Packing thickness dependent plasma emission induced by laser ablating thin-layer microgranular materials

An experimental study on the packing thickness (PT) dependent plasma emission caused by laser ablating thin-layer microgranular samples in air was conducted using three sets of size-selected copper grains (median size d₅₀ = 53 μm, 72 μm, and 100 μm, respectively). For each size-selected case, the PT parameter was tuned from 0.15 to 1.00 mm through varying the amount of grains packed into a vessel with a steel bottom wall and the emission spectra of laser-induced plasma were measured at various PT. It is found that there is a striking threshold phenomenon in the measured behavior of PT-dependent plasma emission. Specifically, when PT is less than a threshold PT_th, the emission intensity exhibits an exponential decreasing with incremental thickness; however, when it exceeds PT_th, the emission intensity becomes almost constant. It is also found that the PT_th slightly depends on grain size but the ratio of PT_th to d₅₀ seems to be size independent. Combining the mechanical fundamentals of granular materials, we interpreted the findings by considering a PT-dependent effect of the vessel's bottom on the formation circumstance of a laser-induced plasma. This work has practical significance in assessing a threshold thickness above which laser-induced breakdown spectroscopy, as an analytical technique to quantify elements embedded in microgranular materials, is viable regardless of PT difference.