Self-absorption in laser-induced plasmas in simulated Martian atmospheric conditions

ChemCam on board the Mars rover Curiosity is the first LIBS instrument on Mars. In 2020,two more LIBS instruments will follow [1,2]. Thus, understanding the capabilities andchallenges of LIBS in Mars’ atmosphere is of high interest. A general challenge of LIBS is self-absorption (SA). The effect of SA depends on the atmospheric conditions. In Earth’satmosphere SA affects the intensities of the emission lines strongly and breaks the linearityof calibration curves which limits their accuracy [3]. While SA is expected to be lower inMartian conditions, no extensive studies on this have been reported. In this study weinvestigate SA in LIBS spectra acquired in simulated Martian atmospheric conditions. LIBSspectra of salts are obtained with two different setups: A high-resolution LIBS setup with ahigh spectral range and a plasma imaging setup allowing for spatially resolvedmeasurements of the full plasma at a selected small spectral range. In measurements madewith the plasma imaging setup, the effect of SA on the Ca(II) doublet at 393 nm and 396 nmcan be observed. Fig. 1A shows the intensity ratio of the two emission lines for line-of-sightmeasurements through the plasma plume at different distances from the plasma axis. Withnegligible SA the ratio would be 2.1. We find a strong influence of SA, especially for largeeffective path lengths through the plasma center. For the calculation of the SA effect inspectra of the high-resolution LIBS setup, the plasma is modelled as a two-layer plasma inlocal thermal equilibrium. Temperature, electron density, and effective path length of eachlayer are varied and the resulting simulated spectra are compared to the measurements tofind the best fit, see Fig. 1B. From the values of the best fit we find that many strong emissionlines are significantly affected by SA, even though the effect is not obvious from the lineprofiles because of the instrumental broadening.