# Characterization of the soft X-ray spectrometer PEAXIS at BESSY II

## TL;DR

PEAXIS at BESSY II is a versatile soft X-ray spectrometer capable of high-resolution RIXS and XPS measurements, with demonstrated capabilities on NiO and liquid acetone, and plans for further resolution improvements.

## Contribution

This paper provides a comprehensive characterization of the PEAXIS spectrometer, including its performance, capabilities, and planned upgrades, advancing soft X-ray spectroscopy techniques.

## Key findings

- Achieved a total energy resolution of <40 meV at ~400 eV
- Demonstrated momentum-resolved RIXS measurements on NiO
- Validated instrument performance with vibrational studies in liquid acetone

## Abstract

The performance of the recently commissioned spectrometer PEAXIS for resonant inelastic soft X-ray scattering (RIXS) and X-ray photoelectron spectroscopy (XPS) and its hosting beamline U41-PEAXIS at the BESSY II synchrotron are characterized. The beamline provides linearly polarized light from 180 eV - 1600 eV allowing for RIXS measurements in the range of 200 eV - 1200 eV. The monochromator optics can be operated in different configurations for the benefit of either high flux, providing up to $10^{12}$ photons/s within the focal spot at the sample, or high energy resolution with a full width at half maximum of <40meV at an incident photon energy of ~400 eV. This measured total energy resolution of the RIXS spectrometer is in very good agreement with the theoretically predicted values by ray-tracing simulations. PEAXIS features a 5 m long RIXS spectrometer arm that can be continuously rotated about the sample position by 106{\deg} within the horizontal photon scattering plane, thus enabling the study of momentum-transfer-dependent excitations. To demonstrate the instrument capabilities, d-d excitations and magnetic excitations have been measured on single-crystalline NiO. Measurements employing a fluid cell demonstrate the vibrational Progression in liquid acetone. Planned upgrades of the beamline and the RIXS spectrometer that will further increase the energy resolution by 20 - 30% to ~100meV at 1000 eV incident photon energy are discussed.