Pressure effects on simultaneous optical and acoustic data from laser-induced plasmas in air: implications to the differentiation of geological

Alvarez-Llamas, P. Purohit, J. Moros, J. J. Laserna, Applied Spectroscopy, submitted (2 June 2021)

Abstract:

As a source of analytical information, laser-induced plasmas have demonstrated an important level of versatility both from the points of view of possible application scenarios and the diversity of sample parameters that may be addressed at once. Still, as well-established as laser-induced breakdown spectroscopy (LIBS) is, there is room for further expansion of the data a single data series provide. The shockwave generated alongside the plasma is an intimately-linked, yet often neglected additional input for characterization in traditional LIBS. The present work introduces a dual LIBS-Acoustics sensor that takes advantage of the analysis of the acoustic spectrum yielded by shockwaves produced on different geological samples to enhance the discrimination power of LIBS in materials featuring similar optical emission spectra. Six iron-based minerals were tested at a distance of 2 meters using 1064 nm laser light and under pressure values ranging from 7 to 1015 mbar. The selection of these experimental parameters was guided by the increasing interest in employing the LIBS-Acoustics tandem in standoff applications such as space exploration, as evidenced by the inclusion of a microphone in the SuperCam instrument onboard the Perseverance rover in the MARS2020 mission. Authors evaluated the use of acoustic signals both in the time-domain and frequency-domain in sensitive cases for the separation of minerals exhibiting LIBS spectra featuring almost the same emission lines using PCA schemes for each pressure setting. Thus, we report herein the impact of the surrounding pressure level upon this diagnostic tool. Results reported herein are the first steps into a new hyphenation technique featuring LIBS. Overall, this paper highlights the potential still to be uncovered in combining the different phenomena occurring during a LIBS event.

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