High resolution X-Ray Spectrometry

Crystal spectrometers [1] can be employed in the hard X-ray range to determine relevant atomic data such as transition probabilities when a sufficiently high energy resolution is needed. In the soft X-ray range, the need for high resolution detection systems is even more pronounced due to the lower absolute differences in characteristic energies. In contrast to Wavelength-Dispersive Systems (WDS), the most interesting features of superconducting energy-dispersive X-ray detectors are the broader energetic range that can be detected simultaneously and the higher solid angle of detection in particular when combined with a polycapillary optic [2]. However, most superconducting energy-dispersive X-ray detectors show relevant drawbacks with respect to practical applications in XRS such as line splitting, additional lines associated with contact and substrate events [2-4] and unstable energy scales, restricting them to only very few promising XRS applications (such as X-ray Absorption Fine Structure, XAFS, applications) in which a low intensity line, being energetically close to a high intensity line of a relevant matrix element, is of interest. In view of these relevant restrictions, PTB built a WDS based on a spherical grating and a CCD camera. This WDS system has been calibrated and the first XRS experiments were initiated. Its resolving power reaches about 500. The WDS has been used to determine transition [5] and Coster-Kronig probabilities as well as sub-shell fluorescence yields of transition metals, thus contributing to the main advantage of reference-free X-ray spectrometry, which is its straightforward applicability to probing novel materials for which no appropriate reference materials exist. In addition, the WDS was employed in complementary X-ray Emission Spectrometry (XES) and XAFS investigations [1] on the speciation of different Ti compounds performed in both the soft and hard x-ray ranges.
The high resolution of this spectrometer in the soft X-ray range enables the chemical speciation of transition metal compounds by probing the absolute energy positions of L-shell fluorescence lines or the relative chemical shift of those lines which is caused by an increasing or decreasing oxidation state of the metal. In addition, satellite lines, characteristic for covalent bonding, occur with different relative intensity and can serve as an indicator for the chemical composition of the sample material. XES information on the chemical state is complementary to the one obtained from high resolution XAFS [1].