be expected to exhibit horizontal reflection symmetry for observation angles up to 200 μrad. Our calculations are in agreement with recent measurements of the on-axis brightness of the prototype APS-CHESS undulator operated in the tapered configuration, and with results from well-established non-FFT programs.

A novel method of bending crystals to log spiral shape

Crystals bent to logarithmic spirals have been used in applications where large acceptance of divergent x rays from a small source is needed. Small acceptances can be easily handled with cylindrically bent crystal. In cases where larger acceptances are needed differential bending may be adequate. However, the largest acceptance is obtained from a flat crystal plate when bent into the logarithmic spiral shape that allows the crystal to present the same angle of incidence to the beam emanating from a point source. The bending of such crystals generally can be accomplished by forcing the crystal into the required shape with a metal frame. A preliminary theoretical study of crystal shapes that when bent will naturally form logarithmic spiral shapes has been done. This study was done for the case of a cantilever beam with fixed end and a normal force applied to a point at the other end. A fixed thickness-variable width beam was considered. The crystal profile in width (direction perpendicular to the diffraction plane) can be spatially varied to achieve this spiral shape. Equations for both cases will be given.

Practical aspects of undulator radiation properties

It is important to be able to accurately predict the spectral and angular distribution of undulator radiation properties when designing beamlines at new synchrotron radiation facilities or when performing radiation experiments at already existing beamlines. In practice, the particle beam emittance and beam energy spread must be taken into account in modeling these properties. The undulators fabricated today are made with small RMS phase errors, making them perform almost as ideal devices. Calculation tools for numerical modeling of undulator radiation sources (ideal and nonideal) will be discussed, and the excellent agreement with experimentally obtained absolute spectral flux measurements of undulator A at the Advanced Photon Source verifies the high accuracy of the computer codes and the high quality of the undulators being built today. Our focus here is on flux properties useful in practical beamline designs, and the chosen examples demonstrate the versatility of computer programs available to beamline designers and experimentalists.

A dedicated soft x‐ray beamline for x‐ray absorption spectroscopy (abstract)

We will present the design, capabilities, and characteristics of the recently commissioned Beamline X‐11B at the National Synchrotron Light Source. This line has been designed to optimally function in the ‘‘soft’’ x‐ray regime (1–5 keV), but can easily be used up to the design limit of the storage ring. The beamline, which shares a common vacuum with the storage ring, is equipped with a double crystal fixed‐exit UHV monochromator. The monochromator crystals are mounted on rotating carousels that can each carry four crystals, permitting crystal changes to cover different energy ranges without breaking vacuum. The incident radiation carousel is water cooled and the second carousel can be heated, allowing thermal equilibration of the crystals. The line can be equipped with a UHV backend or a ‘‘fast‐turnaround’’ backend. The UHV backend can be used for surface EXAFS and other surface techniques (XPS, Auger) on line. The fast‐turnaround backend has been designed to work under less stringent vacuum demands, per...