Modular X-ray emission spectrograph Design for the ALS

We are in the process of designing and building a modular spectrometer with the goal of being portable for use at a diverse group of endstations at the ALS, LCLS and TLS. The basic design is based upon experience from several years of use with similar designs at the ALS. For detector positions of less than 1.3 m, the design incorporates a composite cantilevered swing arm. Two optical elements are used in the X-ray emission spectrograph (the HettrickUnderwood design): a spherical mirror for focusing the X-rays onto an in-vacuum CCD detector and a variable line spacing (VLS) plane grating for energy monochromatization. The grating will be operated in the constant incident angle mode, thus variation in the detection photon energy is accomplished by moving the detector along the focal plane. Relatively large source-size resolution allows the spectrograph to be used at the ALS and LCLS beamlines where tightly focused beam won’t be available, while small detector-pixel-size resolution gives a good resolving power over the operating photon energy range (~500 up to 1,000 eV). Large acceptance angles enhance the detection efficiency for RIXS measurements, which often have a very small cross-section. The momentum-resolved capability, available through the combination of mounting spectrographs at different scattering ports and rotating the entire qRIXS endstation relative to incident photon beam offers the opportunity to perform truly momentum-resolved RIXS in the soft X-ray regime. The modular design of X-ray emission spectrograph gives the flexibility to tailor for specific applications that may require either high throughput or high resolution. We present the status of our current design. Project Description Although there are several very high energy resolution soft x-ray RIXS systems currently being developed at light sources around the world, none of them can perform the truly momentumresolved RIXS spectroscopy to study the elementary excitations in three-dimensional correlated materials. Furthermore, the project serves to demonstrate the first femtosecond time-resolved qRIXS at the LCLS to establish the scientific cases for the FEL-based light sources. Thus, the qRIXS spectrometer at LBNL is designed to be compact, modular and flexible, instead of trying to achieve the highest energy resolution. The qRIXS spectrometer will cover a large energy range from 100 eV to 1.2 keV, and its compact size allows us to incorporate several of them (three units in the current plan, but the endstation can accommodate up to five units) to cover a large horizontal angular range simultaneously. The modular design also provides flexibility for the spectrograph to be operated either in the inside or outside order, and can be tailored for specific classes of science by making minimal modifications on up and downstream components. Scientific Justification Resonant inelastic x-ray scattering spectroscopy (RIXS) is a measurement technique which uses photons with energies tuned to elemental absorption edges to create the direct electronic orbital transitions, and reveal the electron dynamics in femtosecond time and nanometer length scales. With the capability to vary the momentum transfer (Δq) between incident and emitted photons in the scattering plane (Δq=kf-ki), as well as to analyze the energy loss (ΔE) in the process (ΔE=ωf-ωi), the momentum-resolved RIXS (or qRIXS) has been used to measure the dispersion relations of low energy collective modes, such as phonons, magnons and orbitons, that are critically linked to the emergent material properties like high t e m p e r a t u r e s u p e r c o n d u c t i v i t y, c o l o s s a l magnetoresistance and multiferroicity [1-7]. The core of soft x-rays qRIXS instrumentation is the grating-based x-ray emission spectrograph. The physical dimension of spectrograph is often determined by the trade-off between resolving power 5,000 and drops down to ~3,300 at 1,000eV. Spectrometer Endstation Layout The optical design of this qRIXS spectrograph is similar to several spectrometers designed (Hettrick-Underwood design) and in operation at the ALS. The design incorporates a spherical pre-mirror and a VLS plane grating. To facilitate alignment and serviceability, all position sensitive component are mounted on a common vacuum chamber lid. Both optics are mounted on a common cradle with the pivot point locations machined to achieve the required tolerance of .0005” for the relative placement of the optics.