Resonant inelastic X-ray scattering (RIXS) is a key technique for probing charge-neutral collective excitations in materials, playing a vital role in fundamental research across materials science, chemistry, and condensed matter physics. Although soft and hard X-ray RIXS instrumentation has advanced the study of 3 d and 5 d transition metal (TM) materials, the tender X-ray range (2000–3000 eV), which covers most 4 d TM systems, lagged behind due to the lack of high-performance energy-dispersive optics.
In a study published in Light: Science & Applications , researchers from Diamond Light Source (United Kingdom), Tongji University (China), and University of Science and Technology of China, have demonstrated that the combination of the multilayer and the grating optics can vastly boost the efficiency of RIXS at the tender X-ray region. By matching the Bragg condition of the multilayer with the grating diffraction, three graded multilayer coatings were successfully designed and deposited onto the grating optics developed for the RIXS spectrometer at Diamond Light Source. X-ray measurement revealed an efficiency enhancement of 12 to 25 times in comparison to the standard single layer coated grating across the entire tender X-ray energy range (2- 3 keV). This new development shortens the typical acquisition time of one RIXS spectrum from hours to minutes. It establishes I21 as the first grating-based RIXS facility covering both soft and tender X-ray regions (280 – 3000 eV).
Conventional single-layer coated gratings (SLG) and crystal optics work well as energy-dispersive optics for the soft X-ray and hard X-ray regions, respectively (Fig. 1a). However, the diffraction efficiency of SLG drops to below 5% at the tender X-ray region (Fig.1b). On the other hand, crystal optics, though can deliver a good energy resolution, are not easily adaptable for the broad range of 4 d TM elements due to the narrow Darwin width of natural crystals. To solve the issue, Ke-Jin Zhou and Qiushi Huang et al . innovatively designed a lateral-graded multilayer in combination with a spherical variable-line-spacing (VLS) grating:
“This is the first grating-based RIXS spectrometer that can work efficiently in the tender X-ray region. The spherical variable-line-spacing (VLS) grating scheme features a variation of incident angle across its surface significantly larger than the natural bandwidth of multilayers. To match the VLS grating profile, a lateral-graded multilayer, instead of a constant d -spacing multilayer, was designed and deposited onto the entire lateral direction of a VLS grating. The lateral gradient of the multilayer is optimized to precisely match the local incident angle as a function of the lateral grating position, ensuring fulfillment of the generalized Bragg condition across the entire grating surface. Remarkably, the multilayer grating provides a high efficiency of about 60% at e.g . 2.8 keV which is more than an order of magnitude higher than the conventional SLG. To full cover 2-3 keV energy range, three lateral-graded multilayers are fabricated using the same principle.”
The authors stated that the MLG technology can be applied in the beamline monochromator that would yield further improvement of the total photon flux. In future, the resolving power could be improved by increasing the grating line density or by working at higher diffraction orders, paving the way for a high efficiency and high energy-resolution Tender X-ray RIXS. The MLG technology is also extendable to a much broader scientific applications in various X-ray spectroscopy techniques.
Light Science & Applications
TRIXS: a multilayer grating solution towards highly efficient resonant inelastic tender X-ray scattering