Operation of an X-ray spectrometer based on a spherical variable line spacing grating is analyzed using dedicated ray-tracing software allowing fast optimization of the grating parameters and spectrometer geometry. The analysis is illustrated with op
tical design of a model spectrometer to deliver a resolving power above 20400 at photon energy of 930 eV (Cu L-edge). With this energy taken as reference, the VLS coefficients are optimized to cancel the lineshape asymmetry (mostly from the coma aberrations) as well as minimize the symmetric aberration broadening at large grating illuminations, dramatically increasing the aberration-limited vertical acceptance of the spectrometer. For any energy away from the reference, we evaluate corrections to the entrance arm and light incidence angle on the grating to maintain the exactly symmetric lineshape. Furthermore, we evaluate operational modes when these corrections are coordinated to maintain either energy independent focal curve inclination or maximal aberration-limited spectrometer acceptance. The results are supported by analytical evaluation of the coma term of the optical path function. Our analysis gives thus a recipe to design a high-resolution spherical VLS grating spectrometer operating with negligible aberrations at large acceptance and over extended energy range.
We describe the concepts and technical realization of the high-resolution soft-X-ray beamline ADRESS operating in the energy range from 300 to 1600 eV and intended for Resonant Inelastic X-ray Scattering (RIXS) and Angle-Resolved Photoelectron Spectr
oscopy (ARPES). The photon source is an undulator of novel fixed-gap design where longitudinal movement of permanent magnetic arrays controls not only the light polarization (including circular and 0-180 deg rotatable linear polarizations) but also the energy without changing the gap. The beamline optics is based on the well-established scheme of plane grating monochromator (PGM) operating in collimated light. The ultimate resolving power E/dE is above 33000 at 1 keV photon energy. The choice of blazed vs lamellar gratings and optimization of their profile parameters is described. Due to glancing angles on the mirrors as well as optimized groove densities and profiles of the gratings, high photon flux is achieved up to 1.0e13 photons/s/0.01%BW at 1 keV. Ellipsoidal refocusing optics used for the RIXS endstation demagnifies the vertical spot size down to 4 um, which allows slitless operation and thus maximal transmission of the high-resolution RIXS spectrometer delivering E/dE better than 11000 at 1 keV photon energy. Apart from the beamline optics, we give an overview of the control system, describe diagnostics and software tools, and discuss strategies used for the optical alignment. An introduction to the concepts and instrumental realization of the ARPES and RIXS endstations is given.
A spectrometer for resonant inelastic X-ray scattering (RIXS) is proposed where imaging and dispersion actions in two orthogonal planes are combined to deliver full two-dimensional map of RIXS intensity in one shot with parallel detection in incoming
hvin and outgoing hvout photon energies. Preliminary ray-tracing simulations with a typical undulator beamline demonstrate a resolving power well above 11000 in both hvin and hvout near a photon energy of 930 eV, with a vast potential for improvement. Combining such a spectrometer - nicknamed hv2 - with an XFEL source allows efficient time-resolved RIXS experiments.
