Molecular emission in laser-induced breakdown spectroscopy: An investigation of its suitability for chlorine quantification on Mars

Abstract

The intensity of the molecular CaCl emission in LIBS spectra is examined in order to evaluate its suitability for the detection of chlorine in a Martian environment. Various mixtures resembling Martian targets with varying Cl content are investigated under simulated Martian conditions. The reactions leading to the formation of CaCl are modeled based on reaction kinetics and are used to fit the measured CaCl band intensities. MgCl bands are also investigated as potential alternatives to CaCl, but no MgCl bands can be identified in samples containing both Mg and Cl. The study confirms that CaCl is well suited for the indirect detection of chlorine, but finds a strong dependence on the concentrations of Ca and Cl in the sample. Spectra from samples with a high chlorine concentration can have low-intensity CaCl emission due to a deficiency of Ca. A qualitative estimate of the sample composition is possible based on the ratio of the band intensity of CaCl to the intensity of Ca emission lines. Time-resolved measurements show that the CaCl concentration in the plasma is highest after about 1 µs.

The SuperCam Instrument Suite on the NASA Mars 2020 Rover: Body Unit and Combined System Tests

“…2018 ) or studying molecular emission lines of CaCl (Vogt et al. 2018 ) or CaO. Finally, it should be possible to collect atomic emissions largely to the exclusion of molecular emissions.…”

Mars Science Laboratory Observations of Chloride Salts in Gale Crater, Mars

“…We apply standard ChemCam data preprocessing, removing the first five laser shots, which are contaminated by dust and subject to surface effects (as detailed in Wiens et al, ). We use the Cl emission line at 837.8 nm that increases monotonically with Cl content regardless of cation (Anderson, Ehlmann, et al, ) rather than the molecular emission from CaCl, which is complex and not easily used for direct quantification (Vogt et al, ). To quantify Cl, we fit the local region (831–841 nm) using methods described by Thomas et al () and report the fit Cl peak area.…”

The SuperCam Instrument Suite on the Mars 2020 Rover: Science Objectives and Mast-Unit Description

“…Molecular emission of simple diatomic molecules occurs as well due to the recombination of plasma species in colder plasma regions. In Martian atmospheric conditions, the maximum intensity of these molecular bands is observed to last slightly later than that of ionic and atomic emission and stays intense for a longer period of time due to the low-temperature dependence (Vogt et al 2018). If the plasma emission is collected over the entire plasma lifetime, the continuum signal can simultaneously exhibit atomic emission lines, and sometimes molecular bands from simple molecules.…”