Dispersion-compensated Rowland spectrometer: implications for uranium VB-RIXS

The total energy resolution (ΔE$_{tot}$) of a valence-band resonant inelastic X-ray scattering (VB-RIXS) instrument serves as an important point of reference in an otherwise complex field. Since VB-RIXS is a flux-limited technique, a pragmatic approach to reducing ΔE$_{tot}$ is often required—the specifications of a spectrometer should be matched with a comparable incident bandwidth (ΔE$_i$) and the source size contribution (focal point) should be negligible. Although it advocates for a good efficiency, this approach is in many places already limited by count-rates. Here we follow a recent trend emerging in soft X-ray VB-RIXS and look at the performance of our tender X-ray Rowland spectrometer (Gretarsson et al., 2020) when being exposed to a source with a large linear dispersion (higher flux). Detailed ray tracing work, performed at the U M5-edge (3551 eV), finds that the intrinsic resolution of the Rowland spectrometer (ΔE$_a$) can be obtained if the linear dispersion of the source matches the spectrometer's, but opposite in sign—here ΔE$_i$ does not matter. This finding is supported by experimental data where ΔE$_{tot}$ = 48 meV (ΔE$_a$ = 44 meV) was recently achieved. Furthermore, we demonstrate that the dispersion rate can be tuned, ensuring the method's applicability to other atomic edges.