Devon R. Mortensen

Direct Observation of 4f Intrashell Excitation in Luminescent Eu Complexes by Time-Resolved X-ray Absorption Near Edge Spectroscopy

We report time-resolved X-ray absorption near edge structure (TR-XANES) measurements at the Eu L3 edge upon photoexcitation of several Eu(III)-based luminescent lanthanide complexes. We find an unambiguous signature of the 4f intrashell excitation that occurs upon energy transfer from the photoactive organic antennas to the lanthanide species. Phenomenologically, this observation provides the basis for direct investigation of a crucial step in the energy transfer pathways that lead to sensitized luminescence in lanthanide-based dyes. Interestingly, the details of the TR-XANES feature suggest that the degree of 4f-5d hybridization may itself vary depending on the excited state of the Eu(III) ion.

A Laboratory-based Hard X-ray Monochromator for High-Resolution X-ray Emission Spectroscopy and X-ray Absorption Near Edge Structure Measurements

We report the development of a laboratory-based Rowland-circle monochromator that incorporates a low power x-ray (bremsstrahlung) tube source, a spherically bent crystal analyzer, and an energy-resolving solid-state detector. This relatively inexpensive, introductory level instrument achieves 1-eV energy resolution for photon energies of ∼5 keV to ∼10 keV while also demonstrating a net efficiency previously seen only in laboratory monochromators having much coarser energy resolution. Despite the use of only a compact, air-cooled 10 W x-ray tube, we find count rates for nonresonant x-ray emission spectroscopy comparable to those achieved at monochromatized spectroscopy beamlines at synchrotron light sources. For x-ray absorption near edge structure, the monochromatized flux is small (due to the use of a low-powered x-ray generator) but still useful for routine transmission-mode studies of concentrated samples. These results indicate that upgrading to a standard commercial high-power line-focused x-ray tube or rotating anode x-ray generator would result in monochromatized fluxes of order 10(6)-10(7) photons/s with no loss in energy resolution. This work establishes core technical capabilities for a rejuvenation of laboratory-based hard x-ray spectroscopies that could have special relevance for contemporary research on catalytic or electrical energy storage systems using transition-metal, lanthanide, or noble-metal active species.

A versatile medium-resolution x-ray emission spectrometer for diamond anvil cell applications

We present design and performance details for a polycapillary-coupled x-ray spectrometer that provides very high collection efficiency at a moderate energy resolution suitable for many studies of nonresonant x-ray emission spectroscopy, especially for samples of heavy elements under high pressures. Using a single Bragg analyzer operating close to backscattering geometry so as to minimize the effect of the weak divergence of the quasicollimated exit beam from the polycapillary optic, this instrument can maintain a typical energy resolution of 5 eV over photon energies from 5 keV to 10 keV. We find dramatically improved count rates as compared to a traditional higher-resolution instrument based on a single spherically bent crystal analyzer.

Benchtop Nonresonant X-ray Emission Spectroscopy: Coming Soon to Laboratories and XAS Beamlines Near You?

Recently developed instrumentation at the University of Washington has allowed for nonresonant x-ray emission spectra (XES) to be measured in a laboratory-setting with an inexpensive, easily operated system. We present a critical evaluation of this equipment by means of K\b{eta} and valence-level XES measurements for several Co compounds. We find peak count rates of ~5000/s for concentrated samples and a robust relative energy scale with reproducibility of 25 meV or better. We furthermore find excellent agreement with synchrotron measurements with only modest loss in energy resolution. Instruments such as ours, based on only conventional sources that are widely sold for elemental analysis by x-ray fluorescence, can fill an important role to diversify the research applications of XES both by their presence in non-synchrotron laboratories and by their use in conjunction with XAFS beamlines where the complementarity of XAFS and XES holds high scientific potential.

Recent tests of x-ray spectrometers using polycapillary optics

Polycapillary optics provide a promising approach for coupling highly-divergent x-ray emission or inelastic scattering to high-resolution crystal analyzers. We present recent results looking at the application of polycapillary collimators to emission spectrometers. The first application uses a collimating optic and a flat crystal to provide a tunable x-ray fluorescence detector. At high-flux synchrotron radiation sources there is sufficient flux (~1013 ph/sec) to allow application of X-ray Absorption Spectroscopy (XAS) to ppb concentrations if the fluorescence signal can be isolated from an intense background. The polycapillary based analyzer easily achieves the <106 background reduction needed for such measurements. It has the additional advantage of being confocal, only collecting the signal from a small volume at the optic focus, effectively eliminating background from sample substrates, windows, or air scattering. Second, the same type of analyzer can be used for higher-resolution emission spectroscopy if operated close to 90° Bragg angle, and we report results of the commissioning of a user-available instrument suitable for few-eV resolution emission spectroscopy, including the demonstration of roughly order-of-magnitude improved measurement times compared to use of a traditional, single spherically-bent crystal analyzer. As part of this effort, we have developed a process for enhancing the integral reflectivity of Si analyzer crystals through plastic deformation at high temperatures.

Benchmark results and theoretical treatments for valence-to-core x-ray emission spectroscopy in transition metal compounds

We report measurement of the valence-to-core (VTC) region of the K-shell x-ray emission spectra from several Zn and Fe inorganic compounds, and their critical comparison with several existing theoretical treatments. We find generally good agreement between the respective theories and experiment, and in particular find an important admixture of dipole and quadrupole character for Zn materials that is much weaker in Fe-based systems. These results on materials whose simple crystal structures should not, a prior, pose deep challenges to theory, will prove useful in guiding the further development of DFT and time-dependent DFT methods for VTC-XES predictions and their comparison to experiment.

A Rebirth of Laboratory XANES and XES

We have recently launched a reinvestigation of laboratory-based measurement of x-ray absorption near edge structure (XANES) and high-resolution x-ray emission spectroscopy (XES). Driven by the roughly one-hundred-fold improvement in the efficiency of high resolution x-ray optics compared to the 1970s, we have demonstrated that surprisingly impressive performance is now possible with laboratory XES instruments and that quite useful performance is possible for laboratory XANES. For applications in the 5 keV10 keV energy range, i.e., appropriate for 3d transition metal K edges and lanthanide L edges, we find XES performance that is intermediate between what can be achieved at bending-magnet and insertion-device beamlines at third generation synchrotron light sources. In the same energy range, we find that high quality XANES measurements can be performed in transmission mode on concentrated samples with the present instrument and should be possible, in many cases, in fluorescence mode after a planned upgrade brings the monochromatic beam flux up to approximately 10M/sec.