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Suitability of Molecular Emission in Laser-induced Breakdown Spectroscopy for the Quantification of Chlorine under Martian Conditions

Introduction: The ChemCam instrument suite aboard the Mars Science Laboratory uses laser-induced breakdown spectroscopy (LIBS) at stand-off distances for the elemental analysis of Martian targets [1]. The detection of halogens like chlorine is a challenge for LIBS, as the emission lines are typically of low intensity and high concentrations are necessary [2]. For this reason, an indirect detection via molecular emissions of simple molecules that form inside the plasma has been proposed as a new way to greatly enhance the level of detection (LOD) for halogens [3]. In the case of chlorine, the molecular emissions of calcium monochloride (CaCl) were found to be useful for the identification of chlorine in ChemCam spectra [4, 5]. The strongest CaCl bands, located at 593 nm and 618 nm in the spectra, are well within ChemCam’s spectral range. Previous studies have shown that the LOD for chlorine can significantly improve when using CaCl bands instead of Cl emission lines [6]. However, the necessary circumstances under which good results can be obtained from molecular bands have not yet been studied extensively. In particular, there have been no in-depth studies of CaCl bands under Martian conditions so far. This study investigates the relationship between the concentrations of calcium and chlorine and the intensity of CaCl emissions for mixtures that resemble Martian targets. The reactions inside the plasma are modeled in order to find the parameters which have the highest influence on the band intensity. The intensity of the CaCl bands is also compared to that of the strongest magnesium monochloride (MgCl) band, which is outside of ChemCam’s spectral range. Methods: A sample of pure magnesium chloride (MgCl2) was used for the investigation of the MgCl band. Calcium chloride (CaCl2), potassium chloride (KCl) and sodium chloride (NaCl) were used for the investigation of the CaCl band. In order to create samples with varying amounts of chlorine and calcium, these salts were mixed with other substances at different weight ratios. CaCl2 was mixed with magnesium sulfate (MgSO4) and Martian regolith simulant JSC Mars-1A (JSC) [7]. KCl and NaCl were mixed with calcium sulfate (CaSO4) and with JSC. JSC contains about 7 wt% of CaO (or 25,000 ppm of Ca), so that it should be possible to detect CaCl bands even for mixtures with NaCl and KCl. All samples were pressed into pellets of about 1 g. In total, 81 samples were created. Measurements were made with the LIBS system at Deutsches Zentrum für Luftund Raumfahrt (DLR), Berlin, Germany, which uses a Q-switched Nd:YAG laser at 1064 nm with pulse lengths of 8-10 ns operated at a repetition rate of 10 Hz. The laser energy was set to 22 mJ/pulse. The beam was focused onto the surface of the sample, which was placed inside a simulation chamber filled with Mars-analog atmosphere at 6.5 mbar. The plasma emissions were measured with an echelle spectrometer (LTB Aryelle Butterfly), which uses a time-gated ICCD camera (Andor iStar). The spectra cover a wavelength range from 270 nm to 850 nm with a resolution of 11 to 34 pm. Various sets of delay time and integration time were used in this study, but in the following there will be a focus on the measurements with a gate similar to that of ChemCam (no delay, 3 ms integration time). A simple numerical reaction rate model was used to fit the measurement data. The model simulates the time-dependent concentration for Ca, Cl and CaCl inside the plasma. The expected band intensity is then calculated by integrating over the gate time of the measurement.