Unraveling Li I 670.8 nm self-reversal and atomic distribution inhomogeneity in laser ablation plumes under varying argon pressures

Laser Induced Breakdown Spectroscopy (LIBS) offers a promising rapid detection method for ^6,7Li isotopic analysis. The Li I 670.8 nm provides the largest isotopic shift in the UV-VIS spectral region ($\approx$15.8 pm). However, Li 670.8 nm transition is a resonance doublet and possesses very high transition probability, making it susceptible to self-absorption and self-reversal, which can affect line width, shape, and overall analytical capabilities. Environmental factors, such as the pressure and type of ambient gas, significantly influence the physical conditions of plasma and its hydrodynamics, thereby impacting the self-reversal phenomenon. In this study, we investigate plasmas generated via nanosecond laser ablation of a LiAlO₂ target with natural isotopic abundance in an inert gas (Ar) environment at pressures ranging from 0.1 to 100 Torr. Complementary fast-gated Li monochromatic imaging and spatially-integrated optical emission spectroscopy were used to explore the relationship between Li atomic distribution and emission gradients in laser-produced plasmas and self-reversal in Li I spectral features. Results highlight that self-reversal is prominent at later times in plasma evolution (greater than 5 μs) for Ar pressure levels between 1 and 100 Torr, due to a significant buildup of Li ground level population at the plume-background gas interface. Conversely, at lower pressures (0.1 Torr or less), rapid plasma expansion without a strong plume-background interface results in the absence of self-reversal in the Li I spectral profiles.