Detecting sulfur on the Moon: The potential of vacuum ultraviolet laser-induced breakdown spectroscopy

Abstract

Laser-induced breakdown spectroscopy (LIBS) receives increasing interest in the field of space exploration. Some volatile elements important to planetary research are, however, challenging to detect with conventional LIBS because their strongest emission lines lie in the vacuum-UV (VUV) spectral range below 200nm. We demonstrate the potential of LIBS detection in the VUV range from 125 to 190 nm on one of the relevant challenging elements, S, in a lunar context. With our laboratory set-up, we detected S in lunar analogues at concentrations ranging from 0.5 to 4.0 at\% in a time-integrated configuration. The experiments were conducted in high vacuum ($10^{-3}$ Pa) with laser parameters realistic for a space instrument (1064 nm, 25 mJ, 6 ns). The results indicate improved detection capabilities for S in a lunar geological context as compared to LIBS in the more common UV/VIS/NIR spectral range. We found limits of detection in the order of 0.5at S for all investigated sample series. Normalization by total intensity and by an internal standard, in our case by the emission signal of the O I triplet at 130 nm, was tested as well, but did not yield a noticeable improvement for our data. Besides S, we were also able to detect Al, Fe, O, Si and Ti from the lunar analogue matrix.