Diffraction gratings metrology and ray-tracing results for an XUV Raman spectrometer at FLASH.

Siarhei Dziarzhytski,Michael Rübhausen,Holger Weigelt,Frank Siewert,Grzegorz Gwalt,Andrey Sokolov,Tino Seliger,Günter Brenner

doi:10.1107/s1600577517013066

Abstract

The extreme-ultraviolet double-stage imaging Raman spectrometer is a permanent experimental endstation at the plane-grating monochromator beamline branch PG1 at FLASH at DESY in Hamburg, Germany. This unique instrument covers the photon energy range from 20 to 200 eV with high energy resolution of about 2 to 20 meV (design values) featuring an efficient elastic line suppression as well as effective stray light rejection. Such a design enables studies of low-energy excitations like, for example, phonons in solids close to the vicinity of the elastic line. The Raman spectrometer effectively operates with four reflective off-axial parabolic mirrors and two plane-grating units. The optics quality and their precise alignment are crucial to guarantee best performance of the instrument. Here, results on a comprehensive investigation of the quality of the spectrometer diffraction gratings are presented. The gratings have been characterized by ex situ metrology at the BESSY-II Optics Laboratory, employing slope measuring deflectometry and interferometry as well as atomic force microscopy studies. The efficiency of these key optical elements has been measured at the at-wavelength metrology laboratory using the reflectometer at the BESSY-II Optics beamline. Also, the metrology results are discussed with respect to the expected resolving power of the instrument by including them in ray-tracing studies of the instrument.

Highlights

The plane-grating monochromator beamline PG1 (Gerasimova et al, 2011; Dziarzhytski et al, 2016) at the soft X-ray/ extreme-ultraviolet (XUV) free-electron laser FLASH in Hamburg (Ackermann et al, 2007; Tiedtke et al, 2009) is permanently equipped with a unique high-resolution XUV double-stage Raman spectrometer, dedicated to inelastic soft X-ray scattering (IXS) experiments in the spectral region from 20 to 200 eV (Rubhausen et al, 2004; Rusydi et al, 2014)
The obtained results from metrology demonstrated some efficiency degradation and deviations from the optics specifications, as will be discussed in the following. These findings were implemented into ray-tracing package SHADOW (Cerrina & Sanches del Rio, 2010) to quantify their influence on the performance of the XUV Raman spectrometer (x3)
The higher spatial frequency range as measured by means of the atomic force microscope (AFM) has an impact on the efficiency and spectral purity provided by the grating

Summary

Introduction

The plane-grating monochromator beamline PG1 (Gerasimova et al, 2011; Dziarzhytski et al, 2016) at the soft X-ray/ extreme-ultraviolet (XUV) free-electron laser FLASH in Hamburg (Ackermann et al, 2007; Tiedtke et al, 2009) is permanently equipped with a unique high-resolution XUV double-stage Raman spectrometer, dedicated to (resonant) inelastic soft X-ray scattering (IXS) experiments in the spectral region from 20 to 200 eV (Rubhausen et al, 2004; Rusydi et al, 2014). Off-axis parabolic mirrors of the spectrometer have a sagittal slope error below 1 arcsec Such values of the optical quality parameters were chosen during the design phase of the spectrometer and pursuit in fabricating to minimize unwanted specular deflections of the rays from their ideal path resulting in reduction of the spectrometer resolution. These values represent the technical limits of parabola production at that time and have been chosen in the closed discussion with the manufacture. These findings were implemented into ray-tracing package SHADOW (Cerrina & Sanches del Rio, 2010) to quantify their influence on the performance of the XUV Raman spectrometer (x3)

Gratings metrology

Ex situ metrology

In situ metrology

Ray tracing

Conclusion

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