CaCl and CaF emission in LIBS under simulated Martian conditions

Abstract Chlorine and fluorine play an important role in the geological history of Mars due to their high concentration in Martian magmas and their influence on the generation and evolution of Martian basalts. Chlorine-bearing salts could also facilitate the formation of eutectic brines that could be important for the fluvial history of Mars. The LIBS instruments of ChemCam and SuperCam can detect emission lines of Cl and F, but the intensity of these emission lines is comparatively low, making it difficult to quantify them correctly. A promising alternative is the quantification by molecular emission of diatomic molecules like CaCl and CaF, which can be observed as intense molecular bands in LIBS spectra if Ca is also present. However, the nonlinear dependence of the band intensity on the concentrations of both elements needs to be considered. In this study, we expand upon our previous analysis of molecular bands by investigating samples which produce CaCl bands, CaF bands, or both. We find that the highest CaCl band intensities are found in samples containing more Ca than Cl, while the strongest CaF bands are found in samples with roughly equal concentrations of Ca and F. Both observations can be described by the model that we present here. We also find that the CaCl band is significantly stronger for a sample containing CaCl2 than it is for a sample containing the same concentrations of Ca and Cl in separate bonds. The opposite is true for the CaF band, which is significantly weaker for the sample containing CaF2 bonds than it is for the sample that does not contain CaF2 bonds. These matrix effects are partially attributed to fragmentation during the ablation process and differences in the dissociation energies. Furthermore, we observe that CaF formation is not affected by competing CaCl formation, while CaCl is strongly affected by competing CaF formation. All measurements are done in simulated Martian atmospheric conditions in order to assist the analysis of Martian LIBS data.