Effect of Bond Strength on Al and Si Ablation and Intensity in Laser-Induced Breakdown Spectroscopy (LIBS)

LIBS analysis uses a focused laser pulse to ablate atoms and form a plasma. As the plasma cools, excited electrons relax into lower-energy orbitals, releasing photons, which are collected through an optic fiber, diffracted, and recorded on a CCD camera. Wavelength intensity is proportional to the number of atoms of the element in the plasma, corrected for factors such as plasma temperature and orbital transition probability.

In LIBS analysis of silicate minerals, Si peaks are less intense than expected; the reason for this is not yet known. Because Si typically forms strong bonds, we speculate that bond strengths affect ablation. To test for non-stoichiometric ablation, we compare the differences in Al and Si intensity peaks in the aluminum-silicate polymorphs. All Si in the polymorphs is tetrahedral. In kyanite, all Al is octahedral (AlO₆). In andalusite, Al is in AlO₆ and AlO₅ polyhedra. In sillimanite, Al is in AlO₆ and AlO₄. Al-O bonds in sillimanite are the strongest of the three minerals; the Al-O bonds in kyanite are weakest. If ablation is affected by bond strength, Al peak intensities will follow: ky > and > sill.

Eight samples of the polymorphs were analyzed by an Ocean Optics 2500 LIBS using a Nd-YAG laser (wavelength 1064 nm, laser energy of 153 mJ). Intensity ratios for major emission lines for Si at 251.6 and 288.2 nm and for Al at 394.4 and 396.2 nm were calculated (averages of 200 shots per sample). For Al 394.4/Si 251.6 and Al 396.2/Si 251.6, intensity ratios are consistent with non-stoichiometric ablation (kyanite: 19.0-28.5 and 24.6-37.6, respectively; andalusite: 3.8-6.4 and 5.3-8.9; sillimanite: 2.9-3.2, 3.5-3.7). For Al 394.4/Si 288.2 and Al 396.2/Si 288.2, ratios are much higher for kyanite (8.7-12.7 and 11.3-16.8, respectively) than for andalusite and sillimanite, which are similar (andalusite: 2.4-2.8 and 3.2-3.9; sillimanite: 3.0-5.6 and 3.4-6.9).