Spatio-temporal evolution of C₂ and CN emissions in laser-induced shale plasma under inert atmospheres: implications for LIBS-based TOC analysis

In situ TOC analysis of shale by LIBS is complicated by the coexistence of organic carbon, carbonate carbon, and a heterogeneous mineral matrix. In this study, the spatio-temporal evolution of C₂ and CN emissions in laser-induced shale plasma was investigated under inert atmospheres. Spatio-temporal optical emission spectroscopy, narrow-band filtered imaging, plasma diagnostics, and numerical modeling were combined to clarify the origin and evolution of molecular emissions. Under the present experimental conditions, C₂ and CN emissions from carbonate reference materials were below the detection limit. The clear molecular emissions were observed from shale and coal. This result indicates that the molecular emissions from the investigated shale samples were dominated by organic-carbon-related species. A plasma chemical reaction model based on local thermodynamic equilibrium was established. The model reproduced the late-stage tendency of molecular enrichment in cooler peripheral regions, which is consistent with thermodynamic recombination during plasma expansion. In contrast, the early emission from the plasma core was not fully captured by the equilibrium model. A calibration using five shale samples showed that the early 1–5 µs molecular-emission window provided a better correlation with TOC than the late 10–30 µs window. These results provide mechanistic guidance for selecting observation windows in LIBS-based shale TOC analysis.