# Sc/SiC/Al Multilayer Optimization for Li K Spectroscopy
## 4 Conclusions  
We showed that, by optimizing the thickness of the layers constituting a periodic multilayer, as well as the number of periods of the stack, it is possible to turn a structure initially designed to work at near-normal incidence for imaging purposes into a structure working at much lower glancing angles and use it for spectroscopy purposes. The multilayer was first characterized via laboratory-based reflectivity measurements performed in the hard X-ray range and then studied with the use of both synchrotron-based reflectometer and theoretical calculations. The GIXR curve demonstrated the good structural quality of the stack and its fit showed smooth interfaces with roughnesses that did not exceed 0.5 nm. The reflectivity measurements of the Sc/SiC/Al multilayer in the lithium K emission range revealed a relatively high reflectance of the proposed multilayer and its good spectral selectivity. Indeed, the peak reflectance was nearly 40% at the photon energy of 54 eV at near-normal incidence and the bandwidth was about 2.6 eV. This system optimized for the WDS analysis of Li K emission bands shows that the periodic Sc/SiC/Al multilayer can provide a reflectance of up to 35.4% at 30\({}^{\circ}\) and a bandwidth of about 4 eV. The main parameter for the optimization was the change in the multilayer period, fixed to 25.2 nm to work in the Li K spectral range. Thus, such a new periodic multilayer could be inserted as the dispersive element in bent crystal spectrometers working following the Bragg law and equipping electron microprobes or scanning electron microscopes. They could provide a cheap and simple way for the analysis of lithium in various solid samples: Li-ion batteries, lithium ores, minerals, and light alloys, etc. Owing to the combination of peak reflectance and bandwidth, the proposed Sc/SiC/Al periodic multilayer mirror can be used for the elemental quantification \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Multilayer** & **E, eV** & \(\theta\), \({}^{\circ}\) & **d, nm** & **Rsim, \%** & **Rexp, \%** & **Bandwidth, eV** & **Ref.** \\ \hline (Sc/SiC/Al)\({}_{40}\) & 55.2 & 30 & 25.2 & 35.4 & – & 3.9 & This work \\ \hline (Be/Si/Al)\({}_{30}\) & 51.6 & 30 & 29 & 45.8 & 31.8 & 3.5 & [11] \\ \hline (Co/Mg)\({}_{30}\) & 49.6 & 45 & 17 & 56.5 & 42.6 & 8.0 & [18] \\ \hline \end{tabular}
\end{table} 
Table 2: Comparison of multilayers’ characteristics, simulated (Rsim) and experimental (Rexp) reflectance in the range of the Li K emission. \(\theta\) is the grazing angle. of the Li K emission band, that is, for the determination of the Li mass fraction, where only the measured intensity is important. Conceptualization, K.H. and P.J. ; methodology, K.H. and P.J. ; software, [PERSON] and E.M.; validation, [PERSON], E.M., F.D., A.G. and P.J. ; formal analysis, [PERSON], E.M. and A.G.; investigation, [PERSON], E.M., F.D., A.G. and P.J. ; resources, [PERSON], [PERSON], [PERSON] and [PERSON]; data curation, [PERSON], [PERSON] and [PERSON]; writing--original draft preparation, [PERSON], [PERSON], [PERSON], A.G. and P.J. ; writing--review and editing, [PERSON], [PERSON], F.D., A.G. and P.J. ; visualization, [PERSON], [PERSON] and P.J. ; supervision, [PERSON] and P.J. ; project administration, [PERSON], A.G. and P.J. ; funding acquisition, [PERSON] All authors have read and agreed to the published version of the manuscript. This research was partially funded by Agence Nationale de la Recherche in the framework of the SQLX Project (ANR-20-CE29-0022). The raw data supporting the conclusions of this article will be made available by the authors on request. A CC-BY public copyright license has been applied by the authors to the present document and will be applied to all subsequent versions up to the Author Accepted Manuscript arising from this submission, in accordance with the grant's open access conditions. Measurements were performed under Elettra proposal number 20225009.